Dissertationen zum Thema „Films suspendus“

Les films librement suspendus de cristaux liquides thermotropes en phase smectique sont des systemes lamellaires de taille finie, parfaitement ordonnes, dont les couches, constituees par les molecules mesogenes, sont paralleles aux deux surfaces libres. Le film est tendu sur un cadre rectangulaire possedant deux bords mobiles. Il est raccorde a son support par un surplus de materiaux appele le menisque qui est un reservoir de molecules. Trois quantites caracterisent un film: le nombre de couches n, que l'on peut varier d'une quantite macroscopique (>2000) a seulement 2, la tension tau et le nombre de molecules n contenues dans les couches. Nous etudions les proprietes thermodynamiques de ces membranes smectiques ; et plus particulierement l'influence des parametres tau, n et n sur le diagramme de phases, ainsi que les effets d'une contrainte parallele au plan des couches. La these presente quatre nouvelles methodes experimentales qui permettent de mesurer, a une couche moleculaire pres, l'epaisseur d'une membrane smectique, de varier le nombre de couches un par un, de moduler la tension du film, et d'imposer une contrainte mecanique parallele aux plans des couches smectiques. Trois resultats principaux ont ete obtenus. Premierement, le systeme thermodynamique a considerer lors d'une perturbation mecanique est compose du film et du menisque. Deuxiemement, l'epaisseur du film ainsi que la tension interviennent dans le diagramme de phase de la membrane. Ainsi, nous avons montre, d'une part, que la transition du premier ordre sma/smc#* devenait continue sous une epaisseur critique, estimee a 90 couches pour le compose etudie. D'autre part, nous avons constate qu'une variation de la tension peut induire une transition de phase. Enfin, en comprimant et dilatant le film, nous avons pu estimer la valeur du coefficient d'elasticite parallele au plan des couches dans la phase cristalline smb. Nous avons aussi montre que la valeur du coefficient d'elasticite des phases smectiques est fonction de l'existence ou non d'un angle d'inclinaison des molecules par rapport a la normale aux couches

Ce travail presente une etude experimentale et theorique des films librement suspendus du cristal liquide 8cb en phase smectique a, ainsi que de leurs dislocations coin. Dans ces films, les couches sont parfaitement ordonnees, paralleles au plan du film, et leur nombre peut varier de seulement 2 (6nm) a plusieurs centaines (quelques m). Ces films constituent des systemes tres interessants pour etudier les effets d'epaisseur finie et les proprietes des dislocations coin en milieu confine. Stabilite des films smectiques : l'etude d'un film smectique doit s'accompagner de l'etude du menisque qui le relie au cadre qui le supporte, et avec qui il peut echanger de la matiere. Nous avons montre que ce menisque a un profil circulaire, qu'il fixe la pression a l'interieur du film et qu'il ne se raccorde pas au plan du film tangentiellement. L'observation de l'angle de raccordement permet une mesure directe des interactions entre les surfaces libres du film. Nous avons ainsi montre qu'a faible epaisseur les surfaces libres s'attirent a cause de la presence d'un exces d'ordre smectique en surface. A cause de cet effet, un film assez mince est stable jusqu'a une temperature superieure a la temperature de transition smectique a - nematique, d'autant plus elevee que le nombre de couches est faible. En chauffant le film lentement nous avons ainsi observe des sequences de transitions d'amincissement. Nous avons explique toutes ces observations experimentales a l'aide d'un modele de type champ moyen de landau - ginzburg - de gennes. Etude des dislocations coin : une analyse de forme de boucles de dislocation dans un film vertical nous a permis de mesurer leur tension de ligne en fonction de l'epaisseur du film et de la temperature. Nous avons aussi etudie les dynamiques de croissance d'une boucle dans un film horizontal et de nucleation grace a une pointe chauffante, ce qui nous a permis de mesurer leur mobilite et de mettre en evidence les echanges de matiere avec le menisque.

Les films librement suspendus de cristal liquide 8CB en phase smectique A sont très stables du fait de leur structure lamellaire. En effet, l'élasticité des couches compense la pression capillaire imposée par le ménisque qui le borde et qui joue le rôle de réservoir de matière. Lors de ce travail, nous nous sommes intéressés à la dynamique de ces films. Nous avons mis en évidence que le ménisque se comporte
comme un réservoir dissipatif.

En régime quasi-statique, lorsque les échanges de matière entre le film et le ménisque sont lents, les écoulements de perméation autour des dislocations coin qui le composent sont à l'origine de sa perméabilité finie. Un modèle hydrodynamique montre que la dissipation est localisée à son entrée, dans la zone bien orientée et qu'elle dépend fortement de l'épaisseur du film. Dans les films fins, le ralentissement des boucles de dislocation et dans les films épais, la dynamique de relaxation de deux ménisques reliés par un même film ont permis de confirmer expérimentalement ce modèle.

La dynamique d'effondrement d'une bulle a permis de caractériser la dissipation dans des régimes d'écoulements plus rapides en mettant en évidence un écart à la loi de Laplace (statique). Les expériences montrent qu'après une étape conduisant à la déstructuration du ménisque, ce qui le rend plus perméable, la bulle peut s'effondrer, la matière s'engouffrant dans le ménisque avec un comportement rhéofluidifiant. De plus, la nucléation et la croissance d'îlots peut rendre le processus moins dissipatif, ce qui est essentiellement observé dans les films fins.

Les cristaux liquides smectiques peuvent former des films librement suspendus analogues aux films de savon. En equilibre, ces films ont une epaisseur homogene. En meme temps, la tension du film reste constante sur toute sa surface. Ainsi, les vibrations des films smectiques librement suspendus avec des amplitudes faibles repondent tout-a-fait aux criteres d'application de la theorie lineaire. Les films smectiques librement suspendus sont tres robustes, et nous pouvons, par le biais d'une excitation periodique, les mettre en vibration avec une amplitude arbitraire. Ainsi, ces films permettent d'etudier experimentalement les vibrations dans le regime non lineaire. Il s'avere que les vibrations avec une forte amplitude provoquent les instabilites du menisque vis a vis de la surface du film ainsi que le mouvement des iles sur sa surface. De plus, il est tout a fait possible d'introduire des objets etrangers dans des films smectiques librement suspendus. Ca peut etre, par exemple, une fibre qui perce le film dans un endroit donne. L'analogie qui existe entre l'equation de schrodinger bidimensionnelle et l'equation d'onde permet de faire l'analogie entre les films smectiques en vibration et les billards quantiques. En particulier, un film smectique en vibration perce par une fibre fine est analogue a un billard quantique avec une perturbation ponctuelle. Ainsi, nous avons pu etudier du point de vue experimental les points diaboliques dans des billards quantiques avec une perturbation ponctuelle, a travers le spectre de resonance des films smectiques appropries. Les films smectiques peuvent supporter sur leur surface une inclusion massive, comme par exemple, une petite bille. Une telle bille est captee par la surface du film, mais elle peut neanmoins glisser librement sur la surface de ce dernier. Le spectre de vibration du film depend de la position de l'inclusion, qui peut elle-meme bouger sous l'effet de ces vibrations. Il s'avere que la bille choisit une telle position que la frequence de la resonance du systeme film+bille corresponde a la frequence de l'excitation.

Ces travaux concernent l’étude expérimentale des textures se développant dans les ménisques de films smectiques de cristaux liquides thermotropes. Ces fluides complexes nous servent de système modèle pour l’étude des couplages élasto-capillaires qui se manifestent dans certaines conditions par des déformations périodiques de l’interface cristal liquide-air. Dans un premier temps, à partir de l’étude détaillée de la topographie de l’interface par une méthode interférométrique, nous caractérisons les différents types de structures, puis, identifions les principaux paramètres impliqués dans le processus d'apparition des défauts dans le ménisque de films libres suspendus. Ces résultats nous servent de base pour discuter des mécanismes physiques qui génèrent les différents types de défauts observés. Nous regardons en particulier, comment la diminution d’épaisseur de couche associée à une transition de phase va induire une instabilité mécanique responsable de l’apparition des ondulations de l’interface. Dans un deuxième temps, afin d’aller plus loin dans la compréhension des mécanismes, nous avons utilisé des films minces de cristaux liquides déposés sur des substrats solides microstructurés par des plots fabriqués par des techniques photolitographiques. Une telle géométrie permet de générer un ménisque autour de chaque plot et surtout de faire varier un plus grand nombre de paramètres comme par exemple l’ancrage sur le substrat via un traitement chimique de surface. L’ensemble de nos résultats apportent un éclairage nouveau sur la caractérisation et la compréhension des déformations spécifiques aux ménisques de fluides complexes
The present work is an experimental study of the textures that appear in the meniscus of free standing smectic films with thermotropic liquid crystals. These complex fluids serve as model systems to investigate elasto-capillary phenomena which, under certain conditions, manifest themselves through periodic deformations of the liquid crystalair interface. In the first part of the thesis, we focus our attention on meniscus structures whose interfacial topographies are thoroughly characterized thanks to an in-house optical interferometry technique. Our study allows us to identify the main parameters involved in the development of meniscus structures and to discuss the physical mechanisms that are likely to be responsible for their formation. In particular, we show how a phase transition-induced layer shrinkage triggers a mechanical instability leading to interfacial undulations of the smectic free surface. In the second part of the manuscript, we address the case of thin liquid crystal films deposited on solid patterned solid substrates. The latter consist of regular arrays of microposts fabricated through photolithographic techniques. Such a geometry allows a meniscus to be formed around each micropost and makes it possible to examine the influence of other parameters such as the anchoring conditions on the solid substrate. The results gathered so far are able to shed some light on the characterization and the understanding of the specific deformations and textures that appear in the menisci of complex fluids

This thesis presents a novel architecture for a sensing element fabricated from a conducting polymer and a bioderived membrane. The thin film device provides controlled, selective ion transport from a chemical concentration and produces measurable electrical signals, ion storage, and small scale actuation. A chemical gradient applied across a bioderived membrane generates ion flow through protein transporters in the presence of a gating signal. A conducting polymer undergoes ion ingress/egress in the presence of an electrical and chemical potential, which causes a change on the polymers conformal backbone. A ligand (or) voltage gated protein in the bioderived membrane results in ion transport through the bioderived membrane. Integrating the two electroactive materials provides a unique architecture which takes advantage of their similarities in ionic function to produce a device with controlled and selective ion transport. The chemoelectromechanical device is one that couples chemical, electrical, and mechanical potentials through number of ions, dielectric displacement, and strain. The prototype consists of a stacked thin conducting polymer film and bioderived membrane which form three aqueous chambers of varying ionic concentrations. The top chamber contains an electrolytic solution, and the bottom chamber contains deionized water adjacent to the conducting polymer. The current that passes through a conducting polymer for an applied electrical signal is based on the level of doping/undoping and therefore can be used as a method of sensing protein function in the sensing element. This architecture results in a sensing element applicable in real time chemical sensors, volatile organic compound detectors, and bioanalytical sensors. The conducting polymer layer is formed from polypyrrole (PPy) doped with sodium dodecylbenzenesulfonate (NaDBS), and the bilayer lipid membrane is formed from 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) reconstituted with the protein alamethicin. The magnitude of current required to span a 175 µm pore was empirically found to be 326.5 A/cm2 and is based on electrode condition, electrode surface area, pyrrole concentration, and electrical potential. A micron-scale pore through a silicon substrate is spanned by a thin PPy(DBS) layer, forming a bridge which supports the bioderived membrane. The bioderived membrane is reconstituted with alamethicin, a voltage-gated protein extracted from trichoderma viride. Ion transport experiments were performed to characterize the PPy(DBS) layer and the bioderived membrane and are represented as electrical equivalents for subsequent analysis. The equivalent impedance of polypyrrole was calculated to be 1.7847±0.1735Ωcm2 and capacitance was calculated to be 1.2673±0.1823µF/cm2. The equivalent impedance of a bioderived membrane was calculated to be 1.654±1.9894MΩcm2, capacitance was calculated to be 1.1221± 0.239µF/cm2, and alamethicin resistance was calculated to be 1.025± 0.7228MΩcm2. Thus, using impedance measurements in the conducting polymer layer, it is proposed that a scaled up sensing element can be fabricated using the suspended polypyrrole supported bioderived membrane.

Electrokinetic phenomena in liquid foams is at a junction between two areas. On one side is the investigation of liquid foam drainage, and on the other side is electrokinetics of surface driven flow on solid-liquid interfaces. However, the electrokinetic phenomena in liquid foam films significantly lack understanding. Therefore, the novelty of the thesis is to address the mentioned gap in three stages. The outcome has potential applications in a novel separation approaches of biological molecules such as proteins and DNA. In the first stage, the electrokinetic flow of a sufficiently thick (180 μm) free liquid film was investigated using cationic and anionic surfactants by confocal micron-resolution particle image velocimetry (μ-PIV). The reverse of the surface charge resulted in a shift in charge of the electrical double layer at the free liquid film interface, which caused the direction of the electroosmotic velocity to reverse. In each surfactant type used, the fluid velocity profiles were measured at different depths of the free liquid film (different z-planes). It was found how the fluid velocity varied with depth. Numerical simulations of the electroosmotic flow in the same system were also performed using Finite Element Method to understand the flow dynamics. A reasonably good agreement was found between the numerical simulations and the experimental results validating the model. In the second stage, instead of flow visualisation particles, rhodamine B (RB) and fluorescein isocyanate (FICT) dye were added to the free liquid film. Under the initial conditions of pH 7.2, RB is a neutral dye, and FICT has a -2 charge. Under an imposed electric field pH variations were detected and an interesting flow profile was observed. The CFD model developed earlier (stage one) was modified to include the local pH variation. The behaviour of the simulated pH had a good agreement with the behaviour of the FICT. Further confirmation of local pH variation was undertaken using extra new experiments which also showed a good agreed with the simulation. In the third stage, a liquid foam electrokinetic separation chamber was designed to extend the study to include practical applications. The first challenge was to achieve a stable foam under external electric field. A polymer-surfactant mixture can solve the stability problem. However, the mixture of polymers required an alkaline pH (>9) condition for the polymer mixture to be soluble in the aqueous system. Lectin and tetramethylrhodamine goat anti-rabbit (IgG) protein mixture with different molecular mass to charge ratio (50 kDa and 150 kDa) were injected near the anode. The system was monitored in three location: (a) in a vicinity of the injection region, (b) between the two electrodes and (c) in a vicinity of the cathode. In the region (a), a decay of the luminescence intensity of the fluorescein of the two proteins was noted with varying rate. In region (b), an increase followed by a decrease in fluorescein intensity of the proteins was observed again at a varying rate. In region (c), an increase of the dye concentration was observed and again at a different rate. The observed difference was caused by difference of the electrophoretic velocity of the two proteins. The setup proved that proteins could be separated based on their electrophoretic mobility inside a liquid foam. The findings from the thesis show the ability to manipulate fluid flow within a free liquid film, and inside a liquid foam system by an external DC electric field, is not only interesting academically but has potential application in a novel separation approach of biological molecules and beyond. The result show, with the correct surfactant formulation, it possible to make a stable foam under an electric field which can be set up for separation of proteins using foam electrokinetics.

Thesis (Ph. D.)--Ohio State University, 2003.
Title from first page of PDF file. Document formatted into pages; contains xix, 172 p.; also includes graphics. Includes bibliographical references (p. 168-172).

Orientador : Ioshiaki Doi
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação
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Mestrado

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1992.
Includes bibliographical references (leaves 109-116).
by Jeffrey Yen Pan.
Ph.D.

A once-through flowing rig built entirely from 316L stainless steel (SS316L) was used to study dissolution and particulate release behaviour of corrosion films on SS316L surfaces in simulated PWR primary coolant. Coolant of pH25C 9-11 was prepared using LiOH in nitrogen-sparged ultrapure water, and pumped at 0.1-2 g/min, under ~100 bar pressure and temperatures up to 300 °C. Flow velocity was at most 6.4 mm/s in 1⁄4” tubing, and 0.13 mm/s in 1” bore tubing where the bulk of the volume resided. Levels of the five most prevalent alloying elements – Fe, Cr, Ni, Mo, Mn – were analysed in rig effluent samples using ICP-MS. Nitrocellulose filters (0.05-3.0 μm) were used at the point of sampling to assess the contribution from particulate and colloidal matter. In most runs, SS316L surfaces of the rig acted as the sole source of corrosion products. Levels of Fe, Cr, and Ni were generally in the low ppb range, consistent with solubility, though transients to higher levels occurred. Levels of Mo and Mn varied with time and flow rate, sometimes exceeding 100 ppb, and cumulative release, particularly of Mo, appeared to be greater than could be accounted for by non-selective oxidation, suggesting leaching from the SS316L subsurface.

Liquid crystals exhibit a great variety of phases which differ one from another by their structure and physical properties. Near a phase transition, these mesoscopic phases are very sensitive to the surface interactions showing interfacial phenomena which are extraordinarily variable and interesting. The interaction between boundary surfaces and liquid crystal molecules results in surface layers with properties strongly different from the interior ones. For example, it is observed experimentally the unusual phenomenon of layer thinning upon heating. Above the bulk Smetic A - Isotropic transition temperature, the film is thinned to a low number of layers as the temperature increases. The temperature of these layer thinning transitions are related to the film thickness by means of a simple power-law. The characteristic exponent, μ, measured from experiments using distinct compounds, varies slightly in the range 0.6 < μ < 0.8. In this work, we investigate how surface effects modify the order parameter profiles in the vicinity of the Smectic-A-Isotropic (Sm A - I) transition in freestanding liquid crystal films using an extended mean-field approach which allows to incorporate explicitly the anchoring of the surface layers in the effective potential. We observed the contribution of the surface anchoring to the film stability. We found that there is a characteristic surface anchoring above which the bulk layers are less ordered than the surface ones. We also determine the characteristic exponent which governs the transition temperature dependence with the film thickness. We show that the different values of the exponents are related with the ratio between the size of the central rigid portion of the molecules and the typical layer spacing, as well as to the range of film thicknesses investigated. Our results qualitatively reproduce the main experimental findings concerning the layer thinning transitions in free-standing smectic films.
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Cristais líquidos exibem uma grande variedade de fases que diferem umas das outras por suas propriedades físicas e estruturais. Próximo a uma transição de fase, estes compostos são bastante sensíveis às interações superficiais apresentando fenômenos interfaciais que são extraodinariamente variados e interessantes. A interação entre as surperfícies limitantes e as moléculas de cristal líquido produz camadas superficiais com propriedades muito diferentes das camadas interiores do filme. Por exemplo, observa-se experimentalmente o fenômeno não-usual da redução de camadas ocasionado pelo aquecimento do filme. Acima da temperatura de transição esmética-A - Isotrópica da porção interior do filme, a amostra é reduzida para um número menor de camadas com o aumento da temperatura. A temperatura dessas transições por redução de camadas se relaciona com a espessura do filme por meio de uma lei de potência simples. O expoente característico, μ, medido experimentalmente usando compostos distintos, varia ligeiramente no intervalo 0.6 < μ < 0.8. Neste trabalho, investigamos como os efeitos de superfície modificam os perfis dos parâmetros de ordem na vizinhan¸ca da transição Esmética-A-Isotrópica (EsmA- I) em filmes de cristal líquido livremente suspensos, usando uma aproximação de campo médio estendida que permite incorporar explicitamente o ancoramento das camadas superficiais no potencial efetivo. Observamos a contribuição do ancoramento das superfícies para a estabilidade do filme. Encontramos que há um ancoramento superficial característico acima do qual as camadas da porção central do filme são menos ordenadas que as camadas da superfície. Também determinamos o expoente característico que governa a dependência da temperatura de transição com a espessura do filme. Mostramos que os diferentes valores dos expoentes estão relacionados com a razão entre o comprimento da porção central (rígida) das moléculas e o espaçamento típico entre as camadas, bem como com a espessura dos filmes estudados. Nossos resultados reproduzem qualitativamente os principais dados experimentais relacionados com as transições por redução de camadas em filmes esméticos livremente suspensos.

In the present work, we study the effects of external fields and surfaces on liquid-crystalline systems. Initially, we investigate the effects caused by the addition of ferroelectric nanoparticles in a nematic liquid crystal. Using the technique of molecular dynamics simulation, we estimate the density of liquid crystal molecules, the orientational order parameter, and the polar and azimuthal angle profiles as a function of the distance to the center of the immersed nanoparticle for different temperatures of the system. We observe that the presence of ferroelectric nanoparticles enhance the nematic order of the liquid-crystalline medium changing many properties above the transition temperature Tn — Iso. In the second moment, we determine the interaction mediated by elastic deformations between colloidal nanoparticles adsorbed on the surface of free-standing smectic films. In these films, the elastic-mediated force between the adsorbed particles have a long-range character, which contrasts with the character of short-range in films on a solid substrate. The study of phase transitions and critical phenomena in free-standing smectic films has been a subject of many theoretical and experimental works. The McMillan-Miranstsev mean-field approach was used to show that the additional orientational order imposed by the surface anchoring stabilizes the surface-induced smectic siSmA and nematic N phases, leading to he collapse of the tricritical point and to the emergence of a critical end point. Usually strongly anchored free-standing smectic exhibit a stepwise reduction in the thickness when the temperature is raised above the smectic-isotropic bulk transition temperature. In this study, we demonstrate that a layer trinning transition, induced by and external field, can occur in smectic films with negative dielectric anisotropy even below the bulk transition temperature. Using an extended McMillian's model, we show that when the field increases above the bulk transition field, the film thickness reduction is well described by a power law.
Conselho Nacional de Desenvolvimento Científico e Tecnológico
No presente trabalho, estudamos os efeitos de superfícies e campos externos em sistemas líquido-cristalinos. Inicialmente, investigamos os feitos causados pela adição de nanopartículas ferroeléticas em um cristal líquido nemático. Utilizando a técnica de simulação por dinâmica molecular, estimamos a densidade de moléculas do cristal líquido, o parâmetro de ordem orientacional e os perfis dos ângulos polar e azimutal em função da distância ao centro da nanopartícula imersa. Foram consideradas diferentes temperaturas do sistema. Pudemos observar que a presença das nanopartículas ferroelétricas favorece a ordem nemática do meio líquido-cristalino, alterando muitas propriedades do meio acima da temperatura de transição Tn-Iso. Num segundo momento, determinamos a interação, mediada por deformações elásticas, entre nanopartículas coloidais adsorvidas em filmes esméticos livremente suspensos. Nesses filmes, a força de interação entre as partículas adsorvidas tem um caráter de longo alcance, o que contrasta com o comportamento de curto alcance em filmes sobre um substrato sólido. O estudo das transições de fase e fenômenos críticos em filmes esméticos livremente suspensos têm sido objeto de estudo de vários trabalhos teóricos e experimentais. Dentro desse contexto, nós estudamos a transição de fases esmética-nemática (SmA — N) que ocorre no centro de um filme livremente suspenso sujeito a um forte ancoramento superficial, promovido pelo gás ao seu redor. A aproximação de campo médio de McMillan-Mirantsev foi utilizada para mostrar que a ordem orientacional, imposta pelo ancoramento superficial, estabiliza as fases esmética induzida pela superfície (siSmA) e nemática (N), levando ao colapso do ponto tricrítico e ao aparecimento de um ponto crítico terminal. Usualmente, filmes esméticos livremente suspensos e fortemente ancorados apresentam uma redução camada por camada na espessura quando a temperatura aumenta acima da temperatura da transição esmético-isotrópico. Nesse trabalho, nós demostramos que uma transição por redução de camadas, induzida por um campo externo, pode ocorrer em filmes esméticos com anisotropia negativa, mesmo em temperaturas abaixo da temperatura de transição do bulk. Utilizando o mesmo modelo extendido de McMillan, nós mostramos que quando o campo cresce acima do valor do campo crítico, a redução da espessura do filme é bem descrita por uma lei de potência.

碩士
國立中央大學
機械工程學系
104
Graphene is a one atom thickness 2D material that shows remarkable material properties, including high optical transparency, mechanical flexibility, high thermal conductivity, and superior high conductivity. In the last decade, graphene research and its related applications have been attracted intensive attentions. However, the earlier research on graphene device were performed on substrate. The substrate induced carrier scattering, charge impurity doping and corrugation that drastically degrade the intrinsic properties of graphene. Thus the suspended graphene shows superior intrinsic material properties on carrier transport, thermal conductivity and mechanical elasticity. Especially the practical application in ultra-high speed device. Before this study, the suspended graphene membrane made by the inverted floating method can yield the large size about 500μm in diameter; however, the suspended graphene by this approach were suffered from issues of the difficulty for manipulation and complicated process as well as the large amounts of polymer residue on graphene. This study was to develop a simple and reliable route to achieve a large area of suspended graphene. The proposed process including (1) the optimization of graphene growth conditions by atmospheric pressure chemical vapor deposition (APCVD) and (2) the transferring process for suspended graphene by solvent replacing and thermal decomposition method. It was found out that the optimized graphene single crystalline size with high quality could be up to ~50μm. The results shows that the largest suspended graphene membrane over 1,500μm in diameter can be obtained by stacking and transferring 5-layered graphene on a holy substrate. The XPS characterization shows that the extremely low oxygen functional groups of 4～6% on graphene membrane after thermal annealing can be achieved, suggesting the ultra-clean and high quality of suspended graphene can be made from our approach. To study the intrinsic properties and application of our suspended graphene membrane, the devices integrated with suspended graphene were fabricated. The results shows that the carrier mobility on suspended graphene is enhanced up to 154% when compared with the substrate supported graphene. In addition, the capacitive pressure sensor made by our ultra-large suspended graphene membrane, showing a superior high sensitivity and excellent signal linearity than conventional capacitive pressure sensors. The sensitivity of 15.15 aF / Pa were measured which is increased about 422~ 770% than silicon-based material. The developed method for ultra-large suspended graphene pave the way for the potential applications on electromechanical actuator, ultra-sensitive chemical/bio sensors as well as the high-frequency electronic devices.

碩士
和春技術學院
電機工程研究所
95
This thesis propose a way to make the Bulk Micromachining substrate of MEMS by wet etching. The sacrificial layer be derived by photo-resist coating techniques of MEMS. The aluminum thin films were deposited on silicon substrate by evaporator, and then sacrificial layer be removed, the suspended aluminum thin films were derived. The suspended aluminum thin film was proceeded the high temperature metal oxidizing. We can get the aluminum oxide thin films and analysis the selective effect of suspended films properties.

Bubbles are ubiquitous influencing a multitude of biological processes in natural and industrial environments; this influence is especially relevant during and after bubble rupture. Indeed, the influence of a bubble can extend well beyond its lifetime via the droplets produced when it ruptures. These droplets are known to effectively transport nearby particulates including bacteria and viruses into the surroundings, which in addition to affecting human health can influence global climate by acting as cloud condensation nuclei. Further, the bubble's rupture is a violent event that has been linked to decreased cell viability in bioreactors. However, in all these applications many of the studies have taken an empirical approach, making the results difficult to generalize. Here we combine theory and experiment to investigate the static and dynamic interactions between bubbles and the surrounding microorganisms at a free interface. Our first study focuses on the equilibrium shape a bubble forms after reaching the surface of a liquid. Existing literature is limited to a bubble resting on a flat interface; for example, the surface of a pool or calm lake. However, there are instances where this assumption no longer applies -- a bubble bursting on a raindrop, for example. By relaxing this assumption, we show how a curved boundary alters the final shape of the bubble. Our next study focuses on the enrichment of particulates in the cap of a bursting bubble. As a bubble rises to a free surface, particulates in the bulk liquid are frequently transported to the surface by attaching to the bubble's interface. When the bubble ruptures, a fraction of these particulates are often ejected into the surroundings in film droplets with particulate concentrations higher than the liquid from which originate. However, the precise mechanisms responsible for this enrichment are unclear. By simultaneously recording the drainage and rupture events with high-speed and standard photography, we directly measure the concentrations in a thin bubble film. Based on our results, we develop a physical model and provide evidence that the enrichment is due to a combination of scavenging and film drainage. Our next study focuses on the conditions necessary for a jet droplet to be produced. Past research shows that droplet production is halted when either gravitational or viscous effects are significant. Through systematic experimentation we uncover an intermediate region where both effects are significant, leading to an early end of droplet production. By numerically decoupling the gravitational effects into before and after rupture, we find that the equilibrium shape is responsible for the existence of this intermediate region. Our last study focuses on quantifying the localized stresses produced during spontaneous bubble bursting. Directly simulating each bubble and its effect on the suspended cells in a bioreactor is currently infeasible. Here we illustrate how the results of past works, which disagree by several orders of magnitude for similarly sized bubbles, are primarily a result of the chosen numerical mesh, not the underlying physics. By implementing a particle tracking method, we eliminate this mesh dependence and quantify the extent or volume effected by a single bubble bursting event. Based on our results, we develop a generalizable framework that could be integrated into existing models as a parameterization, removing the need to simulate both phases.
2019-07-09T00:00:00Z

MEMS microphones have been a research topic for the last two and half decades. The state-of-the-art comprises surface mount MEMS microphones in laptops, mobile phones and tablets, etc. The popularity and the commercial success of MEMS microphones is largely due to the steep cost reduction in manufacturing afforded by the mass scale production with microfabrication technology. The current MEMS microphones are de-signed along the lines of traditional microphones that use capacitive transduction with or without permanent charge (electret type microphones use permanent charge of their sensor element). These microphones offer high sensitivity, stability and reasonably at frequency response while reducing the overall size and energy consumption by exploiting MEMS technology. Conceptually, microphones are simple transducers that use a membrane or diaphragm as a mechanical structure which deflects elastically in response to the incident acoustic pressure. This dynamic deflection is converted into an electrical signal using an appropriate transduction technique. The most popular transduction technique used for this application is capacitive, where an elastic diaphragm forms one of the two parallel plates of a capacitor, the fixed substrate or the base plate being the other one. Thus, there are basically two main elements in a microphone { the elastic membrane as a mechanical element, and the transduction technique as the electrical element. In this thesis, we propose and study novel design for both these elements. In the mechanical element, we propose a simple topological change by introducing slits in the membrane along its periphery to enhance the mechanical sensitivity. This simple change, however, has significant impact on the microphone design, performance and its eventual cost. Introduction of slits in the membrane makes the geometry of the structural element non-trivial for response analysis. We devote considerable effort in devising appropriate modeling techniques for deriving lumped parameters that are then used for simulating the system response. For transduction, we propose and study an FET (Field Effect Transistor) coupled micro-phone design where the elastic diaphragm is used as the moving (suspended) gate of an FET and the gate deflection modulated drain current is used in the subthreshold regime of operation as the output signal of the microphone. This design is explored in detail with respect to various design parameters in order to enhance the electrical sensitivity. Both proposed changes in the microphone design are motivated by the possibilities that the microfabrication technology offers. In fact, the design proposed here requires further developments in MEMS technology for reliably creating gaps of 50-100 nm between the substrate and a large 2D structure of the order of a few hundred microns in diameter. In the First part of the thesis, we present detailed simulations of acoustic and squeeze lm domain to understand the effect slits could bring upon the behaviour of the device as a microphone. Since the geometry is nontrivial, we resort to Finite element simulations using commercial packages such as COMSOL Multiphysics and ANSYS in the structural, acoustic and Fluid-structure domains to analyze the behaviour of a microphone which has top plate with nontrivial geometry. On the simulated Finite element data, we conduct low and high frequency limit analysis to extract expressions for the lumped parameters. This technique is well known in acoustics. We borrow this technique of curve Fitting from the acoustics domain and apply it in modified form into the squeeze lm domain. The dynamic behaviour of the entire device is then simulated using the extracted parameters. This helps to simulate the microphone behaviour either as a receiver or as a transmitter. The designed device is fabricated using MEMSCAP PolyMUMPS process (a foundry Polysilicon surface micromachining process). We conduct vibrometer (electrostatic ex-citation) and acoustic characterization. We also study the feasibility of a microphone with slits and the issues involved. The effect of the two dissipation modes (acoustic and squeeze lm ) are quantified with the experimentally determined quality factor. The experimentally measured values are: Resonance is 488 kHz (experimentally determined), low frequency roll-off is 796 Hz (theoretical value) and is 780 Hz as obtained by electrical characterization. The first part of this thesis focusses on developing a comprehensive understanding of the effect of slits on the performance of a MEMS microphone. The presence of slits near the circumference of the clamped plate cause reduction in its rigidity. This leads to an increase in the sensitivity of the device. Slits also cause pressure equalization between the top and bottom of the diaphragm if the incoming sound is at relatively low frequencies. At this frequency, also known as the lower cutoff frequency, the microphone's response starts dropping. The presence of slits also changes the radiation impedance of the plate as well as the squeeze lm damping below the plate. The useful bandwidth of the microphone changes as a consequence. The cavity formed between the top plate and the bottom fixed substrate increases the stiffness of the device significantly due to compression of the trapped air. This effect is more pronounced here because unlike the existing capacitive MEMS microphones, there is no backchamber in the device fabricated here. In the second part of the thesis, we present a novel subthreshold biased FET based MEMS microphone. This biasing of the transistor in the subthreshold region (also called as the OFF-region) offers higher sensitivity as compared to the above threshold region (also called as the ON-region) biasing. This is due to the exponentially varying current with change in the bias voltage in the OFF-region as compared to the quadratic variation in the ON-region. Detailed simulations are done to predict the behaviour of the device. A lumped parameter model of the mechanical domain is coupled with the drain current equations to predict the device behaviour in response to the deflection of the moving gate. From the simulations, we predict that the proposed biasing offers a device sensitive to even sub-nanometer deflection of the flexible gate. As a proof of concept, we fabricate fixed-fixed beams which utilize CMOS-MEMS fabrication. The process involves six lithography steps which involve two CMOS and the remaining MEMS fabrication. The fabricated beams are mechanically characterized for resonance. Further, we carry out electrical characterization for I-V (current-voltage) characteristics. The second part of the thesis focusses on a novel biasing method which circumvents the need of signal conditioning circuitry needed in a capacitive based transduction due to inbuilt amplification. Extensive simulations with equivalent circuit has been carried out to determine the increased sensitivity and the role of various design variables.

MEMS microphones have been a research topic for the last two and half decades. The state-of-the-art comprises surface mount MEMS microphones in laptops, mobile phones and tablets, etc. The popularity and the commercial success of MEMS microphones is largely due to the steep cost reduction in manufacturing afforded by the mass scale production with microfabrication technology. The current MEMS microphones are de-signed along the lines of traditional microphones that use capacitive transduction with or without permanent charge (electret type microphones use permanent charge of their sensor element). These microphones offer high sensitivity, stability and reasonably at frequency response while reducing the overall size and energy consumption by exploiting MEMS technology. Conceptually, microphones are simple transducers that use a membrane or diaphragm as a mechanical structure which deflects elastically in response to the incident acoustic pressure. This dynamic deflection is converted into an electrical signal using an appropriate transduction technique. The most popular transduction technique used for this application is capacitive, where an elastic diaphragm forms one of the two parallel plates of a capacitor, the fixed substrate or the base plate being the other one. Thus, there are basically two main elements in a microphone { the elastic membrane as a mechanical element, and the transduction technique as the electrical element. In this thesis, we propose and study novel design for both these elements. In the mechanical element, we propose a simple topological change by introducing slits in the membrane along its periphery to enhance the mechanical sensitivity. This simple change, however, has significant impact on the microphone design, performance and its eventual cost. Introduction of slits in the membrane makes the geometry of the structural element non-trivial for response analysis. We devote considerable effort in devising appropriate modeling techniques for deriving lumped parameters that are then used for simulating the system response. For transduction, we propose and study an FET (Field Effect Transistor) coupled micro-phone design where the elastic diaphragm is used as the moving (suspended) gate of an FET and the gate deflection modulated drain current is used in the subthreshold regime of operation as the output signal of the microphone. This design is explored in detail with respect to various design parameters in order to enhance the electrical sensitivity. Both proposed changes in the microphone design are motivated by the possibilities that the microfabrication technology offers. In fact, the design proposed here requires further developments in MEMS technology for reliably creating gaps of 50-100 nm between the substrate and a large 2D structure of the order of a few hundred microns in diameter. In the First part of the thesis, we present detailed simulations of acoustic and squeeze lm domain to understand the effect slits could bring upon the behaviour of the device as a microphone. Since the geometry is nontrivial, we resort to Finite element simulations using commercial packages such as COMSOL Multiphysics and ANSYS in the structural, acoustic and Fluid-structure domains to analyze the behaviour of a microphone which has top plate with nontrivial geometry. On the simulated Finite element data, we conduct low and high frequency limit analysis to extract expressions for the lumped parameters. This technique is well known in acoustics. We borrow this technique of curve Fitting from the acoustics domain and apply it in modified form into the squeeze lm domain. The dynamic behaviour of the entire device is then simulated using the extracted parameters. This helps to simulate the microphone behaviour either as a receiver or as a transmitter. The designed device is fabricated using MEMSCAP PolyMUMPS process (a foundry Polysilicon surface micromachining process). We conduct vibrometer (electrostatic ex-citation) and acoustic characterization. We also study the feasibility of a microphone with slits and the issues involved. The effect of the two dissipation modes (acoustic and squeeze lm ) are quantified with the experimentally determined quality factor. The experimentally measured values are: Resonance is 488 kHz (experimentally determined), low frequency roll-off is 796 Hz (theoretical value) and is 780 Hz as obtained by electrical characterization. The first part of this thesis focusses on developing a comprehensive understanding of the effect of slits on the performance of a MEMS microphone. The presence of slits near the circumference of the clamped plate cause reduction in its rigidity. This leads to an increase in the sensitivity of the device. Slits also cause pressure equalization between the top and bottom of the diaphragm if the incoming sound is at relatively low frequencies. At this frequency, also known as the lower cutoff frequency, the microphone's response starts dropping. The presence of slits also changes the radiation impedance of the plate as well as the squeeze lm damping below the plate. The useful bandwidth of the microphone changes as a consequence. The cavity formed between the top plate and the bottom fixed substrate increases the stiffness of the device significantly due to compression of the trapped air. This effect is more pronounced here because unlike the existing capacitive MEMS microphones, there is no backchamber in the device fabricated here. In the second part of the thesis, we present a novel subthreshold biased FET based MEMS microphone. This biasing of the transistor in the subthreshold region (also called as the OFF-region) offers higher sensitivity as compared to the above threshold region (also called as the ON-region) biasing. This is due to the exponentially varying current with change in the bias voltage in the OFF-region as compared to the quadratic variation in the ON-region. Detailed simulations are done to predict the behaviour of the device. A lumped parameter model of the mechanical domain is coupled with the drain current equations to predict the device behaviour in response to the deflection of the moving gate. From the simulations, we predict that the proposed biasing offers a device sensitive to even sub-nanometer deflection of the flexible gate. As a proof of concept, we fabricate fixed-fixed beams which utilize CMOS-MEMS fabrication. The process involves six lithography steps which involve two CMOS and the remaining MEMS fabrication. The fabricated beams are mechanically characterized for resonance. Further, we carry out electrical characterization for I-V (current-voltage) characteristics. The second part of the thesis focusses on a novel biasing method which circumvents the need of signal conditioning circuitry needed in a capacitive based transduction due to inbuilt amplification. Extensive simulations with equivalent circuit has been carried out to determine the increased sensitivity and the role of various design variables.