Dissertations / Theses on the topic 'Nanofluidlcs'

Este trabalho trata do estudo teórico do ebulição convectiva de nanofluidos em canais de diâmetro reduzido (denominados de microcanais). Ele aborda, primeiramente, uma análise da literatura sobre a ebulição convectiva de fluidos convencionais em microcanais, na qual são discutidos critérios para a transição entre macro e microcanais e os padrões de escoamentos observados em canais de reduzido diâmetro. Métodos para a previsão das propriedades de transporte de nanofluidos foram levantados da literatura e estudos experimentais da convecção forçada, da ebulição nucleada e da ebulição convectiva de nanofluidos foram discutidos. Um método para a previsão do coeficiente de transferência de calor de nanofluidos em microcanais durante a ebulição convectiva foi proposto baseado em modelos convencionais da literatura ajustados para nanofluidos. O ajuste dos modelos convencionais foi realizado através de análise regressiva de dados experimentais para ebulição nucleada e convecção forçada de nanofluidos levantados da literatura, e da análise crítica de adimensionais que capturassem a influência das nanopartículas no processo de transferência de calor. De maneira geral o método proposto neste estudo apresenta concordância razoável com dados experimentais independentes, referente ao acréscimo do coeficiente de transferência de calor com o incremento da concentração volumétrica de nanopartículas. No entanto, a escassez de estudos experimentais sobre a ebulição convectiva de nanofluidos, especialmente em microcanais, impossibilitou uma análise mais aprofundada do método proposto.
The present work aims the theoretical study of convective boiling of nanofluids in small diameter channels (called microchannel). It discusses an analysis of the literature on convective boiling of conventional fluids in microchannels which presents criteria for the transition between conventional and microchannels and the flow patterns observed in small diameter channels. Methods for predicting the transport properties of nanofluids were compiled from the literature and experimental studies of forced convection, nucleate boiling and convective boiling of nanofluids were discussed. A method for predicting the heat transfer coefficient of nanofluids in microchannels during convective boiling was proposed based on conventional models from literature adjusted to nanofluids. The conventional models fitting was performed by regression analysis of experimental data for nucleate boiling and forced convection of nanofluids compiled from the literature and by critical analysis of dimensionless numbers which enable to capture the influence of nanoparticles on heat transfer process. In general the proposed method in this work presents reasonable agreement with independent experimental data regarding the increase in heat transfer coefficient with increasing nanoparticles volume fraction. However the scarcity of experimental studies on the convective boiling of nanofluids, especially in microchannels, precluded further analysis of the proposed method.

Flow cells are on their way to become a key player for electrical energy storage (EES) thanks to their suitability as load-levelling devices thus contributing to the development of smart grid and to offset the intermittency of renewable energy sources. Until recently flow cells have been limited to Redox Flow Batteries (RFB), where energy storage is given by the redox reactions of dissolved ions. Very recently, new types of “flowable” electrodes have been proposed making use of capacitive storage mechanism (Electrochemical Flow Capacitors or EFCs). Our group has been one of the pioneering labs in this type of novel flow cells based on electroactive nanofluids. The present thesis aimed at harnessing the activity of well-known electroactive species (quinones, graphene, polyoxometalates, LiFePO4) in novel electroactive nanofluids. An important part of our strategy has been the design of hybrid formulations and systems which could combine faradaic (redox) and capacitive (double-layer) storage mechanisms in order to improve the performance of the resulting flow cells. We make an extended review and perspective of the electrochemical flow cell technology and their possible lines of evolution in the introduction of this thesis. With this we introduce the state of the art, the issues to solve and the different solutions proposed. Moreover, we also show our point of view and prospective for this technology and electrical energy storage in general. In the chapter 4, the electrochemical fundamentals of quinones in lithium-organic electrolytes are studied. Quinones electrochemical mechanisms have been widely studied in aqueous media. In this work we study them in an organic electrolyte in an attempt to take advantage of the greater solubility and wider potential windows available in this media. We found and describe in detail several issues preventing the reversible functioning of quinones in Li+ organic electrolytes which in turn preclude their use in flow cells under those conditions. Chapter 5 describes the synthesis, characterization and electrochemical performance of hybrid materials based on reduced graphene oxide (rGO) and polyoxometalates dispersed in an aqueous H2SO4 electrolyte in order to produce a nanofluid. These nanofluids feature low viscosity and show an ultrafast electrochemical response. We demonstrated their functioning as energy storage fluids with full charge and discharge of all solid material dispersed. In chapter 6 a new kind of rGO nanofluid is presented. Instead of using conventional surfactants, we dissolved an aromatic molecule able to stabilize rGO in an aqueous electrolyte. With this approach we achieved a great increase in the stability of the nanofluid. Furthermore, this new nanofluid also showed a great charge transfer capability, as demonstrated by its enabling of the redox activity of LiFePO4 nanoparticles. Thus, thanks to the presence of rGO in the nanofluid, electrons could reach the dispersed nanoparticles and thus be effectively and fully charged and discharged, something not possible in nanofluids containing only LiFePO4 nanoparticles. Graphene synthesis has been also deeply studied as part of this thesis as is shown in the chapter 7. As a result, a new method for the production of graphene by electrochemical exfoliation of graphite has been developed and patented. The patent, a summary of the results obtained and the state of the art of the electrochemical exfoliation method of graphene are presented in this thesis as the last chapter describing research work carried out within the framework of this thesis.
Las celdas de flujo van camino de convertirse en una pieza clave para el almacenamiento de energía eléctrica (EES) gracias a su idoneidad como dispositivos de nivelación de carga, contribuyendo así al desarrollo de una red inteligente que pueda compensar la intermitencia de las fuentes de energía renovables. Hasta hace poco, las celdas de flujo se habían limitado a las baterías de flujo redox (RFB), donde el almacenamiento de energía está dado por las reacciones redox de los iones disueltos. Muy recientemente, se han propuesto nuevos tipos de electrodos líquidos basados en un mecanismo de almacenamiento capacitivo (condensadores de flujo electroquímicos o EFC). Nuestro grupo ha sido uno de los laboratorios pioneros en este tipo de nuevas celdas de flujo basadas en nanofluidos electroactivos. La presente Tesis ha tenido como objetivo aprovechar la actividad de especies electroactivas bien conocidas (quinonas, grafeno, polioxometalatos, LiFePO4) en nuevos nanofluidos electroactivos. Una parte importante de nuestra estrategia ha sido el diseño de formulaciones y sistemas híbridos que pudieran combinar mecanismos de almacenamiento faradaico (redox) y capacitivo (doble capa) para mejorar el rendimiento de las celdas de flujo resultantes. En la introducción de esta tesis se ha realizado una revisión y perspectiva ampliadas de las tecnologías de celdas de flujo electroquímicas y sus posibles líneas de evolución. Con esto se presentan el estado del arte, los problemas a resolver y las diferentes soluciones propuestas para estas tecnologías. Además, también mostramos nuestro punto de vista y perspectivas para estas tecnologías y el almacenamiento de energía eléctrica en general. Por ello esta parte constituye la parte principal de la introducción y una parte fundamental de esta tesis para entender los objetivos, motivaciones y el trabajo realizado. En el capítulo 4 se estudiaron los fundamentos electroquímicos de las quinonas en electrolitos orgánicos con sal de litio. Los mecanismos electroquímicos de las quinonas se han descrito ampliamente en la bibliografía pero en medios acuosos. En este trabajo, los estudiamos en un electrolito orgánico en un intento de aprovechar la mayor solubilidad y las ventanas de potencial más amplias disponibles en este medio. Encontramos y describimos en detalle varios problemas que impiden el funcionamiento reversible de las quinonas en los electrolitos orgánicos con Li+ que, a su vez, impiden su uso en celdas de flujo en esas condiciones. En el capítulo 5 se describe la síntesis, caracterización y rendimiento electroquímico de materiales híbridos basados en óxido de grafeno reducido (rGO) y polioxometalatos dispersos en un electrolito acuoso (H2SO4) para producir un nanofluido. Estos nanofluidos presentan baja viscosidad y muestran una respuesta electroquímica ultrarrápida e hibrida, con contribución tanto capacitiva del rGO como faradaica de los polioxometalatos. Demostrando así su funcionamiento como fluidos de almacenamiento de energía con plena carga y descarga de todo el material sólido disperso. El sexto capítulo presenta un nuevo tipo de nanofluido basado en rGO. En lugar de usar tensioactivos convencionales como en el capítulo descrito anteriormente, disolvimos una molécula aromática capaz de estabilizar el rGO en un electrolito acuoso mediante interacciones de tipo π-π. Con este enfoque logramos un gran aumento en la estabilidad del nanofluido. Además, este nuevo nanofluido también mostró una gran capacidad de transferencia de carga, como lo demuestra el hecho de que permite que se produzca actividad redox de nanopartículas de LiFePO4 (sin recubrimento conductor) simplemente dispersas en el nanofluido. Por lo tanto, gracias a la presencia de rGO en el nanofluido, los electrones podrían alcanzar las nanopartículas dispersas y, por lo tanto, cargarse y descargarse de manera efectiva y completa, algo que no es posible en nanofluidos que contienen solo nanopartículas de LiFePO4. La síntesis de grafeno también se ha estudiado en profundidad en esta tesis tal y como se puede ver en el capítulo 7, dado que el objetivo final es producir materiales que se puedan usar en aplicaciones reales, asegurarse de que los materiales con los que se trabaja se pueden producir en cantidades grandes, mediante métodos escalables y elementos abundantes es también importante. Como resultado, se ha desarrollado y patentado un nuevo método para la producción de grafeno por exfoliación electroquímica de grafito. En esta tesis se presenta la patente, un resumen de los resultados obtenidos y el estado del arte del método de exfoliación electroquímica de grafeno. En esta tesis hemos demostrado el potencial de los nanofluidos en el almacenamiento de energía electroquímica. A partir de los resultados mostrados aquí, podemos inducir conclusiones generales importantes sobre los efectos extendidos de pequeñas cantidades de sólidos en todo el volumen del nanofluido. Hemos demostrado que las dispersiones estables de rGO en agua pueden transferir la carga a través de todo el volumen de nanofluidos, lo que hace que todo el nanofluido actúe como un electrodo supercondensador que almacena la carga a través de un mecanismo capacitivo. De hecho, el nanofluido acuoso rGO mostró una transferencia de carga extremadamente rápida, pudiendo realizar ciclos a 10V·s-1. Gracias a esta rápida transferencia de carga, pudimos cargar y descargar por completo nanopartículas activas redox dispersas de LiFePO4 y detectar claramente picos redox incluso a 25 mV·s-1. Además, al dopar el rGO con especies redox activas moleculares como los polioxometalatos, desarrollamos sistemas híbridos con potencia y capacidad mejoradas con respecto al nanofluido rGO puro. Finalmente, demostramos que los nanofluidos de rGO acuosos pueden mejorar su estabilidad al disolver una molécula aromática (DABA) capaz de estabilizar rGO mediante interacciones π-π manteniendo su buena conductividad eléctrica. Todo esto ha sido posible manteniendo la viscosidad de los nanofluidos desarrollados muy cerca de los disolventes originales, lo que facilitará su aplicación final en dispositivos de flujo real. Por otro lado, la baja concentración de nanopartículas de grafeno podría ser una desventaja para la aplicación de estos materiales en dispositivos de alta densidad de energía. Por lo tanto, aumentar la carga de nanopartículas electroactivas es un objetivo importante. En resumen, hemos diseñado y preparado nanofluidos basados en grafeno pero también en híbridos de grafeno. Hemos mostrado en esta descripción general cómo estos novedosos materiales de nanofluidos pueden presentar rendimientos sobresalientes incluso en el caso de sistemas muy diluidos. Hemos demostrado efectos no lineales, que conducen a propiedades notables con pequeñas cantidades de grafeno dispersas en los nanofluidos. Por lo tanto, nuestro trabajo subraya el sólido potencial de estos sistemas para el almacenamiento de energía.

Los fluidos de transferencia de calor, y en particular los nanofluidos, se pueden considerar un elemento esencial en diversos sectores industriales y su rendimiento es clave para una adecuada aplicación en tecnologías que van desde la gestión térmica y la refrigeración, a la generación de energía solar térmica y eléctrica mediante el uso de intercambiadores de calor. Estas industrias necesitan fluidos de transferencia de calor con un rango de temperatura del líquido más amplio y mejores prestaciones en la transferencia de calor que los fluidos convencionales. Todos los fluidos parecen beneficiarse de la dispersión de nanopartículas sólidas, tanto aquellos usados en aplicaciones de baja temperatura y temperatura ambiente, como aquellos que funden a más alta temperatura (p. ej. sales fundidas). La dispersión de nanopartículas conduce a la obtención de nanofluidos que con frecuencia presentan mejores conductividades térmicas y/o calores específicos en comparación con los fluidos base. Sin embargo hay algunas excepciones. En la bibliografía podemos encontrar resultados contradictorios acerca de la mejora de las propiedades térmicas en nanofluidos, lo cual hace que sea necesario un estudio de estos materiales en mayor profundidad. Por otra parte, la naturaleza líquida de estos materiales plantea un verdadero desafío, tanto desde el punto de vista experimental como en relación al marco conceptual. El trabajo que se presenta en esta tesis ha abordado dos retos diferentes relacionados con los fluidos de transferencia de calor y los nanofluidos. En primer lugar, se llevó a cabo un estudio riguroso y sistemático de las propiedades térmicas, morfológicas, reológicas, de estabilidad, acústicas y vibracionales de nanofluidos de grafeno en disolventes orgánicos. Observamos un gran aumento de la conductividad térmica de hasta un 48% y un aumento del 18% en la capacidad calorífica de los nanofluidos de grafeno en N,N-dimetilacetamida (DMAc). También se observó una mejora significativa en los nanofluidos de grafeno en N,N-dimetilformamida (DMF) del orden del 25% y 12% para la conductividad térmica y la capacidad calorífica, respectivamente. El desplazamiento de varias bandas del espectro Raman de DMF y DMAc hacia altas frecuencias (máx. ~ 4 cm-1) al aumentar la concentración de grafeno, sugirió que éste tiene la capacidad de afectar a las moléculas de disolvente a larga distancia, en términos de energía vibracional. En paralelo, las simulaciones numéricas basadas en la teoría funcional de la densidad (DFT) y dinámica molecular (MD) mostraron una orientación paralela de DMF hacia el grafeno, favoreciendo la interacción π-π y contribuyendo a la modificación de los espectros de Raman. Además, se observó un orden local de las moléculas de DMF alrededor del grafeno, lo que sugiere que tanto este tipo especial de interacción como el orden local inducido pueden contribuir a la mejora de las propiedades térmicas del fluido. También se realizaron estudios similares en nanofluidos de grafeno disperso en 1-metil-2-pirrolidona, sin embargo, no se observó ninguna modificación de la conductividad térmica o de los espectros de Raman. Todas estas observaciones juntas sugieren que existe una correlación entre la modificación de los espectros vibracionales y el aumento de la conductividad térmica de los nanofluidos. En vista de los resultados, se discutieron y descartaron algunos de los mecanismos propuestos para explicar la mejora de la conductividad térmica en nanofluidos. La segunda línea de investigación se centró en el desarrollo y caracterización de nuevas formulaciones de sales fundidas con baja temperatura de fusión y alta estabilidad térmica. Con este propósito, se sintetizaron dos nuevas formulaciones de seis componentes basadas en nitratos con una temperatura de fusión de 60-75 °C y una estabilidad térmica de aprox. 500 °C. Por otro lado, la complejidad de las muestras llevó a establecer una serie de métodos experimentales que se proponen para la detección del punto de fusión de estos materiales como una alternativa a la calorimetría convencional, estas técnicas son: espectroscopia Raman, técnica 3ω y transmisión óptica.
Heat transfer fluids and nanofluids constitute an important element in the industry and their performance is key to the successful application in technologies that go from heat management and cooling to heat exchangers in thermal-solar energy and electricity generation. These industries demand heat transfer fluids with a wider liquid temperature range and better thermal performance than the conventional fluids. From low-temperature fluids to high-temperature molten salts, these fluids seem to benefit from the dispersion of solid nanoparticles, leading to nanofluids which frequently feature improved thermal conductivities and/or specific heats as compared with the bare fluids. However, there are some exceptions. Contradictory reports make it necessary to study these materials in greater depth than has been usual. Yet, the liquid nature of these materials poses a real challenge, both from the experimental point of view and from the conceptual framework. The work reported in this thesis has tackled two different challenges related to heat transfer fluids and nanofluids. In the first place, a careful and systematic study of thermal, morphological, rheological, stability, acoustic and vibrational properties of graphene-based nanofluids was carried out. We observed a huge increase of up to 48% in thermal conductivity and 18% in heat capacity of graphene-N,N-dimethylacetamide (DMAc) nanofluids. A significant enhancement was also observed in graphene-N,N-dimethylformamide (DMF) nanofluids of approximately 25% and 12% for thermal conductivity and heat capacity, respectively. The blue shift of several Raman bands (max. ~ 4 cm-1) with increasing graphene concentration in DMF and DMAc nanofluids suggested that graphene has the ability to affect solvent molecules at long-range, in terms of vibrational energy. In parallel, numerical simulations based on density functional theory (DFT) and molecular dynamics (MD) showed a parallel orientation of DMF towards graphene, favoring π–π stacking and contributing to the modification of the Raman spectra. Furthermore, a local order of DMF molecules around graphene was observed suggesting that both this special kind of interaction and the induced local order may contribute to the enhancement of the thermal properties of the fluid. Similar studies were also performed in graphene-N-methyl-2-pyrrolidinone nanofluids, however, no modification of the thermal conductivity or the Raman spectra was observed. All these observations together suggest that there is a correlation between the modification of the vibrational spectra and the increase in the thermal conductivity of the nanofluids. In light of these results, the mechanisms suggested in the literature to explain the enhancement of thermal conductivity in nanofluids were discussed and some of them were discarded. The second line of research focused on the development and characterization of novel molten salts formulations with low-melting temperature and high thermal stability. In this regard, two novel formulations of six components based on nitrates with a melting temperature of 60-75 °C and a thermal stability up to ~ 500 °C were synthesized. Moreover, the complexity of the samples led to establish a series of experimental methods which are proposed for the melting temperature detection of these materials as an alternative to conventional calorimetry. These methods are Raman spectroscopy, three-omega technique, and optical transmission.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 217-226).
Diagnostic tests are essential to medical practice. In vitro diagnostics is a market worth US$ 40-45 billion. Diagnostic tests are usually conducted in centralized laboratories, equipped with expensive instrumentation and staffed with trained personnel. An important part of clinical diagnosis involves protein and DNA sensing. Significant effort is made to make protein and DNA sensing more accessible and affordable, through micro and nano-technologies. However, typical commercial and academic devices for molecular sensing suffered needs for external equipment, high cost and large form factors. In this work, we propose a self-contained point-of-care platform based on complementary metal oxide semiconductor (CMOS). CMOS platform has the capability of pattern features at the scale of nanometers. Important electronic functions in bio-sensing, such as amplifiers, counters and drivers are routinely implemented in CMOS. With the introduction of photonic and nanofluidic functionalities in this thesis, a CMOS chip can potentially perform biomolecular sensing without the aid of external equipment, hence becoming true lab-on-chip devices. This thesis presents the methods developed to introduce nanofluidic and photonic devices in commercial CMOS chips. We first introduce a method to fabricate nanofluidic channels in CMOS by using the transistor gate polysilicon as a sacrificial layer. A nanochannel with critical dimension of 100nm and length of 200 [mu]m is fabricated. Actuation and separation of bio-molecules in the nanochannel with electrophoresis is demonstrated. We then incorporate avalanche photodiodes (APD) in CMOS. Additionally, a packaging method is introduced to work with CMOS chips with size of a few square millimeters. With components mentioned above, clinical applications, such as gene mapping for virus identification and protein separation for cancer diagnosis and monitoring, could potentially run on a chip without external equipment.
by Huaiyu Meng.
Ph. D.

The study of superfluid helium has been carried out mainly by physicists. In recent years, taking advantage of the potentialities presented by superfluid helium nanodroplets (HeNDs) as inert matrices at very low temperatures (0.37 K), the chemical community became involved in its application to high-resolution spectroscopy. More recently (early 2000s), this community began to be involved in research using HeNDs to investigate chemical reactivity in this quantum solvent. As for the theoretical studies on the dynamics of physicochemical processes in HeNDs, they have been possible about five years ago and the number of theoretical dynamics studies, despite their interest, is very scarce. The main objective of this thesis is to contribute to the development of the research in this area. To introduce the reader into the topic, Chapter 1 is divided into four sections: the first one describes the properties of helium, the second one considers the history of the discovery and research carried out on the superfluidity phenomenon, the third one outlines the properties of superfluid helium nanodroplets, and the last one gives an overview of the applications and fields of study implying HeNDs. The theoretical and numerical methods used to describe superfluid liquid helium are detailed in Chapter 2. In the first section attention has been paid on the density functional theory (DFT) and its time dependent extension for real-time simulations (TDDFT). The second section describes the main density functionals used and the third section is aimed to present the numerical methods employed to perform the TDDFT calculations. The following four chapters contain the original studies carried out in this thesis. The investigation of the capture process of a Ne atom by a HeND can be found in Chapter 3. Here, the atom is treated using classical mechanics and the influence of energy and angular momentum is examined for a wide set of initial conditions. The microscopic mechanism, energy and angular momentum exchanges and vortex formation have been extensively analysed. The present contribution corresponds to the first systematic analysis of the influence of angular momentum in the capture process and vortex formation. Chapter 4 represents a natural evolution from Chapter 3 and describes the formation of a neon dimer or neon adduct inside a superfluid helium nanodroplet, treating both atoms classically. Analogously as in the previous chapter, angular momentum has also been taken into consideration and the mechanism, energy an angular momentum exchanges and vortex formation are analysed. These two chapters complement and extend two previous investigations of our group where the Ne atoms were treated using standard quantum mechanics at zero angular momentum. The contents of Chapter 4 correspond to the second theoretical investigation on bimolecular reaction dynamics in HeNDs. The following two chapters use a full quantum hybrid approach to explore rotational and vibrational energy relaxation dynamics. Chapter 5 corresponds to the first theoretical study reported so far on the rotational energy relaxation dynamics of molecules in HeNDs. This process has been studied using several isotopes of the H2 molecule (fast rotors) and considering a set of initial excitations and nanodroplet sizes. The last investigation (Chapter 6) is centred on the study of the vibrational energy relaxation in HeNDs. Thus, the influence of the energy gap between the vibrational levels, molecule-helium interaction energy and nanodroplet size on the vibrational relaxation dynamics has been analysed, taking as a reference the I2@(4He)100 doped nanodroplet which was recently studied in our group. To the best of our knowledge it is the first time that the influence of these key factors has been examined. Finally, in Chapters 7 and 8 the main conclusions and a summary in Catalan are presented.
Les nanogotes d’heli superfluid (HeNDs) són matrius inerts i nanoreactors ideals a baixa temperatura (0.37 K). Això ha atret l’atenció de químics doncs permeten realitzar espectroscopia d’altra resolució, estudiar la reactivitat i sintetitzar en condicions especials. L’estudi teòric de la dinàmica de processos en HeND ha estat possible tan sols fa cinc anys i, tot i el seu interès, n’hi ha molt pocs estudis. L’objectiu d’aquesta tesi és contribuir a la recerca en aquesta àrea. El Capítol 1 descriu les propietats de l’heli, la història de la superfluïdesa i les propietats i aplicacions de les HeNDs. La teoria del funcional de la densitat (DFT) i l’extensió de la mateixa depenent del temps (TDDFT), els principals funcionals per HeNDs i els mètodes numèrics es presenten al Capítol 2. Els següents capítols contenen els estudis originals d’aquesta tesi. En el Capítol 3 s’investiga la captura de Ne en una HeND on l’àtom es tracta clàssicament. El mecanisme, els intercanvis d’energia i moment angular i la formació de vòrtexs s’han analitzat àmpliament. Aquest és el primer anàlisi rigorós de la influència del moment angular en la captura i formació de vòrtexs. El Capítol 4 descriu la formació de Ne2/Ne-Ne en HeND tractant ambdós àtoms clàssicament. El mecanisme, bescanvis d’energia i moment angular i formació de vòrtexs també s’han estudiat. És el segon estudi sobre reaccions bimoleculars en HeNDs. Els Capítols 3 i 4 complementen i amplien dues investigacions del nostre grup on els àtoms es van tractar quànticament amb moment angular zero. En els propers dos capítols es consideren les relaxacions rotacional i vibracional utilitzant enfocs quàntics híbrids. El Capítol 5 correspon al primer estudi teòric de la relaxació rotacional de molècules en HeNDs, i s’han considerat varis isòtops de H2 i excitacions inicials i mides de nanogota. El Capítol 6 detalla la influència de la separació energètica vibracional, interacció molècula-heli i mida de nanogota en la relaxació vibracional en HeNDs, agafant com a referència el sistema I2@(4He)100. És el primer cop que s’examina l’efecte d’aquestes propietats clau. Els Capítols 7 i 8 presenten les principals conclusions i un resum en català, respectivament.
Las nanogotas de helio superfluido (HeNDs) son matrices inertes y nanoreactores ideales a baja temperatura (0.37 K). Esto ha atraído a los químicos pues posibilitan realizar espectroscopia de alta resolución, así como estudiar de la reactividad y síntesis en condiciones especiales. La dinámica teórica de procesos en HeND ha sido posible tan sólo hace cinco años y, a pesar de su interés, todavía hay muy pocos estudios. Esta tesis pretende contribuir a la investigación en esta área. El Capítulo 1 describe las propiedades del helio, la superfluidez y las propiedades y aplicaciones de las HeNDs. La teoría del funcional de la densidad (DFT) y su extensión dependiente del tiempo (TDDFT), los principales funcionales para HeNDs y los métodos numéricos se presentan en el Capítulo 2. Los siguientes capítulos contienen los estudios originales de esta tesis. En el Capítulo 3 se investiga la captura de Ne en una HeND donde el átomo se trata clásicamente. El mecanismo microscópico, intercambios de energía y momento angular y formación de vórtices se han analizado ampliamente. Este es el primer análisis detallado de la influencia del momento angular en la captura y la formación de vórtices. El Capítulo 4 describe la formación de Ne2/Ne-Ne en HeND tratando ambos átomos clásicamente. El mecanismo, intercambios de energía y momento angular y formación de vórtices también se han estudiado. Los Capítulos 3 y 4 complementan y amplían dos investigaciones de nuestro grupo donde los átomos se trataron cuánticamente con momento angular cero. En los dos capítulos siguientes se estudian las relajaciones rotacional y vibracional utilizando enfoques cuánticos híbridos. El Capítulo 5 corresponde al primer estudio teórico de la relajación rotacional de moléculas en HeNDs, y se han considerando varios isótopos de H2, excitaciones iniciales y tamaños de nanogota. El Capítulo 6 detalla la influencia de la separación energética, interacción molécula-helio y tamaño de nanogota en la relajación vibracional en HeND, habiéndose tomando como referencia el sistema I2@(4He)100. Es la primera vez que se examina el efecto de estas propiedades clave en la dinámica. Los Capítulos 7 y 8 presentan las principales conclusiones y un resumen en catalán, respectivamente.

Dans notre vie quotidienne, le transfert de la chaleur et de l’énergie constitue la base de nombreux processus industriels. L’épuisement progressif des énergies fossiles conduit à améliorer et optimiser les rendements de ces échanges par de nouveaux procédés. Pour cela, une idée d’améliorer la performance thermique des fluides dans les échangeurs de chaleur a été proposée pour réduire l’énergie consommée pour l’échange de chaleur. Cette idée est basée sur l’introduction des nanoparticules solides qui présentent des propriétés thermiques beaucoup plus importantes que les liquides caloporteurs dans ces derniers, en obtenant un nanofluide. Cette introduction a pour effet d’augmenter la conductivité thermique du fluide mais d’autre part provoque une augmentation défavorable de sa viscosité qui résulte en une augmentation de la puissance de pompage. Alors il faut faire un compromis entre la stabilité, la conductivité thermique et la viscosité des nanofluides. Dans cette étude, des nanofluides à base de graphène à quelques couches et un fluide commercial, Tyfocor® LS, ont été préparés dans la gamme de concentration massique 0,05-0,5% en utilisant trois surfactants différents. Une étude complète sur ces nanofluides est présentée, y compris la synthèse des feuillets de graphène, la préparation des nanofluides et l’étude de leur stabilité, ainsi que l’évaluation expérimentale de leurs propriétés thermophysiques en fonction de la concentration en graphène, du type de surfactant utilisé et de la température dans la gamme 283,15-323,15 K. Finalement, sur la base de ces résultats et par une approche qualitative, le potentiel applicatif des nanofluides dans des systèmes énergétiques est déterminé pour sélectionner le meilleur candidat. Les résultats ont montré une bonne amélioration de la performance thermique par rapport aux fluides de base dans la gamme de température testée et surtout le nanofluide de la série du surfactant Pluronic® P-123 de concentration massique 0,25%
In our daily lives, the heat and energy transfer forms the basis of many industrial processes. The gradual depletion of fossil fuels leads to improving and optimizing the efficiency of these exchanges through new processes. To this end, the idea of improving the thermal performance of fluids in heat exchangers has been proposed forward to reduce the energy consumed for heat exchange. This idea is based on the introduction of solid nanoparticles, which have much greater thermal properties than heat-transfer fluids in the latter, obtaining a nanofluid. This introduction has the effect of increasing the thermal conductivity of the fluid but on the other hand causes an unfavorable increase in its viscosity, which results in an increase in pumping power. So a compromise has to be made between the stability, thermal conductivity and viscosity of nanofluids. In this study, few layer graphene based nanofluids and a commercial fluid, Tyfocor® LS, were prepared in the weight concentration range 0.05-0.5% using three different surfactants. A complete study on these nanofluids is presented, including the synthesis of the graphene sheets, the preparation of the nanofluids and the study of their stability, as well as the experimental evaluation of their thermo-physical properties as a function of the graphene concentration, the type of surfactant used and the temperature in the range 283.15-323.15 K. Finally, on the basis of these results and through a qualitative approach, the potential application of nanofluids in energy systems is determined in order to select the best candidate. The results showed a good improvement of the thermal performance compared to the base fluids in the tested temperature range and especially the nanofluid of the Pluronic® P-123 surfactant series with a mass concentration of 0.25%

Cette thèse décrit diverses situations liées au transport fluidique aux nano-échelles. Le premier chapitre est une introduction à la nanofluidique qui contient une revue des longueurs caractéristiques, des forces et des phénomènes présents aux nano-échelles. Le deuxième chapitre est une étude de l'impact de la géométrie sur la perméabilité hydrodynamique d'un nanopore. Inspirée par la forme des aquaporines, cette étude suggère une optimisation possible pour des canaux biconiques. Le troisième chapitre est une étude du remplissage capillaire dans des canaux sub-nanométriques en carbone. Cette étude montre l'importance de la pression de disjonction induite par la structure du fluide sur le remplissage. Le quatrième chapitre est une étude d'une diode nanofluidique, un composant connu pour imiter le comportement d'une diode à semi-conducteur. On montre qu'un fort couplage entre l'eau et la dynamique des ions entraîne une rectification du flux d'eau à l'intérieur de la diode. Le cinquième et dernier chapitre est une étude de l'origine du bruit rose (1=f) communément observé lors des mesures de courant ionique dans les nanopores
This thesis discusses various situations linked to transport at the nanoscale. The first chapter is an introduction to nanofluidics, containing a review of characteristic lengths, forces, or phenomena existing at the nanoscale. The second chapter is a study of the impact of geometry on the hydrodynamic permeability of a nanopore. This study, inspired by the shape of aquaporins, suggests a possible optimisation of permeability for bi-conical channels. The third chapter is a study of capillary filing inside subnanometric carbon channels which highlights the importance of the disjoining pressure induced by the fluid structuring inside the nanochannel. The fourth chapter is a study of nanofluidic diode, a component known to mimic the behaviour of semiconductor diode. The study highlights a strong coupling between water and ion dynamics which leads to a water flow rectification inside the diode. The fifth and last chapter is a study of the origin of commonly observed pink noise (1=f) in ionic current measurements through nanopores

Dissipadores de calor baseados em microcanais são apresentados como solução para a remoção de fluxos de calor elevados em espaços restritos, pois proporcionam elevados coeficientes de transferência de calor quando comparados a canais convencionais. Tais trocadores também proporcionam elevadas razões entre a área superficial em contato com o refrigerante por unidade de volume do dissipador. Além dos microcanais, a utilização de nanofluidos também se apresenta como tecnologia com potencial de incremento do coeficiente de transferência de calor. Os nanofluidos consistem na adição de nanopartículas a um fluido base visando alterar suas propriedades de transporte termodinâmicas. Neste contexto, o objetivo do presente estudo é avaliar o coeficiente de transferência de calor para escoamentos monofásicos e ebulição convectiva de nanofluidos aquosos no interior de microcanais. Para isto, foram realizados experimentos em canais com diâmetro de 1,1 mm e comprimento de 200 mm para água deionizada, nanofluidos de alumina com diâmetros de 20-30 e 40-80 nm, nanofluidos de dióxido de silício com diâmetros de 15 e 80 nm, e nanofluidos de cobre com diâmetro de 25 nm. Estas soluções foram ensaiadas para concentrações volumétricas de nanopartículas de 0,001, 0,01 e 0,1, velocidades mássicas de 200, 400 e 600 kg/m2s e fluxos de calor de 20 a 350 kW/m2. A análise dos resultados revelou que a adição de nanopartículas a água deionizada proporciona o incremento do número de Nusselt para escoamentos monofásicos, principalmente na região inicial do tubo. Concluiu-se que os efeitos da adição de nanopartículas a um fluido base no coeficiente de transferência de calor durante a ebulição convectiva estão relacionados ao recobrimento da superfície com uma camada porosa. A deposição de nanopartículas com diâmetro inferior a 30 nm resultou na redução do coeficiente de transferência de calor e das instabilidades térmicas do escoamento em relação a água deionizada. O coeficiente de transferência de calor e as instabilidades térmicas não apresentaram variações significativas da deposição de nanopartículas com diâmetro superior a 40 nm. Por meio da análise da textura das superfícies recobertas e do critério de nucleação proposto por Kandlikar et al. (1997) concluiu-se que tal comportamento encontra-se associado aos efeitos do acabamento superficial na densidade de cavidades de nucleação ativas.
Microchannels based heat exchangers were introduced as a solution to high heat flux removal in restrict spaces due to their high heat transfer coefficients compared to heat exchangers based on conventional channels. The high ratio of surface are per volume is an additional advantage to microchannels in relation to conventional channels. Beside the microchannels technology, the nanofluids also present itself as a technique with potential to increase the heat transfer coefficient. Nanofluids consist of a solution containing nanoparticles dispersed in a base fluid with the goal to improve its thermodynamic and transport properties. In this context, the objective of the present study is to evaluate the heat transfer coefficient for single-phase flow and convective boiling of aqueous nanofluids inside microchannels. Experiments were performed for channels with internal diameter of 1.1mm and 200 mm long for DI-water, nanofluids containing alumina- (nanoparticles diameters of 20-30 and 40-80 nm), silicon dioxide (nanoparticles diameters of 15 and 80 nm), and copper (nanoparticles diameter of 25 nm). These solutions were evaluated for volumetric concentrations of 0.001, 0.01 and 0.1%, mass velocities of 200, 400 and 600 kg/m2s and heat fluxes from 20 to 350 kW/m2. The analysis of the results revealed that the addition of nanoparticles to DI-water causes an increment in the Nusselt number for single phase flows, especially at the inlet of the tube. The results for flow boiling indicated that the effects of adding nanoparticles to the base fluid are related to the deposition on the heating surface of a nanoparticles porous layer due to the boiling process. The deposition of nanoparticles smaller than 30 nm promoted a reduction of the heat transfer coefficient compared to DI-water on a clean surface, and thermal instabilities were minimized. For the deposition of nanoparticles larger than 40 nm these parameters did not presented significant variations in comparison to DI-water. A combined analysis of the surfaces finishing and the criterion of Kandlikar et al. (1997) for bubble nucleation revealed that such behaviors are correlated to the effects of the surface texture associated to the boiling process on the density of active nucleation cavities.

Orientador: Elaine Maria Cardoso
Resumo: O avanço de novas tecnologias, associado à minimização dos custos de fabricação e instala-ção, constitui um grande desafio para a área de refrigeração, uma vez que a geração de calor tem aumentado gradativamente nos últimos anos. Assim, a busca de novos fluidos com pro-priedades térmicas superiores aos comumente usados tornou-se indispensável para melhorar a eficiência energética. Nas últimas décadas os nanofluidos - dispersões de partículas de escala nanométrica (1 a 100nm) em um fluido-base - têm atraído especial interesse não somente da comunidade acadêmica, mas também da indústria em áreas como: a microeletrônica, microflu-ídica, transporte, manufatura, assistência médica, entre outras. O melhor desempenho térmico e a vasta gama de aplicações fazem dos nanofluidos potenciais substitutos dos refrigerantes utilizados em diversos segmentos da engenharia. Dentro desse contexto, o presente trabalho teve como objetivos: o estudo teórico e experimental da influência das propriedades termofísi-cas e concentração de nanofluidos, bem como, das características geométricas da superfície aquecedora sobre o ângulo de contato e a molhabilidade. Também, atenção foi dada à prepa-ração e caracterização dos nanofluidos (Al2O3-água e Fe2O3-água), por meio da análise expe-rimental da condutividade térmica e da viscosidade dinâmica para diferentes concentrações; uma bancada experimental, para aquisição de imagens de gota séssil, foi construída a fim de viabilizar as análises de ângulo de conta... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The advance of new technologies, associated to the minimization of manufacturing and installation costs, presents a great challenge for the refrigeration area, since the heat generation has increased in recent years. Thus, the search for new fluids with thermal properties higher than those commonly used has become indispensable to improve energy efficiency. In recent decades, nanofluids-dispersions of nanometer-scale particles (1 to 100 nm) in a base fluid - have attracted special interest not only from the academic community but also from industry in areas such as microelectronics, microfluidics, transport, manufacturing, medical assistance, among others. In this context, the present work had the following goals: the theoretical and experimental study of the influence of thermophysical properties and nanofluid concentration, as well as the geometric characteristics of the heating surface on the contact angle and wetta-bility. Attention was also given to the preparation and characterization of nanofluids (Al2O3-water and Fe2O3-water) by the experimental analysis of thermal conductivity and dynamic viscosity for different concentrations; an experimental apparatus for the acquisition of sessile droplet images was designed in order to analyze the contact angle and wettability; and a computational routine was developed to obtain the drop profile and the surface-fluid interaction for the different nanofluids and surfaces used. Based on database, it was possible to evaluate the pre... (Complete abstract click electronic access below)
Doutor

El proceso de secado por atomización interviene en numerosas aplicaciones industriales. Un campo de especial interés, en el que se utiliza dicho proceso es la obtención de gránulos nanoestructurados obtenidos a partir de materias primas de tamaño nanométrico. En este trabajo se ha llevado a cabo un estudio del proceso de secado de gotas de nanofluidos, en un levitador acústico. El estudio se ha extendido al secado de gotas de suspensiones conteniendo mezclas de nanopartículas y micropartículas. Se ha analizado y modelado la influencia de las variables de interés en la cinética de secado, el empaquetamiento de las partículas en el interior del gránulo, su microestructura interna y su resistencia mecánica. Finalmente, los resultados obtenidos en el secado de gotas individuales en el levitador acústico han sido validados a escala de planta piloto mediante el secado por atomización de diferentes suspensiones.
The spray drying process is present in numerous industrial applications. A field of special interest, in which said process is used, is the obtaining of nanostructured granules from nano-sized raw materials. In this work, a study of the drying process of nanofluid droplets has been carried out in an acoustic levitator. The study has been extended to the drying of suspension droplets containing mixtures of nanoparticles and microparticles. The influence of the variables of interest on the kinetics of drying, the packing of the particles inside the granule, its internal microstructure and its mechanical resistance has been analyzed and modeled. Finally, the results obtained in the drying of individual droplets in the acoustic levitator have been validated at pilot plant scale by spray drying of different suspensions.

Este trabalho trata da caracterização de propriedades termodinâmicas e de transporte de nanofluidos baseados em nanopartículas de alumina em água para diferentes concentrações. Suspensões estáveis foram elaboradas por meio de um agitador ultrassônico. As seguintes propriedades foram analisadas: i) condutividade térmica com o método da sonda-linear; ii) viscosidade dinâmica através do reômetro do tipo cone e placa e iii) ângulo de contato com base em registros fotográficos de gotas em uma superfície plana e o tratamento de imagem através de um programa elaborado em LabVIEW. Procedimentos foram utilizados visando validar os métodos experimentais adotados, entre eles a comparação com resultados para fluidos puros. Além do estudo experimental, foi realizada uma análise crítica da literatura sobre condutividade térmica e viscosidade dinâmica de nanofluidos. Com base nesta análise, os resultados experimentais foram comparados a dados empíricos da literatura e métodos de previsão de propriedades desenvolvidos para nanofluidos e para suspensões de particulado sólido em líquido. De uma maneira geral, os resultados levantados neste estudo para condutividade térmica e viscosidade dinâmica de nanofluidos foram significativamente superiores a maioria dos dados experimentais da literatura e aos resultados proporcionados pelos métodos de previsão. Entretanto, para nanofluidos com composições distintas de nanopartículas de alumina em água, comportamentos similares ao do presente estudo também são observados na literatura. No caso do ângulo de contato, verificou-se seu decréscimo com o incremento da concentração de nanopartículas. Tal resultado coincide com a bibliografia consultada, segundo a qual a molhabilidade do nanofluido se eleva com o incremento da concentração de nanopartículas.
The present study concerns the characterization of thermodynamic and transport properties of nanofluids based on alumina nanoparticles in deionized water. Stable suspensions were obtained using an ultrasonic homogenizer (Sonicator). The following properties were measured: i) thermal conductivity using the linear probe method, ii) dynamic viscosity through a cone-plate rheometer iii) contact angle, based on photographic of nanofluid drops on a flat surface and image processing through a program based on LabVIEW. The methods and experimental procedures were validated by performing measurements properties of pure fluids with well known characteristics. Besides the experimental study, it was performed a comprehensive literature review on thermal conductivity and dynamic viscosity of nanofluids. Experimental results were compared against the data from the literature and the respective predictive methods developed for suspensions of nanofluids and micro solid particles in liquid. Generally speaking, the nanofluid thermal conductivity and dynamic viscosity measured in the present study were higher than the empirical values from the literature and the values given by predictive methods. However, it should be highlighted that although for different compositions of nanofluids behaviors similar to the one observed in this study are also reported in the literature. In case of contact angle, it was found that its value decreases with increasing the nanoparticle volumetric concentration. Such results is coincident with literature reports according to which the nanofluid wettability, given in terms of the contact angle, increases with increasing the nanoparticle concentration.

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Nas últimas décadas surgiu a necessidade de dissipar maiores quantidades de energia térmica, fato que acarretou no aumento do número de estudos em ebulição nucleada e convectiva com o objetivo de produzir trocadores de calor cada vez mais eficientes e compactos. A busca de produtos cada vez mais eficientes e compactos e a procura de novas técnicas para melhorar a transferência de calor, garantindo a integridade física do equipamento, continuam crescendo e a tendência é que continuará assim nos próximos anos. Uma das técnicas que está sendo amplamente pesquisada na comunidade cientifica é o uso de nanofluidos. Os nanofluidos foram desenvolvidos com o intuito de melhorar a condutividade e a difusividade térmica em relação aos fluidos tradicionais. Muitos experimentos com nanofluidos têm sido desenvolvidos nos últimos anos, mas ainda existem muitas divergências a respeito do efeito desses fluidos sobre o fenômeno de ebulição. Dentro deste contexto, o presente trabalho tem como objetivo a análise teórico-experimental do efeito de superfícies nanoestruturadas e da concentração do nanofluido, a ser depositado sobre a superfície aquecedora, sobre o coeficiente de transferência de calor em regime de ebulição nucleada. Para tanto, testes foram realizados para fluxos de calor que correspondem ao regime de ebulição nucleada da água deionizada, à temperatura de saturação (Tsat = 99 °C) e à pressão atmosférica (patm = 98 kPa), sobre superfícies aquecedoras de cobre com diferentes rugosidades. As superfícies nanoestruturadas foram produzidas por deposição de nanopartículas de maguemita, por meio do processo de ebulição da solução Fe2O3-água deionizada para diferentes concentrações mássicas previamente estabelecidas. As superfícies foram submetidas a ensaios metalográficos, de molhabilidade e de rugosidade permitindo a avaliação das modificações estruturais, topográficas e químicas das superfícies, antes e após os testes no regime de ebulição nucleada. Os resultados para o coeficiente de transferência de calor foram relacionados com as características geométricas e morfológicas das superfícies de teste, levando em consideração os aspectos relacionados à interação fluido/superfície, como, o ângulo de contato e a molhabilidade.
In the last decade, the necessity to dissipate large quantities of heat energy increased, thus leading to an increase on the number of studies in nucleate pool boiling and flow boiling with the aim of producing more compact and efficient heat exchangers. The search for increasingly efficient and compact products and for new techniques to improve the heat transfer, ensuring the physical integrity of the equipment, keep growing and it will remain so in the next years. One of the techniques being widely researched in the scientific community is the use of nanofluids. The nanofluids have been developed in order to improve the thermal conductivity and diffusivity compared to traditional fluids. Although many experiments with nanofluids have been developed in recent years, there are still many differences related to the effects of these fluids on the pool boiling phenomenon. In this context, this work aims to analyze the effects of nanostructured surfaces and different nanofluid concentrations, which are deposited on the heating surface, on the heat transfer coefficient during the nucleate boiling regime. Therefore, tests were performed to heat fluxes values corresponding to the nucleate boiling regime for deionized water, at saturation temperature (Tsat = 99 °C) and atmospheric pressure (patm = 98 kPa), on copper heating surfaces with different roughness values. The nanostructured surfaces were produced by maghemite nanoparticle deposition, which is achieved by boiling selected mass concentrations of a Fe2O3-deionized water nanofluid. Prior and after each boiling test, the characteristics of the test surfaces were evaluated by applying the metallographic, wettability and surface roughness tests. The results for the heat transfer coefficient were related to the geometrical and morphological characteristics of the test surfaces, taking into account the aspects of the flu-id/surface interaction such as, the contact angle and wettability.
FAPESP: 2014/07949-9

Orientador: Elaine Maria Cardoso
Resumo: Nas últimas décadas surgiu a necessidade de dissipar maiores quantidades de energiatérmica, fato que acarretou no aumento do número de estudos em ebulição nucleada e convectivacom o objetivo de produzir trocadores de calor cada vez mais eficientes e compactos. Abusca de produtos cada vez mais eficientes e compactos e a procura de novas técnicas paramelhorar a transferência de calor, garantindo a integridade física do equipamento, continuamcrescendo e a tendência é que continuará assim nos próximos anos. Uma das técnicas que estásendo amplamente pesquisada na comunidade cientifica é o uso de nanofluidos. Os nanofluidosforam desenvolvidos com o intuito de melhorar a condutividade e a difusividade térmicaem relação aos fluidos tradicionais. Muitos experimentos com nanofluidos têm sido desenvolvidosnos últimos anos, mas ainda existem muitas divergências a respeito do efeito dessesfluidos sobre o fenômeno de ebulição. Dentro deste contexto, o presente trabalho tem comoobjetivo a análise teórico-experimental do efeito de superfícies nanoestruturadas e da concentraçãodo nanofluido, a ser depositado sobre a superfície aquecedora, sobre o coeficiente detransferência de calor em regime de ebulição nucleada. Para tanto, testes foram realizadospara fluxos de calor que correspondem ao regime de ebulição nucleada da água deionizada, àtemperatura de saturação (Tsat = 99 °C) e à pressão atmosférica (patm = 98 kPa), sobre superfíciesaquecedoras de cobre com dif... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: In the last decade, the necessity to dissipate large quantities of heat energy increased,thus leading to an increase on the number of studies in nucleate pool boiling and flow boilingwith the aim of producing more compact and efficient heat exchangers. The search for increasinglyefficient and compact products and for new techniques to improve the heat transfer,ensuring the physical integrity of the equipment, keep growing and it will remain so in thenext years. One of the techniques being widely researched in the scientific community is theuse of nanofluids. The nanofluids have been developed in order to improve the thermal conductivityand diffusivity compared to traditional fluids. Although many experiments withnanofluids have been developed in recent years, there are still many differences related to theeffects of these fluids on the pool boiling phenomenon. In this context, this work aims to analyzethe effects of nanostructured surfaces and different nanofluid concentrations, which aredeposited on the heating surface, on the heat transfer coefficient during the nucleate boilingregime. Therefore, tests were performed to heat fluxes values corresponding to the nucleateboiling regime for deionized water, at saturation temperature (Tsat = 99 °C) and atmosphericpressure (patm = 98 kPa), on copper heating surfaces with different roughness values. Thenanostructured surfaces were produced by maghemite nanoparticle deposition, which isachieved by boi... (Complete abstract click electronic access below)
Mestre

Mestrado em Engenharia Mecânica
Em resposta à carência social de soluções sustentáveis, a comunidade científica iniciou o estudo acerca dos nanofluidos devido ao seu potencial em sistemas de transferência de calor, relacionada com o aumemto da condutividade térmica dos mesmos, quando comparada com a dos fluidos base, que resultará na miniaturização das soluções tornando-as mais eficientes e menos carentes de recursos naturais. As propriedades de transferência de calor destes fluidos, como a condutividade térmica tem sido alvo de inúmeros estudos ao invés de outras propriedadades termofísicas, como por exemplo a densidade, que têm ficado por estudar e assim, possivelemte uma das razões que podem explicar a baixa aplicação dos nanofluidos na indústria. No âmbito deste trabalho, estudou-se empíricamente o comportamento da densidade de nanofluidos de base aquosa com nanotubos de carbono de parede múltipla e assim avaliar as previões resultantes do único modelo físico para o efeito, o modelo de Pak e Cho. Para a concretização deste estudo, são apresentadas metodologias rigorosas e reprodutíveis para a produção deste tipo de nanofluidos, bem como os métodos mais importantes para a aferição da sua estabilidade coloidal, assegurando assim o rigor da sua produção. Após a obtenção dos nanofluidos estáveis, realizou-se uma análise comparativa conducente a uma base de dados obtida experimentalmente que visa quantificar a influência relativa dos distintos fatores de controlo no modelo preditivo para a densidade existente. Os fatores de controlo alvo de análise são: temperatura, fluido base, geometria da nanopartícula (relação diêmetro - comprimento) e concentração volúmica das nanopartículas. Assim, após a aquisição de uma base de dados de confiança e da quantificação do desvio em relação à literatura, induzido por combinações de fatores de controlo, desenvolveu-se e validou-se um modelo físico-matemático, que possibilitará a previsão mais acertada da densidade para este tipo de nanofluidos.
Looking forward for social needs and environmentally sustainable solutions, the scientific community began the study of nanofluid properties, mainly due to it high potential in heat transfer systems, as the carbon tubes present high thermal conductivity that will pave the road to device miniaturization, with concomitant gain of efficiency and less demanding in terms of consumption of natural resources. The heat transfer properties of the carbon nanotubes based nanofluids, in parallel with it thermal conductivity was been studied in detail, however, other thermophysical properties, such as density lacks for a detailed analysis that precludes the dissemination of the nanofluids in the industrial context. In this work, one makes the empirical study the of water-based multi-walled carbon nanofluids’ density, that were compared using the single physical model to the effect, the Pak and Cho model. To implement the study, rigorous and reproducible methodologies to produce and test the colloidal stability, thus ensuring its rigorous production, are presented. After the production of stable nanofluids, a comparative analysis was made looking ahead to the construction of an experimental database that intends to quantify the relative role of distinct control factors according to the existing predictive model such as temperature, base fluid, nanoparticle’s geometry and concentration. Thus, after the acquisition of a reliable database and quantification of deviations in comparison to the literature results, induced by the control factors combination, a physic and mathematical model was developed and validated, that will ensure the future determination of the density of the kind of nanofluids studied with improved accuracy.

Mestrado em Engenharia Mecânica
A transferência de energia sob a forma de calor é um processo essencial ao funcionamento de uma enorme variedade de equipamentos e indústrias, desde a eletrónica à automóvel. Com o desenvolvimento da nanotecnologia das últimas décadas, várias inovações têm surgido, como é o caso dos nanofluidos. Estas suspensões de nanopartículas em fluidos de permuta, como a água e o etilenoglicol, têm sido consideradas como uma nova classe de fluidos de permuta, com propriedades térmicas superiores relativamente aos fluidos convencionais. A elevada razão de aspeto dos nanotubos de carbono (CNTs) associado ao progresso em técnicas de dispersão, proporcionou o desenvolvimento destas suspensões estáveis e de elevado desempenho térmico. O principal objetivo desta dissertação prende-se com o estudo do comportamento reológico de nanofluidos à base de CNTs. Desta forma, e utilizando para o efeito medições de viscosidade realizadas por investigadores do GRIDS-DIMOULD do Departamento de Engenharia Mecânica da Universidade de Aveiro), foi assim possível relacionar a viscosidade dos nanofluidos com diferentes variáveis, tais como a temperatura, taxa de corte, concentração e geometria dos CNT e fluidos base. A comparação dos resultados obtidos experimentalmente com previsões postuladas pelos modelos reológicos existentes na literatura são assim objeto de estudo detalhado nesta dissertação, Com este trabalho foi possível estabelecer os limites de validade e aplicabilidade dos modelos existentes, para os nanofluidos em estudo, evidenciando os trabalhos futuros que permitirão desmitificar algumas das dúvidas que ainda persistem na previsão.
The transfer of energy in the form of heat is an essential process for the operation of a wide variety of equipment and industries ranging from electronics to automotive. With the development of nanotechnology the last decades, various innovations have emerged, such as the nanofluids. These suspensions of nanoparticles in exchange fluids, such as water and ethylene glycol have been considered as a novel class of fluid exchange with superior thermal properties relative to conventional fluids. The high aspect ratio of carbon nanotubes (CNTs) associated with the progress scattering techniques, provided the development of these stable suspensions and high thermal performance. The main objective of this work relates to the study of the rheological behavior of CNT nanofluids. In this way, and based on viscosity measurements carried out by researchers of GRIDS-DIMOULD at the Mechanical Engineering Department of the University of Aveiro, it was it possible to relate the viscosity of nanofluids with different variables such as temperature, shear rate, concentration and geometry of the CNT and base fluids. The comparison of the results obtained experimentally was established with data predicted by the existing rheological models in the literature. The validity of the latter were therefore detailed studied during this dissertation, and the need for further work was assessed and discussed in order to identify the means to properly derive a generalized rheological model for CNT nanofluids.

The present thesis evaluates the potential of solar nanofluids as direct absorption solar collectors and to generate steam. Nowadays, as energy consumption continues to rise while conventional energy are close to begin exhausted, there is an increasing demand for renewable energy technologies, especially solar thermal. Currently, solar thermal energy systems include numerous losses due to the heat transfer processes, thus, nanofluids have been proposed to enhance the solar collectors efficiency through the direct sunlight absorption. In this work, optical properties of different nanofluids carbon based have been characterised in order to study their suitability as direct absorption solar collectors.
La presente tesis trata de evaluar el potencial de los nanofluidos solares para absorber directamente la radiación solar e intentar generar vapor. Como el consumo de energía continúa aumentando y las fuentes de energía convencionales se están agotando, aparece como excelente alternativa, el uso de las energías renovables, concretamente, la energía solar térmica. Los sistemas actuales de energía solar térmica presentan numerosas pérdidas debido a los procesos de transferencia de calor, por ello, se han propuesto los nanofluidos solares como absorbedores de la radiación y fluidos de transferencia de calor y así, mejorar la eficiencia del proceso de generación de vapor en las centrales solares térmicas. En este trabajo, se ha realizado la caracterización de las propiedades ópticas de diferentes nanofluidos solares basados en nanopartículas de carbón para estudiar su idoneidad en la aplicación solar térmica.

Les transferts de chaleur constituent la base de nombreux processus industriels qui sont présents dans notre vie quotidienne. L’intensification de ces échanges et l’amélioration du rendement sont devenues aujourd’hui une problématique majeure dans le monde industriel, des organismes de réglementation mais aussi de la société dans son ensemble, qui prend conscience de l’épuisement progressif des ressources énergétiques et qui se soucie de l’avenir en matière énergétique. Face à ces enjeux énergétiques et environnementaux, Le défi technologique réside dans le développement de nouveaux processus pour une meilleure gestion de l’énergie. Ce travail de thèse s’inscrit dans ce cadre, et concerne particulièrement les problèmes liés à l’intensification des échanges thermiques dans les échangeurs de chaleur. Les améliorations des échanges thermiques dites ‘passives’ sont une voie déjà largement élaborée et atteignent leurs limites. De nouvelles stratégies d’optimisation doivent donc être étudiées. Une de ces stratégies consiste à améliorer les propriétés thermiques des fluides caloporteurs utilisés dans les systèmes thermiques, notamment dans les échangeurs de chaleur. Des progrès importants en chimie ont permis dès la fin des années 90 de synthétiser des particules de taille nanométrique, qui, dispersées dans un liquide porteur, constituent des nanofluides. Leur synthèse répond au besoin d’améliorer les propriétés thermiques des fluides caloporteurs en y insérant une phase solide de conductivité thermique très élevée. Le fil directeur de ce travail consiste donc à caractériser de manière approfondie le comportement thermique et rhéologique des nanofluides à base de nanotubes de carbone NTC utilisés tout au long de ce travail afin de quantifier les principaux paramètres influençant leurs propriétés thermo-physiques et les phénomènes physiques régissant l’intensification des transferts thermiques induits par ces nanofluides. Une analyse des travaux de recherche antérieurs a été menée dans le but de s’affranchir des différents paramètres qui peuvent influencer le comportement thermique et rhéologique des nanofluides dont on citera les paramètres liés à la composition des nanofluides (fraction volumique des NTC, type de surfactant, rapport d’aspect des NTC), la température, le fluide de base… Suite à cette étude, nous avons mené une étude expérimentale sur les propriétés thermo-physiques des nanofluides testés (conductivité thermique, viscosité dynamique, masse volumique) et sur les performances thermiques dans un échangeur de chaleur. Nous avons présenté également une analyse des résultats de façon à étudier l’influence des paramètres évoqués ci-dessus. Les résultats obtenus sont comparés et discutés vis-à-vis des modèles classiques existants, en proposant des améliorations et des interprétations selon les tendances obtenues. Les résultats prometteurs de cette étude sont très encourageants et montrent que l’utilisation des nanofluides à base de nanotubes de carbone offre clairement une amélioration des performances thermiques par rapport aux fluides de base classiques. Les nanofluides à base de NTC peuvent constituer ainsi un débouché prometteur des transferts thermiques et présentent de bonnes perspectives et développement
Heat transfer is one of the most important industrial processes in our daily lives. Nowadays, the intensification of the heat transfer and the improving of the energy efficiency have become a major problem in industry, regulatory agencies, and also the society that becomes conscious of the progressive exhaustion of the world’s energy resources and cares about the future of energy. Due to these energy and environmental issues, the technological challenge is to develop new processes for better energy management. This work fits in that context and applies particularly the problems associated to the improvements of heat exchanger’s energy efficiency. The conventional methods for increasing the heat transfer in heat exchangers have already been extensively explored and have reached their objective limits. There is therefore an urgent need for new strategies with improved performances. The novel concept of improving the thermal properties of the working fluids used in thermal system, especially in heat exchangers, has been proposed as a means of meeting these challenges. The innovative concept of nanofluids heat transfer fluids consisting of suspended of nanoparticles with very high thermal conductivities has been proposed for these challenges. The aim of this work is therefore to characterize profoundly the thermal and the rheological behavior of nanofluids containing carbon nanotubes CNTs used throughout of this work. This is in order to quantify the main parameters influencing their thermophysical properties and physical phenomena governing the intensification of heat transfer induced by these nanofluids. An analysis of previous researches has been conducted for the purpose of establishing various parameters that may influence the thermal and rheological behavior of nanofluids, which including the parameters related to the composition of nanofluids (volume fraction of CNTs, type of surfactant, aspect ratio of CNTs), the temperature, the base fluid... Following this study, experiments have been carried out on the thermal physical properties of tested nanofluids (thermal conductivity, dynamic viscosity, density) and thermal performances in a heat exchanger. Analyses of the results have been presented in order to study the influence of the abovementioned parameters. The results obtained are compared and discussed vis-à-vis the existing conventional models, suggesting improvements and interpretations according to the trends obtained. The promising results of this study are very encouraging and show that the use of nanofluids containing carbon nanotubes clearly improved the thermal performances compared to the conventional base fluids. The CNT-based nanofluids can thus be a promising candidate for heat transfer and presents good perspective and development

Fluids containing nano-sized structures are being increasingly employed in modern technologies, ranging from lubrication fluids to drug delivery. However, despite their numerous applications, our fundamental understanding of the surface forces mediated by nanofluids is still relatively limited. In particular, due to their nanosize and related characteristics, the applicability of established surface force theories, such as the DLVO theory, remain unclear. With several tunable parameters such as the size, shape and surface chemistry, dendritic macromolecules (or dendrimers) offer a unique model nanofluid for investigating the effect of nanostructures on classic colloidal phenomena. Accordingly small-angle X-ray scattering (SAXS) and X-ray reflectivity (XRR) measurements have been performed to study the interactions between negatively charged poly(amidoamine) (PAMAM) dendrimers in the bulk and at the mica-water interface. In particular, how their interactions can be influenced by the presence of cationic surfactant dodecyl trimethyl ammonium bromide (DTAB) have been studied. Using a version of the surface force apparatus (SF A), the surface forces mediated by these dendrimer-surfactant mixtures under nano-confinement and shear have then also been directly measured. In further work the effect of the surface chemistry on the interactions in dendrimer nanofluids has been studied in two systems: i) as a comparison to the above system, the interactions between positively charged dendrimers with an anionic surfactant have been characterised both in the bulk and at the mica interface using SAXS and XRR, and ii) the interactions between three different surface chemistries of P AMAM dendrimers in an ionic surfactant mesophase have investigated using SAXS. Overall, the results presented demonstrate the tunability of inter-dendrimer interactions via their intramolecular architecture, which in turn may be harnessed to control and tailor the physical properties of dendrimer nanofluids. Such interactions bear fundamental importance to the application of dendrimers, as well as contribute to a better understanding of the surface forces mediated nanofluids.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Este trabalho surgiu a partir da necessidade de produzir avanços em projeto que trata do desenvolvimento de um porta-ferramentas refrigerado internamente através de um fluido em mudança de fase e, na tentativa de minimizar a alta temperatura na ferramenta de corte através desse sistema de circulação. A utilização de nanofluidos surgiu como uma alternativa para a otimização da transferência térmica entre fluido e ferramenta de corte. A pesquisa consiste em avaliar a influência da adição de nanopartículas de prata numa solução de etilenoglicol e água deionizada, e também, da adição de nanopartículas de hematita (Fe2O3) no fluido refrigerante R141b. Em ambos os casos, as nanopartículas possuíam formato esférico, diâmetro médio de 30nm e foram avaliadas em concentrações. Além disso, as duas soluções foram submetidas a um campo elétrico na região de transferência térmica para analisar a influência do efeito eletrohidrodinâmico e, por fim, considerando as propriedades magnéticas da hematita, este nanofluido foi testado sob influência de um campo magnético. Os testes mostraram que as nanopartículas realmente influenciaram as propriedades dos fluidos e, por consequência, a quantidade de calor transferido. O nanofluido Ag/ETG+H2O(l) (0,023 vol%) resultou num incremento de 11% no valor do coeficiente de transferência térmica convectivo (h) quando sujeito ao campo elétrico. Para o caso do nanofluido Fe2O3/R141b, o valor de h aumentou em 30,3%, porém, quando sob efeito do campo magnético ou elétrico, o coeficiente foi prejudicado, resultando num valor menor que o do controle. Ao final, tem-se a proposta de um possível modelo desse porta-ferramentas.
This work arose from the need to produce advances in design development of an internally cooled toolholder through a phase change fluid. In order to minimize the high temperature in the cutting tool by this circulation system, using nanofluids emerged as an alternative to optimize heat transfer between the fluid and the cutting tool. The research consists in evaluate the influence of addition of silver nanoparticles in an ethylene glycol and deionized water solution, and also the addition of hematite nanoparticles (Fe2O3) in the refrigerant R141b. In both cases, nanoparticles had spherical shape, diameter of 30nm, and they were evaluated in different concentrations. Moreover, both nanofluids were subjected to an electric field in the heat transfer region to evaluate the influence of electrohydrodynamic effect and, finally, considering the magnetic properties of hematite, this nanofluid was tested under the influence of a magnetic field. The tests have shown that the nanoparticles really influence the properties of the fluids and, therefore, the amount of heat transferred. The nanofluid Ag/ETG+H2O(l) also presented a positive influence of the electric field, further enhancing the value of the convective heat transfer coefficient (h) in 11% (0,039 vol%). In the case of Fe2O3/R141b nanofluid, the h value increased 30.3%. However, when the nanofluid was under magnetic or electric effect, the value of h was deteriorated, resulting in a lesser value than the control. As conclusion, a new toolholder prototype is presented.

Mestrado em Engenharia Mecânica
É do conhecimento geral que a existência de fluidos com boa performance térmica é fundamental numa panóplia de sectores industriais, incluindo a produção de energia, indústria química, automóvel, entre outras. Contudo, são as limitações térmicas dos fluidos convencionais que comprometem a eficiência energética e, portanto a redução de tamanho, dos próprios permutadores de calor. De modo a melhorar a performance térmica dos fluidos convencionais, equaciona-se a adição de nanotubos de carbono (CNTs). Assim, com o este estudo, pretende-se contribuir para a redução significativa do tamanho, peso e custos envolvidos em sistemas de transferência de calor e, deste modo, contribuir efectivamente para a resolução de um dos maiores entraves actuais à miniaturização de equipamentos desta natureza. Neste estudo, variáveis como fracção volumétrica dos CNTs e temperatura do fluido são tidas em consideração. O nanofluido propriamente dito é obtido pela adição, a fluidos convencionais (neste caso, água) de nanotubos tratados quimicamente. A dispersão do nanotubo no fluido base, de modo a inferir homogeneidade, será garantida recorrendo a agitação por ultra-sons. A relação entre variáveis como viscosidade dinâmica, condutividade térmica, densidade e tensão superficial são, pois, objecto de estudo detalhado. ABSTRACT: The present proposal addresses the improvement of thermal characteristics of conventional fluids, having into account the great need for economy dematerialization and energy efficiency in industrial processes and systems, to achieve a higher level of environmental control and consequently a more sustainable development. Studying (and understanding) nanofluids, using available and innovative experimental and computational techniques, is the basis of the research towards the development of custom-designed nanofluids with enhanced properties and functions. Possible applications include more efficient cooling and heating in new and critical applications, like environment control, electronics, nuclear and biomedical instrumentation and equipment, transportation and industrial cooling, heat management, therefore promoting the eco design for energy efficiency. The heat transfer characteristics of conventional fluids obstruct their performance enhancement compromising both energy efficiency and compactness of heat exchangers. This work addresses the improvement of thermal characteristics of conventional fluids by the addition of specific nanoparticles (i.e. multiwalled carbon nanotubes, CNTs) in well defined concentrations, in order to obtain enhanced thermal properties of fluids for process intensification and device miniaturization. Thermal conductivity and rheological properties of nanofluids are studied.

This work presents membrane development applicable in nanofluidic devices. These membranes can also be termed suspended thin films, supported on two or more edges. I first discuss motivation and background for developing these structures. Then I derive the formative principles for nanofluidic systems. Following the derivation of the Navier-Stokes and Washburn equations, I discuss applying these theories to planar nanofluidic capillaries and finish the derivation by discussing the forces that drive liquid flow in nanochannels. I next discuss the membrane development process, starting with my work in static height traps, and develop the concept of analyzing nanoparticles using suspended membranes. After reviewing the lessons learned from the double-nanopore project I discuss developing an oxide layer tuned to the needs of a membrane and present the design of an adjustable membrane structure. Afterward, I discuss modeling and simulating the structure, and present a procedure for fabricating robust membranes. I then explain applying the membrane structure to form a nanofluidic pump and document the process for recording and analyzing the pumping characteristics for nanodevices. As part of the pump section I propose a theory and model for predicting the behavior of the pumps. I next present applying active membranes as nanoparticle traps. I document a quick-turn optical profilometry method for charicterizing the devices, then present experimental data involving trapping. Early results show that the device functions as a nanoparticle concentrator and may work well as a size-based trap for nanoparticles. I conclude by summarizing the main contributions made during my course of study and by providing supplemental material to guide future research.

85% des patients atteints de cancer du pancréas présentent au diagnostic des formes avancées de la maladie qui empêchent leur prise en charge thérapeutique efficace. Il est donc urgent de mettre en évidence des marqueurs diagnostics permettant de détecter plus tôt ces cancers, mais également leur rechute, afin d'améliorer leur prise en charge. Les miARNs (micro acides ribonucléiques) sont des biomarqueurs du cancer du pancréas, présentant une valeur clinique démontrée pour la détection précoce des tumeurs et le suivi de la réponse au traitement. Cependant, les méthodes actuelles d'extraction et de détection de ces molécules ne sont pas adaptées à une utilisation clinique. Les nouvelles technologies issues des méthodes de micro et nanofabrication ont le potentiel de permettre la mise en place de tests diagnostiques, offrant un haut degré de portabilité et de robustesse, une lecture en temps réel, et à bas coût. Nous proposons ici une plateforme nanofluidique couplée à une détection en fluorescence permettant la mesure en temps réel d'interactions moléculaires en milieu hyper-confiné. Nous décrivons dans un premier temps la plateforme de détection via un modèle théorique à une dimension basé sur la dynamique moléculaire permettant de prédire la capture spécifique des miARNs dans un nanocanal fonctionnalisé. L'originalité du système réside dans une accroche non homogène des miARNs sur la surface du capteur. Ainsi, nous démontrons que l'étude du profil spatial d'hybridation engendré permet de déterminer l'affinité du miARN capturé avec la séquence sonde en une seule étape, sans lavage. Nous démontrons également l'excellente spécificité du biocapteur qui permet la discrimination rapide (moins de 10 minutes) de SND (single nucleotide difference). Les performances du dispositif pour des applications au plus près des problématiques biologiques dans le cadre de la détection du cancer du pancréas sont enfin discutées : les effets de la préparation d'échantillon types biofluides complexes sur l'extraction de miARNs sont étudiés, puis deux approches permettant la détection de miARNs endogènes sont décrites et comparées, conduisant à la détection de miARNs extraits de cultures cellulaires modèles du cancer du pancréas
85% of patients affected by pancreatic adenocarcinoma (PDA) are diagnosed at an advanced stage, preventing effective care and curative treatments. Therefore, it is urgent to identify reliable biomarkers for the early detection of disease status, including relapse. MiRNAs (micro ribonucleic acids) are biomarkers of PDA, with demonstrated clinical value for early detection of tumors and monitoring of response to treatment. However, current methods of extraction and detection of miRNA are not compatible with clinical use. New technologies derived from micro and nanofabrication methods have the potential to facilitate the implementation of diagnostic tests, by offering a high degree of portability and robustness, short time to results at low cost. Here, we propose a nanofluidic platform coupled to fluorescence detection for the real time measurement of molecular interactions in a confined environment. We first describe the detection platform via a one-dimension theoretical model based on molecular dynamics to predict the capture of miRNAs into biofunctionalized nanochannels. The originality of the system lies in the non-homogeneous hybridization of miRNA targets onto the sensor. We demonstrate that the analysis of the spatial hybridization profile enables the determination of the affinity of the captured miRNA with the probe sequence in a wash-free single step. We then show the rapid discrimination (less than 10 minutes) of single nucleotide difference (SND) using this strategy. The performance of the device in the context of pancreatic cancer detection is discussed: the effect of sample preparation of complex biofluids is studied and two labeling approaches compatible with the detection of endogenous miRNAs are described and compared, leading to the detection of miRNAs extracted from model cell cultures of pancreatic cancer

A nanofluid is the suspension of nanoparticles in a base fluid. Nanofluids are promising for heat transfer enhancement due to their high thermal conductivity. Presently, discrepancy exists in nanofluid thermal conductivity data in the literature, and enhancement mechanisms have not been fully understood yet. In the first part of this study, a literature review of nanofluid thermal conductivity is performed. Experimental studies are discussed through the effects of some parameters such as particle volume fraction, particle size, and temperature on conductivity. Enhancement mechanisms of conductivity are summarized, theoretical models are explained, model predictions are compared with experimental data, and discrepancies are indicated. Nanofluid forced convection research is important for practical application of nanofluids. Recent experiments showed that nanofluid heat transfer enhancement exceeds the associated thermal conductivity enhancement, which might be explained by thermal dispersion, which occurs due to random motion of nanoparticles. In the second part of the study, to examine the validity of a thermal dispersion model, hydrodynamically developed, thermally developing laminar Al2O3/water nanofluid flow inside a circular tube under constant wall temperature and heat flux boundary conditions is analyzed by using finite difference method with Alternating Direction Implicit Scheme. Numerical results are compared with experimental and numerical data in the literature and good agreement is observed especially with experimental data, which indicates the validity of the thermal dispersion model for explaining nanofluid heat transfer. Additionally, a theoretical analysis is performed, which shows that usage of classical correlations for heat transfer analysis of nanofluids is not valid.

The present Master thesis is concerned with forced convection heat transfer in laminar and turbulent flowwith nanofluids. Nanofluids are defined as a colloidal suspension of particles in a base fluid, where theparticles have a characteristic length of less than 100 nm. Experiments were conducted to determine thequalification of nanofluids for laminar and turbulent flow forced convection heat transfer. Theexperiments were conducted in two different devices: Firstly, a stainless steel pipe with an inner diameterof 3.7 mm, heated directly by a DC current in the pipe wall, and secondly, a tubular heat exchanger, whichthe fluid was cooled down in. The tested nanofluids were not only assessed considering Nu/Re, as it hasbeen found to be common in a short literature review, but also by taking into account the pressure drop indifferent ways. A way of considering pressure drop in non-dimensional quantities was introduced that hadnot been seen in literature. In some cases, an opposite assessment for the fluid could be found fromcomparing Nu/Re of base fluid and nanofluid and comparing h/Δp. Difficulties during validation of thetest rig had called for system improvement; an extensive error investigation was conducted on the test rigand the calculation. The error investigation resulted in changes concerning the calculation and the test rig.

In recent years, the study of fluid flow with nanoparticles in base fluids has attracted the attention of several researchers due to its various applications to science and engineering problems. Recent investigations on convective heat transfer in nanofluids indicate that the suspended nanoparticles markedly change the transport properties and thereby the heat transfer characteristics. Convection in saturated porous media with nanofluids is also an area of growing interest. In this thesis, we study the effects of radiation on the heat and mass transfer characteristics of nanofluid flows over solid surfaces. In Chapter 2, an investigation is made into the effects of radiation on mixed convection over a wedge embedded in a saturated porous medium with nanofluids, while in Chapter 3 results are presented for the effects of radiation on convection heat transfer about a cone embedded in a saturated porous medium with nanofluids. The resulting governing equations are non-dimensionalized and transformed into a non-similar form and then solved by Keller box method. A comparison is made with the available results in the literature, and the results are found to be in very good agreement. The numerical results for the velocity, temperature, volume fraction, the local Nusselt number and the Sherwood number are presented graphically. The salient features of the results are analyzed and discussed for several sets of values of the pertinent parameters. Also, the effects of the Rosseland diffusion and the Brownian motion are discussed.
ID: 031001347; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Adviser: Kuppalapalle Vajravelu.; Title from PDF title page (viewed April 18, 2013).; Thesis (M.S.)--University of Central Florida, 2012.; Includes bibliographical references (p. 32-34).
M.S.
Masters
Mathematics
Sciences
Mathematical Science; Industrial Mathematics

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.
Includes bibliographical references (p. 101-106).
The effect of an external gate potential control on the nanofluidic nanochannels was experimentally investigated in this work. Like in the field effect transistors (FET) in microelectronics, molecular transport in micro/nanofluidic channels can be controlled by applying external potentials on the wall of the fluidic channel. In nanofluidic devices, this type of control is expected to be more efficient due to its high surface to charge ratio. We focused on a nanofluidic concentrator to study this effect. We could increase or decrease the concentration rate of the device by increasing or decreasing the surface charge potential (-potential) on the walls of the nanochannels. An increased -potential enhances the electrokinetic effects caused by electrical double layer. Which in turn accelerates the creation of a charge polarization region and improves the concentration capabilities of the device. We also have demonstrated concentration polarization effect, caused by pressure-driven flow in the nanofluidic channel, and showed that this phenomena can also be modulated by changing the gate potential of the nanofluidic devices. The gate potential effect opens the door for closed-loop real-time control of nanofluidic concentrators.
by Arnaud Le Coguic.
S.M.

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2020
Cataloged from student-submitted PDF of thesis.
Includes bibliographical references (pages 56-61).
Water scarcity is one of the largest global challenges, affecting two-thirds of the world population. Water desalination and purification technologies, such as novel membrane processes and materials, are in great demand to produce clean water from contaminated sources or the sea. However, the lack of fundamental understanding of structure-property-performance has hindered the advancement of these techniques. In this study, we address this critical knowledge gap by adapting multiscale computational modeling to better understand the mechanisms of intrinsic molecular interaction in nanofluidic applications. We performed ab initio molecular dynamics to study the nanoscale solvation behavior of selected ions on finite graphene models. The degree of charge transfer between ion and water, and the effect of defects on dynamics and solvation has been investigated. Furthermore, a quantum mechanics/molecular mechanics (QM/MM) model for the accurate description of free energy changes in ion adsorption process has been developed. Lastly, we combined classical molecular dynamics and density functional theory (DFT) to elucidate the dielectric-driven mechanism of ionization behavior in nanoporous polyamide films. We seek to utilize this knowledge for the design of next-generation membranes for separation and water purification.
by Mengyi Wang.
S.M.
S.M. Massachusetts Institute of Technology, Department of Materials Science and Engineering

Lab-on-a-chip devices, also known as micro total analysis systems (μTAS), are implementations of chemical analysis systems on microchips. These systems can be fabricated using standard thin film processing techniques. Microfluidic and nanofluidic channels are fabricated in this work through sacrificial etching. Microchannels are fabricated utilizing cores made from AZ3330 and SU8 photoresist. Multi-channel electroosmotic (EO) pumps are evaluated and the accompanying channel zeta potentials are calculated. Capillary flow is studied as an effective filling mechanism for nanochannels. Experimental departure from the Washburn model is considered, where capillary flow rates lie within 10% to 70% of theoretical values. Nanochannels are fabricated utilizing cores made from aluminum, germanium, and chromium. Nanochannels are made with 5 μm thick top layers of oxide to prevent dynamic channel deformation. Nanochannel separation schemes are considered, including Ogston sieving, entropic trapping, reptation, electrostatic sieving, and immutable trapping. Immutable trapping is studied through dual-segment nanochannels that capture analytes that are too large to pass from one channel into a second, smaller channel. Polymer nanoparticles, Herpes simplex virus type 1 capsids, and hepatitis B virus capsids are trapped and detected. The signal-to-noise ratio of the fluorescently-detected signal is shown to be greater than 3 for all analyte concentrations considered.

Ce travail aborde la problématique des systèmes de refroidissement ou de contrôle thermique industriels. Nous avons particulièrement mis l'accent sur le refroidissement des serveurs informatiques. Une première partie consiste en l'étude des moyens d'amélioration des techniques de refroidissement existantes, tandis que la deuxième partie est une réflexion sur des techniques de refroidissement alternatives potentiellement plus efficaces et répondant aux demandes actuelles du contrôle thermique industriel. Dans le premier chapitre, nous analysons la bibliographie et la théorie relatives aux phénomènes physiques derrière les techniques de refroidissement étudiées. Ensuite, une classification des techniques de refroidissement est proposée en fin de chapitre. Ce chapitre a servi de base pour l'amélioration des technologies de refroidissement existantes et à la réflexion sur de nouvelles techniques plus efficaces. Le second chapitre porte sur l'optimisation d'une plaque froide, destinée au refroidissement des serveurs informatiques, en s'aidant d'un outil numérique et d'essais expérimentaux. Nous avons noté une augmentation des transferts de chaleur dans la plaque froide en utilisant des inserts, notamment ceux en forme de losange disposés en quinconce. A l'inverse, l'utilisation de certains nanofluides en tant que fluides caloporteurs ne semble pas apporter de gain significatif. Dans le troisième chapitre nous détaillons la démarche suivie pour la conception d'un dissipateur de chaleur basé sur une technologie caloducs, destiné au refroidissement des cartes électroniques. En premier lieu, nous présentons le modèle thermohydraulique de dimensionnement d'un caloduc cylindrique ; une étude paramétrique (géométrique, type de fluide,...) nous a permis d'identifier le jeu de paramètres donnant la meilleure performance du caloduc. En second lieu, nous évoquons les tests réalisés sur le dissipateur de chaleur à caloduc qui nous amènent à valider en partie le modèle thermohydraulique développé. Le dernier chapitre porte sur la réalisation et l'étude d'un démonstrateur pour le refroidissement des cartes électroniques par immersion dans un liquide à basse température de saturation. On commence par la mise en place et l'utilisation d'un modèle numérique pour la conception du démonstrateur, puis des tests expérimentaux sont réalisés. Les premiers résultats obtenus en utilisant le SES-36 comme fluide de travail sont assez prometteurs.Mots clés : modélisation, transfert de chaleur, refroidissement, datacenter, liquid-cooling, caloducs, échangeurs, nanofluides, ébullition en vase, simulation numérique

Vapour absorption refrigeration systems (VARSs), which utilise eco-friendly refrigerants (water), can work based on low-grade thermal energy, such as the solar energy. Using solar energy can relief the high electrical load on many national grids around the world as the peak load almost coincides with the high solar intensities time during summer. However, high initial cost, big specific size and low coefficient of performance are the main challenges that face the VARSs. Therefore, improving the efficiencies of the components of a solar refrigeration system, such as the solar collector, generator and absorber, is crucial to improve the overall efficiency of that system and to reduce its size and the cost. To improve the efficiency of a solar VARS, nanofluids are proposed in this work through direct and indirect ways. The direct way is seeding functional nanoparticles in the aqueous solution of a VARS. The expectations are that refrigerant (steam) can be generated efficiently via direct volumetric absorption of the solar energy at the generator and can be absorbed effectively via the Brownian motion of the nanoparticles at the absorber. While, the indirect way is; using aqueous nanofluids in direct solar collectors can harvest the solar energy in an efficient way, saving it in a storage tank. This stored energy is used later to generate the steam in the generator of the VARS. This work aims to investigate fundamentally the applicability of utilising nanofluids for solar absorption refrigeration systems through performing three main studies: Firstly, a comparative study of gold, copper oxide, gold and copper oxide hybrids, and carbon black nanofluids has been conducted to investigate the photo-thermal conversion efficiency. The results have shown that gold nanofluids are not feasible for solar application due to the high cost and low performance comparing to the carbon black nanofluids. Moreover, this study has demonstrated that blending different nanofluids of different narrow spectral absorption peaks can really broaden the effective spectral absorption peak but reduces its value due to the accompanied dilution of the blended nanofluids as the overall volume unavoidably increases. Secondly, a comparative study of solar steam generation among gold nanofluids, carbon black nanofluids and a thin carbon-based porous medium has been conducted. The results have also shown the infeasibility of using gold nanofluids comparing with the carbon black nanofluids due to the high cost and low absorptivity. Furthermore, this study has shown the superiority of using a thin, carbon-based porous medium in producing steam due to its capability to absorb most the solar energy in micro-sized thickness. While, very high nanoparticles concentration is required to trap and absorb the solar energy in such a thin layer, which consequently leads to an instability, high viscosity and high-cost issues. Finally, a study of the steam absorption by and generation from aqueous lithium bromide solutions seeded with carbon black and carbon nanotubes has been conducted. The results have demonstrated that a very low concentration of carbon black nanoparticles can reduce the transparency of the solutions to zero. However, seeding nanoparticles in the solutions has shown the negligible effect on the steam absorption rate, which demonstrated that the Brownian motion of the nanoparticles has a negligible effect on the steam absorption. Although the experiments conducted in this project showed negligible enhancement in the steam absorption, obvious enhancements in the photo-thermal conversion efficiency and steam generation were achieved by using nanofluids. Recommendations are suggested for future work to study other affecting aspects of seeding nanoparticles in aqueous solutions.

Thesis (Ph. D.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Thesis (Ph.D)--Chemical Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Teja, Amyn S.; Committee Member: Abdel-Khalik, Said I.; Committee Member: Meredith, Carson; Committee Member: Nair, Sankar; Committee Member: Skandan, Ganesh. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Les écoulements de fluides complexes à l'échelle sub-micronique est une problématique rencontrée dans des domaines aussi divers que la récupération assistée du pétrole ou la lubrification des surfaces. Un fluide complexe a des propriétés rhéologiques riches, dues à la présence d'objets déformables en solution, comme les pelotes de polymère. Les phénomènes de surface, comme le glissement jouent un rôle important aux petites échelles. La question de l'effet du confinement sur la rhéologie de solutions de polymères est abordée. Nous caractérisons la taille des objets en solution et la rhéologie volumique des fluides. Grâce au développement d'une technique de photobleaching de fluorescence pour la mesure de vitesse d'écoulement dans des canaux sub-microniques, nous déterminons la viscosité effective des fluides en géométrie confinée. Cette approche expérimentale nous permet de montrer que le confinement induit une diminution de la viscosité effective des fluides. Une mesure directe des vitesses et longueurs de glissement est réalisée en microcanaux par vélocimétrie de particules (micro-PIV). Ces données mettent en évidence une réduction du glissement en géométrie confinée, qui est interprétée en termes de modification du mécanisme de glissement. Une distinction entre le comportement volumique et les phénomènes de surface ne permet plus de rendre compte du comportement du fluide à l'échelle sub-micronique. Une étude préliminaire des écoulements de solutions de tensioactifs à l'échelle sub-micronique est également proposée.

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CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Este trabalho teve por objetivo estudar a influencia de materiais nanoestruturados no transporte térmico em meio líquido e sua aplicabilidade como fluidos de dissipação de calor. Foram estudados nanofluidos de base aquosa contendo nanopartículas esféricas de ouro e prata, bem como nanofolhas de grafeno, nanotubos de carbono, fulerenos e nanofios de fulerenos. Para a caracterização estrutural dos nanofluidos foram utilizadas a espectroscopia de extinção/absorção UV-visível, microscopia de força atômica (AFM) e espectroscopia via espalhamento Raman. Com base na teoria de espalhamento de Mie foi possível estimar a distribuição de tamanhos para as nanopartículas metálicas. Com as imagens de AFM foram obtidos tamanhos de partículas, principalmente para as nanoestruturas de carbono. A qualidade e tipo de nanoestrutura de carbono foram definidos com auxílio da espectroscopia Raman. Os nanofluidos foram termicamente caracterizados através da espectroscopia via efeito de lente térmica. Obteve-se aumento na eficiência térmica para os nanofluidos com nanopartículas metálicas. No entanto, o mesmo não ocorreu com as dispersões de carbono nanoestruturado. Assim, mostraram-se como alternativas promissoras de fluidos de resfriamento os nanofluidos com nanopartículas metálicas.
This work was intended to study the influences of nanostructured materials in thermal transport in liquid medium and the applicability of these new materials as heat dissipation media. We studied aqueous nanofluids containing spherical nanoparticles of gold and silver, as well as Graphene nanosheets, carbon nanotubes, Fullerenes and Fullerene nanowires. For structural characterization of these nanofluids, extinction/ absorption UV-visible spectroscopy, atomic force microscopy (AFM) and Raman scattering spectroscopy have been used. Based on Mie scattering theory it was possible to estimate the distribution of sizes for metal nanoparticles. With AFM images were obtained particle sizes, mainly for the nanocarbon structures. The quality and type of nano carbon structure were defined with help of Raman spectroscopy. The heat transfer characteristics were obtained through thermal lens spectroscopy. We obtained thermal transport enhancement for metallic nanofluids. However, the same did not occur with nanocarbon nanofluids. Thus, the metallic nanofluids proved to be promising alternatives as cooling fluids.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 67-71).
The overall goal of this thesis was to exploit the versatility of the polyelectrolyte multilayer (PEM) to fabricate a novel micro/nanofluidic device for patterning bacteria in BioMEMS. Nanofluidic channels offer new opportunities for advanced biomolecule manipulation and separation science because they provide unique capabilities such as ion-perm selectivity and nanometer-sized structures. In order to establish industrial applications for biotechnology and medicine, including separation of biomolecules, drug delivery, and single molecule detection, however, regular planar nanofluidic channels have limited fluidic conductance that results low throughput. Therefore, it would be important to develop a robust engineering platform with precise control of depth to the nanometer scale without channel collapse. Nanochannel-induced fluidic conduction can be enhanced by controlling the channel gap size for increasing electrical double layer (EDL) overlap as well as fabricating high-throughput vertical nanofluidic channels. We have fabricated a vertical nanofluidic channel by anisotropic etching of silicon. The gap size of the vertical nanochannel was as low as 50 nm, as obtained by layer-by-layer deposition of polyelectrolyte. Silicon-to-glass bonding was achieved by electrostatic interaction at lower temperature (180 'C) than conventional anodic bonding temperatures (300-400 C), and even at room temperature (25 C). The second part of this thesis focuses on patterning bacteria on polyelectrolyte multilayers. Patterns of bacteria are of growing interest in biofilm formation and the broader area of microbial ecology. A simple method to create functionalized surfaces for efficient micro-patterning of bacteria is presented, based on the use of micromolding in capillaries (MIMIC) of poly(ethylene glycol)-poly(lactide) diblock copolymer (PEG-PLA) onto polyelectrolyte multilayers. Two different implementations showed excellent selective antibiofouling results for micropatterning of bacteria.
by Hongchul Jang.
S.M.

Numerical analysis on a three-dimensional PTSC receiver’s tube model equipped with conical/wavy turbulators was conducted with various types of nanofluids/hybrid nanofluids. The Navier-Stokes equations were solved using FVM coupled with the MCRT method. The flow, thermal and entropy characteristics of the PTSC’s receiver tube were investigated. This research revealed that the coupling of conical/wavy turbulators and hybrid nanofluids effectively augmented the thermal and exergetic efficiencies and reduced the entropy generation rate compared with pure base-fluid.

La première partie de cette thèse constitue une introduction aux différentes méthodes de conversion d'énergie et de dessalement qui seront évoquées dans cet ouvrage. Dans une deuxième partie, nous montrons que la conductance ionique d'un réseau de nanopores est sous-additive avec le nombre de pores. La contribution individuelle de chaque pore à la conductance globale tend vers une valeur nulle, pour un réseau suffisamment grand. On note que seuls des rapports de longueurs interviennent, et que le choix d'une échelle nanométrique n'a pas d'influence dans l'effet observé. Ensuite, dans une troisième partie, nous mesurons la perméabilité d'un réseau de pores à une échelle macroscopique. Là aussi, l'influence du réseau ne dépend pas de l'échelle du système. La perméabilité évolue en sens inverse de la conductance : elle est augmentée par la présence de pores voisins, mais dans une faible proportion. La quatrième partie se sert des résultats des deux parties précédentes, dans le but de déterminer une loi d'échelle pour la puissance électrique produite par courant d'écoulement et diffusio-osmose, deux méthodes de conversion d'énergie osmotique. On montre que les effets d'entrée ont un effet délétère sur cette conversion ; ils nécessitent des études plus approfondies. La dernière partie est un travail numérique sur un nouveau procédé de dessalement par osmose via une phase gaz, piégée dans des nanotubes hydrophobes. Son intérêt principal est l'utilisation de nanotubes plus gros que les pores des matériaux actuellement utilisés, donc moins susceptibles de s'encrasser. Par dynamique moléculaire, nous étudions la perméabilité et la sélectivité du dispositif
The first part of this thesis is an introduction to the different energy conversion and desalination methods that will be invoked in this work. In a second part, we show that the ionic conductance of a nanopore array is sub-additive with the number of pores. Individal contributions of each pore to the global conductance tend to a null value, if the network is big enough. We note that this phenomenon only involves length ratios, and that working at a nanometric scale does not have any influence. Then, in a third part, we measure the permeability of a pore array at a macroscopic scale. There too, the effect of the array does not depend on the scale of the system. Permeability evolves inversely to conductance: permeability is enhanced by the presence of neighboring pores, but in a smaller proportion than the ionic conductance falls under the same cause. The fourth part uses the results of the two preceding ones, to determine a scaling law for the electric power produced by streaming current and diffusio-osmosis, two methods of osmotic energy conversion. We show that entrance effects have a negative impact on such conversion, more efforts are needed to understand them better and circumvent them. The fifth and last part of this thesis is a numerical work on a new desalination device. It relies on osmosis through a gas phase which is trapped within a hydrophobic nanotube. Its main interest is to use nanotubes bigger than the pores of currently used materials, thus less prone to fouling. We use molecular dynamics methods to study the permeability and selectivity of this device

Nanoscopic pores in biological systems – cells, for example – are responsible for regulating the transport of ionic and molecular species between physiologically distinct compartments maintained by thin plasma membranes. These biological pores are proteinaceous structures: long, contorted chains of chemical building blocks called amino acids. Protein pores have evolved to span a staggering range of shapes, sizes and chemical properties, each crucial to a pore's unique functionality.

Protein pores have extremely well-defined jobs. For instance, pores called ion channels only transport ions. Within this family, there are pores designated to selectively transport specific ions, such as sodium channels for sodium, chloride channels for chloride and so on. Further subdivisions exist within each type of ion channel, resulting in a pantheon of specialized proteins pores.

Specificity and selectivity are bestowed upon a pore through its unique incorporation and arrangement of its amino acids, which in turn have their own unique chemical and physical properties. With hundreds of task-specific pores, deciphering the precise relationship between form and function in these protein channels is a critical, but daunting task. In this thesis, we examine an alternative for probing the fundamental mechanisms responsible for transport on the nanoscale.

Solid-state membranes offer well-defined structural surrogates to directly address the science underlying pore functionality. Numerous protein pores rely on electronic interactions, size exclusion principles and hydrophobic effects to fulfill their duties, regardless of their amino acid sequence. Substituting an engineered and well-characterized pore, we strive to achieve and, thus, understand the hallmarks of biological pore function: analyte recognition and selective transport.

While we restrict our study to only two readily available membrane materials – one a polymer and the other a ceramic – nanofabrication techniques give us access to a virtually limitless combination of pore shapes and sizes. Exploiting this, we investigate the role of pore geometry in mediating the electrostatic and steric interactions responsible for transport on the nanoscale. Through targeted chemical modifications of our homogenous pores, we easily tailor their surface properties to investigate the role of hydrophobic effects in confined environments. Unbound by the physiological limitations of protein structures (such as sensitivity to electrolyte composition and fragility to external forces), our report concludes with the fusion of fabrication and modification considerations to design robust components for nanofluidic circuitry and nanoscopic biosensors.

Mestrado em Engenharia Mecânica
A transferência de energia sobre a forma de calor é um processo indispensável a uma panóplia de indústrias, desde a eletrónica à alimentar. Frequentemente, estes tipos de sistemas energéticos dependem de fluidos para promoverem a transferência de calor, tais como a água, o etileno-glicol ou óleos. Contudo, estes fluidos convencionais apresentam propriedades termo-físicas pobres, limitando o desenvolvimento de sistemas térmicos sustentáveis, bem como a sua miniaturização. Com o intuito de melhorar as propriedades termo-físicas de fluidos de permuta, dispersou-se nanotubos de carbono num fluido convencional (30:70 Etileno-glicol/Água). Estas dispersões são designadas de nanofluídos. Para o estudo da transferência de calor dos nanofluídos produzidos, otimizou-se um aparato experimental dedicado a estudo dos fenómenos de transferência de calor por convecção. Neste aparato, os nanofluídos foram estudados a diferentes taxas de escoamentos, num tubo circular sujeito a um fluxo de calor superficial constante. As relações entre as diferentes variáveis como: propriedades termofísicas, fração volúmica, temperatura e velocidade de escoamento são, assim objeto de estudo detalhado nesta dissertação. Os resultados experimentais sugerem que os nanofluídos permitem taxas de transferência de calor por convecção superiores, quando comparadas com o fluido base. Assim, a dispersão de nanopartículas em fluidos de permuta convencionais, permite o desenvolvimento de sistemas energéticos sustentáveis.
The heat transfer is an essential process to a wide range of industries, from electronics to food production. Often, these types of energy systems rely on fluids to promote the heat transfer, such as water, ethylene glycol or oils. However, these conventional fluids usually have poor thermo-physical properties, limiting the sustainable development of such thermal systems, as well as their miniaturization. In order to improve the thermo-physical properties of heat-exchange fluids, it was dispersed carbon nanotubes in a conventional fluid (Ethylene glycol/Water 30:70). These dispersions are designated as nanofluids. To study the heat transfer of the produced nanofluids, it was optimized a dedicated experimental apparatus to study the phenomena of convection heat transfer. In such apparatus, the nanofluids were studied at different flow rates, in a circular tube subjected to a constant heat flux. The relationships between the different variables such as: thermo-physical properties, volume fraction of the nanoparticles, temperature and flow rate are, therefore, main studying factors in this dissertation. The experimental results suggest that nanofluids allow a superior exchange heat transfer through convection, when compared with the base fluids. Therefore, the dispersion of nanoparticles in conventional heat-exchange fluid enables the development of more sustainable energy systems.

Ces travaux de thèse ont eu deux objectifs: i) le développent de systèmes nanofluidique en utilisant une méthode non-lithographique, peu chère et facilement transposable à l'échelle industrielle ii) la compréhension des phénomènes nanofluidiques au travers des études expérimentales et de modélisation. Des couches minces mesoporeuses, en particulier des structures planaires avec des nanopiliers, ont été utilisé pour des études sur l'infiltration capillaire des liquides dans espaces confiné au niveau nanométrique. En plus des premiers tests pour des applications plus complexes comme des séparations et réactions nanoconfiné. Des structures mesoporeuses non-organisés ont aussi été étudiées pour déterminer la relation entre la nanostructure et la vitesse de remplissage capillaire. A été aussi démontré que pour des porosités avec des forts rétrécissements le remplissage capillaire se produit par l'intermédiaire d'une phase vapeur. Les échantillons ont été préparés par dip-coating. Une méthode de préparation basé sur une substitution de la plus grande parte de la solution à déposer par un fluide inerte a été développé. La méthode permet de réduire fortement le cout de procédé et, par conséquence, de faire des dépôts sur plus grande surface. Un effort dans la modélisation des phénomènes nanofluidiques a aussi été fait pendant cette thèse. Une méthode de simulation qui permet de décrire adéquatement les interactions hydrodynamiques dans un système nano a été utilisée pour simuler un flux électro-osmotique. La méthode, Stochastic Rotational Dynamics, a été valide par confrontation avec des résultats connus et l'influence des certains paramètres de simulation évaluée dans le détail
This thesis had a dual purpose: i) the development of nanofluidic devices through not lithographic, cheap and scalable bottom-up approach ii) the understanding of nanofluidic phenomena both through experiments and simulations. Mesoporous thin films, in particular Pillared Planar Nanochannels (PPNs), were prepared and utilized to study the capillary infiltration of liquids in nanostructures and have been tested for future nanofluidic applications like separations and nanoconfined reactions. Non organized mesoporous films have also been studied to determine the relationship between nanostructure characteristics and infiltration speed. It has been also demonstrated that in the case of porosities with reduced bottle-necks capillary penetration is performed through a vapor mediated mechanism The samples were prepared by dip-coating. A novel method of preparation based on the substitution of a large part of the deposing solution in dip-coating with an inert fluid has been developed in order to strongly reduce the fabrication costs and allow the preparation of larger samples. Moreover advancement in control of the dip-coating technique in “acceleration-mode” to produce thickness gradients has been developed and some potential application linked to fluidics shown. Finally a part of the effort of this thesis has been placed in the modeling of the electro-osmotic phenomenon in nanostructures through a rather novel simulation method, Stochastic Rotational Dynamics, which takes into account the hydrodynamics and the other interactions inside a nanofluidic system. Validations of the method and further investigations in particular nanofluidic conditions have been performed

Branching networks play a major role in a variety of physiological and engineering structures over a range of physical scales. However, increasingly, artificial systems are being tailored towards the nanoscale to reduce costs and improve performance and process control. In this thesis, analytical and numerical models are developed to enable the efficient design and accurate simulation of nanofluidic branching networks, where non-continuum/non-equilibrium effects prohibit the use of common solutions. A hybrid molecular-continuum method is presented for the design and simulation of general high-aspect-ratio nanofl uidic networks. This increases the scope of hybrid techniques in two main ways: 1) the method is generalised to accurately model any nanofluidic network of connected channels, regardless of its size or complexity; 2) by generalising the application of constraints, the geometry or governing pressures can be the output of the method, enabling the design of networks without the need for a costly trial-and-error process. For a variety of constraint combinations, it is shown that the hybrid method converges quickly to the solution of a full molecular dynamics simulation, with relative errors of < 4% for all variables across all cases. Network design is further advanced by the development of a generalised optimisation principle that finds the daughter-parent area ratio maximising flow conductance per unit volume in all branches. Through numerically verified analytical models, it is demonstrated that the common branching principle `Murray's law' is sub-optimal for asymmetric branching (where the local optimisation of each individual channel does not correspond to the global optimum for the network as a whole), while the generalised law presented in this thesis is valid for 1) symmetric and asymmetric branching, 2) slip and plug fl flows, which occur at very small scales, and 3) any cross-sectional shape; making it a powerful tool for nanofluidic biomimetic modelling.