Dissertations / Theses on the topic 'Air-water interface dynamics'

The behaviour of polymeric amphiphiles, specifically systems containing poly( methyl methacrylate), PMMA, and poly(ethylene oxide), PEO, spread at the air water interface is reported. An amphiphilic graft copolymer containing a PMMA backbone and PEO side chains forms the main subject of the study. Films of PMMA spread onto a 0.1 wt.% solution of PEO have also been considered. Due to the presence of a surface excess, both PEO and PMMA populate the surface and the latter system provides a model of the graft copolymer. Monolayer behaviour has been characterised by recording surface pressure isotherms. It has been demonstrated that the behaviour of the graft copolymers is highly composition dependent, exhibiting characteristics associated with the constituent homopolymers. In contrast the surface pressure isotherm for PMMA on PEO solution is quite unlike that for PMMA on water. Details of the structural organisation in these systems have been determined using neutron reflectometry. The surface organisation adopted by the graft copolymers varies as the surface concentration increases. The PEO side chains penetrate the subphase and stretch increasingly whereas the PMMA backbone behaves much like the homopolymer. This is in contrast to the behaviour of PMMA on PEO solution where the PMMA occupies a wider region of the interface. The structure of the adsorbed PEO differs from that of the bare solution but maintains a constant surface excess. Dynamic behaviour has been studied using surface quasi elastic light scattering. For the graft copolymers surface wave evolution is very unusual. The dilational parameters provide evidence, e.g. a negative dilational viscosity, indicating that surface wave propagation is complex and not described fully by current theories. Similar behaviour is exhibited by PMMA on a PEO solution. These features are believed to be associated with subphase penetration and are discussed in this context.

Polyethylene(oxide) (PEO) is an intriguing polymer, it is water-soluble but exhibits surface-active properties. When capped hydrophobically at one end by a fluorocarbon group, a novel polymeric surfactant is generated. This has been synthesised by combining anionic polymerisation and an end-capping reaction using isophorone diisocyanate. Three molecular weight polymers were generated, for each of which a hydrogenous and a deuterated version were required, hi aqueous solution, these polymeric surfactants formed surface excess layers at the air-water interface and their surface organisation and dynamic behaviour has been investigated. Surface tension data was obtained using a digital tensiometer and the surface tension isotherm is dependent on both solution concentration and polymer molecular weight. Using neutron reflectometry, the organisation of such adsorbed polymer films at the air-water interface has been obtained over a range of solution concentrations for each polymer molecular weight. Neutron reflectometry data was analysed by both optical matrix and kinematic approximation methods. Both analyses yield the same description, i.e. a two-layer organisation is observed at the air-water interface for all three molecular weight polymers. The PEO layer thickness and the surface organisation are found to be dependent on both solution concentration and molecular weight. The PEO chains are totally immersed in the subphase and strongly anchored at the surface via the fluorocarbon end group. The dynamic behaviour of each PEO adsorbed surface excess at the air-water interface has been studied using surface quasi-elastic light scattering. A resonance between the capillary and dilational waves is observed and the maximum in damping is independent on surface organisation but is molecular weight and solution concentration dependent. The viscoelastic behaviour of the dilational modulus can be described using a simple Maxwell fluid model, from which a relaxation time has been obtained assuming a single relaxational process.

Malgré un rôle important en physique, biologie et dans les processus industriels tels l’agroalimentaire et la dépollution de l’eau, la dynamique d'une particule colloïdale à proximité d'une interface fluide et ses interactions avec l’interface sont des phénomènes physiques encore débattus.Dans cette thèse, nous explorons la dynamique et l'interaction de particules colloïdales individuelles à proximité d'une interface air-eau à l’équilibre thermique.Afin de mener cette étude sans perturber le système expérimental, nous avons conçu et construit un microscope interférentiel à double onde adapté à l'interface air-eau. Contrairement à d'autres techniques expérimentales, notre configuration permet la mesure précise de la distance absolue entre particule l'interface sans nécessiter d’étalonnage ou d’hypothèse sur l'emplacement de l'interface. Nous avons ainsi pu obtenir des trajectoires hautement résolues de particules en 3D proches de l'interface, permettant la mesure précise des diffusions proche de l’interface et des interactions particules-interface.Le système montre deux profils d’énergie potentielle différents. Deux distances d’équilibre particule-interface sont ainsi observées. La plus grande peut être expliquée par la compétition des interactions de Van der Waals et électrostatiques avec la pesanteur. La distance d’équilibre plus courte ne peut s’expliquer que par la présence d’une interaction attractive supplémentaire. Les origines possibles de cette interaction sont discutées.En utilisant une nouvelle méthode d'analyse des déplacements quadratiques moyens des particules dans un potentiel générique, nous avons pu accéder aux coefficients de friction visqueuse des particules en fonction de la distance à l'interface. De manière singulière, l’interface air-eau se comporte comme une paroi liquide pour le mouvement des particules parallèlement à l’interface et comme une paroi solide pour le mouvement des particules perpendiculaire à l’interface. Ce résultat expérimental peut être partiellement rationalisé en considérant des modèles récents basés sur l’incompressibilité de surface. Cependant, certaines différences entre les expériences et les théories demeurent. Les coefficients de friction visqueuse sont plus importants que les prédictions hydrodynamiques et dépendent de la charge électrique des particules, ce qui suggère un possible rôle des phénomènes électrocinétiques.Enfin, le piégeage des particules à l'interface air-eau et leur angle de contact ont été mesurés tout en modifiant la force ionique de la solution aqueuse et en faisant varier l‘état de surface des colloïdes
Despite the relevance to environmental, biological and industrial processes, the motion of a colloidal particle close to a fluid interface and the way it interacts with the water surface are still largely elusive and intriguing physical phenomena.In this thesis, we explore the motion dynamics and the interaction of individual colloidal particles close to an air-water interface in thermal equilibrium.In order to investigate them without perturbing or altering the experimental system, we designed and built a dual-wave reflection interference microscope working with an air-water interface geometry. Contrary to other established experimental techniques, our set-up allows accurate measurements of the absolute particle-interface distance and thus does not require any calibration or assumption to know the location of the interface. Highly resolved 3D particle trajectories close to the interface were obtained, from which information on particle diffusion close to the interface and particle-interface interactions are obtained.The system shows two different potential energy landscapes resulting in two different equilibrium particle-interface distances. The larger one can be fairly explained by Van der Waals and electrostatic interactions combined with gravity. The shorter one highlights the existence of an unexpected additional attractive interaction. The possible origins of such an interaction are discussed.Using a method of analysis of the particle mean square displacements in a generic potential we developed, we were able to access to particle drag coefficients as a function of the distance from the interface. Peculiarly, the air-water interface acts as a slip boundary for the particle motion parallel to the interface and as a no-slip boundary for the particle motion perpendicular to the interface. This experimental result can be partially rationalized considering recent models based on surface incompressibility. However, some discrepancies between experiments and theories remain. Experimental drag coefficients are larger than the hydrodynamic predictions and depend on the particle electrical charge, pointing therefore to a possible role of electrokinetic phenomena.Finally, the particle trapping at the air-water interface and its contact angle were observed while tuning the ionic strength of the aqueous solution and varying the surface state of the colloids

This thesis describes the techniques of anionic polymerisation and characterisation used in the synthesis of poly(methyl methacrylate)/poly(ethylene oxide) diblock copolymers, the various surface techniques used to examine the interfacial properties of these copolymers spread on water, and the dynamics of these copolymers in solution. The surface techniques used were surface pressure-concentration isotherm studies, neutron reflectivity, surface quasi-elastic light scattering, and ellipsometry. The thermodynamics of micellization and dynamic properties of the copolymer solutions were investigated using light scattering. The diblock copolymers had a target composition of 50:50 mole ratio and M(_W) = 50000. In addition, several copolymers had one or both blocks holly deuterated which was necessary for the neutron reflectivity studies where contrast variation was required to apply the kinematic approximation. Surface pressure isotherms give thermodynamic information about the behaviour of polymer segments at the interface. It has been possible to interpret this behaviour by using neutron reflectivity to obtain information concerning die thickness and distribution of the PMMA and PEO blocks, and water at the interface. The trends in layer thickness have been supported by the ellipsometric measurements and interpretation of the viscoelastic SQELS data has allowed conclusions about the hydrodynamics of the polymer chains at various surface concentrations.

Novel amphiphilic graft copolymers with a backbone of poly(norbornene) (PNB) with poly(ethylene oxide) (PEO) grafts have been synthesised by a combination of ring opening metathesis and anionic polymerisation methods. The polymer has been prepared with hydrogenous and deuterated grafts and with grafts of different degrees of polymerisation. These graft copolymers spread at the air-water and air-PEO solution interface forming thin films and their organisation and dynamic behaviour is discussed. Monolayer behaviour was characterised from surface pressure isotherms and it was demonstrated that the shape of the isotherm is dependent on graft length and on the concentration of PEO in the subphase. Using neutron reflectometry the organisation of such spread films at the air-water interface have been obtained over a range of surface concentrations for each length of PEO graft. Data were analysed by both exact calculation methods and the partial kinematic approximation and the models adopted were verified by applying the model independent Bayesian analysis. All yield the same description i.e. the hydrophobic backbone remains at the uppermost surface while the PEO grafts penetrate the subphase. The PEO layer increases in thickness with increased surface concentration and graft length. In each case the rate of increase with surface concentration was initially rapid but above a critical concentration, a slower rate was observed. In this latter regime the variation of the tethered layer height scales with surface density (ơ) and degree of polymerisation of the graft (N) as, r(_s) = N(^1.06)ơ(^0.33),which agrees well with scaling and self consistent field theory of polymer brushes. The dynamic behaviour of each copolymer film spread on water has been studied using surface quasi-elastic light scattering. A resonance between the capillary and dilational waves is observed at identical surface concentrations for each copolymer film. The viscoelastic behaviour of the dilational mode is reminiscent of Kramers-Kronig relations. The phenomenon of resistive mode mixing was observed in frequency dependency studies.

Cette thèse présente les deux sujets liés à la dynamique d'interfaces: la rhéologie des particules monocouches et de la croissance dendritique des alliages. Dans la première partie, chapitre1-6, les propriétés des monocouches de nanoparticules de silice à l’interface air-eau sont présentées et reliées à la stabilité des mousses faites avec les dispersions aqueuses de ces particules. Les propriétés des couches: évolution de texture, pression de surface, épaisseur, angle de contact des particules et concentration de surface sont caractérisées pour différentes hydrophobicités des particules. La viscoélasticité de compression des couches a été déterminée par trois méthodes basées sur l’utilisation de deux plaques de Wilhelmy dans des cuves Langmuir. Les différences remarquables entre les couches comprimées et les couches déposées ont été trouvés. Les modules présentent un maximum à l'hydrophobicité intermédiaire des particules et dépendent de la vitesse de déformation, de la quantité de particule initiale, de la longueur de la cuve et de l'âge de la couche. La viscoélasticité de cisaillement des couches a été également déterminée. Une analogie fréquence-gradient de vitesse a été trouvée comme chez d’autres solides mous tridimensionnels. La relaxation structurale s’effectue avec un temps de relaxation qui varie comme l’inverse du gradient de vitesse. Un comportement d'auto-guérison des fractures a été observé et lié à la relaxation structurale. Dans la deuxième partie, Chapitre 7, la croissance dendritique rapide d’alliages surfondus ternaires Ni-Co-Cu et quaternaires Ni-Co-Cu-Ge a été étudiée. La surfusion élevée est obtenue par lévitation électromagnétique et des méthodes de flux de verre. Les vitesses de croissance dendritiques ont été mesurées en fonction de la surfusion. Nous proposons une double fonction exponentielle pour décrire la relation entre la vitesse de croissance et la surfusion dans les alliages monophasiques. Un nouveau comportement a été observé : la vitesse de croissance des dendrites est abaissée en cas de séparation de phase et des mécanismes possibles sont proposés
This dissertation presents two topics related to the dynamics of interfaces: rheology of particle monolayers and the dendritic growth of alloys. In the first part, chapter1-6, the properties of silica nanoparticle monolayers at the air-water surface is presented and related to foam stability. The properties of the layers: textural evolution, surface pressure, thickness, particle contact angle with interface and effective surface concentration are characterized with respect to different particle hydrophobicities. The viscoelasticity of the layers are determined by three methods based on two Wilhelmy plates in the Langmuir trough. Remarkable differences between compressed layers and deposited layers have been found. The moduli present the maximum at intermediate particle hydrophobicity and depend on strain rate, initial particle quantity, trough length and age of the layer. The same universal linear and nonlinear behaviour as three-dimensional soft materials is found by a shear rheological study. The structural relaxation has been observed and the corresponding relaxation time has been characterized by SRFS method. A self-healing behavior is observed and a microscopic mechanism is proposed to account for the slow self-healing. The results suggests that the same physical process may involved in self heal as in structural relaxation. In the second part, chapter7, rapid dendritic growth in undercooled liquid ternary Ni-Co-Cu and quarternary Ni-Co-Cu-Ge alloys has been investigated. The high undercooling is obtained by electromagnetic levitation and glass flux methods. The dendritic growth velocities are measured as a function of undercooling. We propose a double exponential function to describe the relationship between growth velocity and undercooling in single phase alloys. A novel behavior that the dendritic growth velocity is reduced by liquid phase separation is found and the possible mechanism is proposed

Adsorption layers of soluble surfactants enable and govern a variety of phenomena in surface and colloidal sciences, such as foams. The ability of a surfactant solution to form wet foam lamellae is governed by the surface dilatational rheology. Only systems having a non-vanishing imaginary part in their surface dilatational modulus, E, are able to form wet foams. The aim of this thesis is to illuminate the dissipative processes that give rise to the imaginary part of the modulus.

There are two controversial models discussed in the literature. The reorientation model assumes that the surfactants adsorb in two distinct states, differing in their orientation. This model is able to describe the frequency dependence of the modulus E. However, it assumes reorientation dynamics in the millisecond time regime. In order to assess this model, we designed a SHG pump-probe experiment that addresses the orientation dynamics. Results obtained reveal that the orientation dynamics occur in the picosecond time regime, being in strong contradiction with the two states model.

The second model regards the interface as an interphase. The adsorption layer consists of a topmost monolayer and an adjacent sublayer. The dissipative process is due to the molecular exchange between both layers. The assessment of this model required the design of an experiment that discriminates between the surface compositional term and the sublayer contribution. Such an experiment has been successfully designed and results on elastic and viscoelastic surfactant provided evidence for the correctness of the model.

Because of its inherent surface specificity, surface SHG is a powerful analytical tool that can be used to gain information on molecular dynamics and reorganization of soluble surfactants. They are central elements of both experiments. However, they impose several structural elements of the model system. During the course of this thesis, a proper model system has been identified and characterized. The combination of several linear and nonlinear optical techniques, allowed for a detailed picture of the interfacial architecture of these surfactants.
Amphiphile vereinen zwei gegensätzliche Strukturelemente in einem Molekül, eine hydrophile Kopfgruppe und ein hydrophobe, meist aliphatische Kette. Aufgrund der molekularen Asymmetrie erfolgt eine spontane Adsorption an der Wasser-Luft Grenzfläche. Die Adsorptionsschicht verändert die makroskopischen Eigenschaften des Materials, z.B. die Grenzflächenspannung wird erniedrigt. Amphiphile sind zentrale Bauelemente der Kolloid- und Grenzflächenforschung, die Phänomene, wie Schäume ermöglichen.

Eine Schaumlamelle besteht aus einem dünnen Wasserfilm, der durch zwei Adsorptionsschichten stabilisiert wird. Die Stabilität der Lamelle wird durch die Grenzflächenrheologie entscheidend geprägt. Die wesentliche makroskopische Größe in diesem Zusammenhang ist das so genannte Grenzflächendilatationsmodul E. Es beschreibt die Fähigkeit des Systems die Gleichgewichtsgrenzflächenspannung nach einer Expansion oder Dilatation der Adsorptionschicht wieder herzustellen. Das Modul E ist eine komplexe Größe, in dem der Imaginärteil direkt mit der Schaumstabilität korreliert.

Diese Arbeit widmet sich der Grenzflächenrheologie. In der Literatur werden zwei kontroverse Modelle zur Interpretation dieser Größe diskutiert. Diese Modelle werden experimentell in dieser Arbeit überprüft. Dies erfordert die Entwicklung neuer experimenteller Aufbauten basierend auf nichtlinearen, optischen Techniken. Mit diesen Experimenten konnte eines der Modelle bestätigt werden.

Throughout the study of polymeric Langmuir monolayers at the air/water (A/W) interface, the Wilhelmy plate and Langmuir-Blodgett (LB) techniques along with Brewster angle microscopy (BAM) have been identified as key methods for acquiring structural, thermodynamic, rheological and morphological information. These techniques along with surface light scattering (SLS), a method for probing a monolayerâ s dynamic dilational rheological properties, will be used to characterize homopolymers, poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG), and a new class of novel polymeric surfactants, telechelic (POSS-PEG-POSS) and hemi-telechelic (POSS-PEG) polyhedral oligomeric silsesquioxane (POSS) derivatives of PEG. PEO with number average molar mass, Mn > ~ 18 kgâ ¢mol-1 form stable spread Langmuir films at the A/W interface, while oligomeric PEG have ï -A isotherms that deviate from high molar mass PEO. Nonetheless, SLS reveals that the dynamic dilational viscoelastic properties of any Mn PEG(PEO) only depend on ï and not Mn. Likewise, POSS-PEG-POSS telechelics exhibit molar mass dependent ï -A isotherms, where low ï regimes (ï < 1 mNâ ¢m-1) have PEG-like behavior, but high ï regimes were dominated by POSS-POSS interactions. SLS studies reveal that the dynamic dilational moduli of POSS-PEG-POSS are greater than either PEO or an analogous POSS compound, trisilanolcyclohexyl-POSS. The ability to control rheological properties and the hydrophilic-lipophilic balance even allows one POSS-PEG-POSS (PEG Mn = 1 kgâ ¢mol-1) to form Y-type LB-multilayer films. For POSS-PEG systems, comparisons at comparable POSS:PEG ratios reveal short PEG chains (PEG Mn ~ 0.5 kgâ ¢mol-1) yield similar viscoelastic properties as POSS-PEG-POSS (PEG Mn ~ 1 kgâ ¢mol-1), while longer PEG chains (PEG Mn ~ 2 kgâ ¢mol-1) yield lower modulus films than comparable POSS-PEG-POSS. These differences are attributed to brush-like PEG conformations in short POSS-PEG versus mushroom-like PEG conformations in long POSS-PEG at the A/W interface. These results provide insight for designing PEG-based amphiphilic nanoparticles with controlled interfacial rheology.
Ph. D.

Améliorer notre connaissance de la structure de l'eau dans l'environnement spécial offert par une interface est essentiel pour la compréhension de nombreux phénomènes naturels et applications technologiques. Pour révéler cette structure interfaciale de l'eau, des techniques capables de fournir des informations microscopiques, de manière sélective, pour cette couche interfaciale (BIL) sont nécessaires. Dans le présent travail de thèse, nous avons donc étudié les interfaces aqueuses au niveau moléculaire, en couplant la modélisation théorique à partir de simulations DFT-MD avec les spectroscopies SFG et THz-IR. En développant de nouveaux protocoles/outils d'investigation associant simulations DFT-MD et spectroscopie SFG, en particulier pour la rationalisation plus complexe des interfaces chargées, nous avons fourni une compréhension globale de l'effet des conditions interfaciales d'hydrophilicité, de pH, de force ionique sur le réseau des liaisons-H formé dans la couche interfaciale BIL, sur ses signatures spectroscopiques et sur son impact sur les propriétés physico-chimiques. Nous avons montré pour la première fois que, dans des conditions suffisamment hydrophobes, l'eau interfaciale crée des réseaux des liaisons-H bidimensionnels, révélé expérimentalement par les spectres THz-IR. Le réseau-2D dicte la dynamique de l'eau interfaciale, le potentiel de surface, l'acidité de surface, la tension superficielle et la thermodynamique d'hydratation des solutés hydrophobes. Cet "ordre horizontal" aux interfaces hydrophobes est opposé à "l'ordre verticale" obtenu aux interfaces hydrophiles. Nous avons aussi révélé comment les ions et les conditions de pH modifient ces arrangements structuraux
Improving our knowledge on water H-Bonded networks formed in the special environment offered by an interface is pivotal for our understanding of many natural phenomena and technological applications. To reveal the interfacial water arrangement, techniques able to provide detailed microscopic information selectively for the interfacial layer are required. In the present thesis work, we have hence investigated aqueous interfaces at the molecular level, by coupling theoretical modeling from DFT-MD simulations with SFG & THz-IR spectroscopies. By developing new investigation protocols/tools, coupling DFT-MD simulations and SFG spectroscopy, in particular for the more complex rationalization of charged interfaces, we have provided a global comprehension of the effect of various interfacial conditions (hydrophilicity, pH, ionic strength) on the HB-Network formed in the interfacial layer (BIL), on its spectroscopic signatures and on its impact on physico-chemical properties. We have shown for the first time that, in sufficiently hydrophobic conditions, BIL interfacial water creates special 2-Dimensional HB-Networks, experimentally revealed by one specific THz-IR marker band. Such 2D-network dictates HBs and orientational dynamics of interfacial water, surface potential, surface acidity, water surface tension and thermodynamics of hydration of hydrophobic solutes. Such "horizontal ordering” of water at hydrophobic interfaces is found opposite to the “vertical ordering” of water at hydrophilic interfaces, while coexistence of the two orders leads to disordered interfacial water in intermediate hydrophilic/hydrophobic conditions. Both DFT-MD and SFG further revealed how ions & pH conditions alter these BIL-water orders

Amphiphilic molecules contain a hydrophilic headgroup and a hydrophobic tail. The headgroup is polar or ionic and likes water, the tail is typically an aliphatic chain that cannot be accommodated in a polar environment. The prevailing molecular asymmetry leads to a spontaneous adsorption of amphiphiles at the air/water or oil/water interfaces. As a result, the surface tension and the surface rheology is changed. Amphiphiles are important tools to deliberately modify the interfacial properties of liquid interfaces and enable new phenomena such as foams which cannot be formed in a pure liquid.

In this thesis we investigate the static and dynamic properties of adsorption layers of soluble amphiphiles at the air/water interface, the so called Gibbs monolayers. The classical way for an investigation of these systems is based on a thermodynamic analysis of the equilibrium surface tension as a function of the bulk composition in the framework of Gibbs theory. However, thermodynamics does not provide any structural information and several recent publications challenge even fundamental text book concepts.

The experimental investigation faces difficulties imposed by the low surface coverage and the presence of dissolved amphiphiles in the adjacent bulk phase. In this thesis we used a suite of techniques with the sensitivity to detect less than a monolayer of molecules at the air-water interface. Some of these techniques are extremely complex such as infrared visible sum frequency generation (IR-VIS SFG) spectroscopy or second harmonic generation (SHG). Others are traditional techniques, such as ellipsometry employed in new ways and pushed to new limits. Each technique probes selectively different parts of the interface and the combination provides a profound picture of the interfacial architecture.

The first part of the thesis is dedicated to the distribution of ions at interfaces. Adsorption layers of ionic amphiphiles serve as model systems allowing to produce a defined surface charge. The charge of the monolayer is compensated by the counterions. As a result of a complex zoo of interactions there will be a defined distribution of ions at the interface, however, its experimental determination is a big scientific challenge. We could demonstrate that a combination of linear and nonlinear techniques gives direct insights in the prevailing ion distribution. Our investigations reveal specific ion effects which cannot be described by classical Poisson-Boltzmann mean field type theories.

Adsorption layer and bulk phase are in thermodynamic equilibrium, however, it is important to stress that there is a constant molecular exchange between adsorbed and dissolved species. This exchange process is a key element for the understanding of some of the thermodynamic properties. An excellent way to study Gibbs monolayers is to follow the relaxation from a non-equilibrium to an equilibrium state. Upon compression amphiphiles must leave the adsorption layer and dissolve in the adjacent bulk phase. Upon expansion amphiphiles must adsorb at the interface to restore the equilibrium coverage. Obviously the frequency of the expansion and compression cycles must match the molecular exchange processes. At too low frequencies the equilibrium is maintained at all times. If the frequency is too fast the system behaves as a monolayer of insoluble surfactants. In this thesis we describe an unique variant of an oscillating bubble technique that measures precisely the real and imaginary part of the complex dilational modulus E in a frequency range up to 500 Hz. The extension of about two decades in the time domain in comparison to the conventional method of an oscillating drop is a tremendous achievement. The imaginary part of the complex dilational modulus E is a consequence of a dissipative process which is interpreted as an intrinsic surface dilational viscosity. The IR-VIS SFG spectra of the interfacial water provide a molecular interpretation of the underlying dissipative process.

Amphiphile Moleküle vereinen zwei gegensätzliche Strukturelemente. Sie bestehen aus einer polaren oder ionischen Kopfgruppe und einem unpolaren Molekülteil, häufig einer Kohlenwasserstoffkette. Die vorliegende molekulare Asymmetrie bewirkt eine spontane Adsorption der Amphiphile an der Wasser/Luft Grenzschicht. Als Folge verändern sich Oberflächenspannung und Grenzflächenrheologie. Amphiphile Moleküle werden benutzt, um die Eigenschaften flüssiger Grenzflächen zu verändern und begegnen uns z.B. in Form von Seifen oder anderen waschaktiven Substanzen im täglichen Leben.

Der erste Teil dieser Doktorarbeit widmet sich der Verteilung von Ionen an geladenen flüssigen Grenzflächen. Adsorbtionsschichten ionischer Amphiphile bieten Modellsysteme zur Untersuchung dieses klassischen Bereiches der Kolloid- und Grenzflächenforschung. Durch die Adsorption der Amphiphile in der Grenzschicht werden definierte Oberflächenladungen erzeugt, welche durch die angrenzenden Gegenionen in der Sublage kompensiert werden.

In dieser Arbeit wird gezeigt, dass eine Kombination aus linearen und komplexen nichtlinearen optischen Methoden, die experimentelle Bestimmung der Verteilung der Gegenionen an geladenen Grenzflächen ermöglicht. Unsere Messungen zeigen ionenspezifische Effekte, die sich nicht in Reihenfolge des Periodensystems ordnen lassen. Insbesondere wurde ein Phasenübergang in der Verteilung der Gegenionen von einem Zustand, in dem sich die Ionen in der Sublage befinden, zu einem Zustand bestehend aus direkt kondensierten Ionen beobachtet. Dieser Phasenübergang geschieht innerhalb einer geringen Erhöhung der Oberflächenladung und lässt sich nicht mit klassischen Theorien beschreiben.

Der zweite Teil dieser Arbeit widmet sich der Stabilität von Schaumlamellen. Eine Schaumlamelle ist ein dünner Wasserfilm, der durch die Adsorption von oberflächenaktiven Molekülen an beiden Seiten stabilisiert wird. In Zusammenhang von Schäumen muss zwischen zwei Prozessen unterschieden werden: Der Schaumbildung und der Schaumstabilität. Die zugrundeliegenden Mechanismen der Schaumbildung sind weitestgehend verstanden, die der Schaumstabilität jedoch noch nicht.

Um die Stabilität von Schäumen zu untersuchen, müssen Nichtgleichgewichtszustände erzeugt und die anschließende Relaxation in das Gleichgewicht beobachtet werden. In dieser Arbeit wurde ein neues Verfahren entwickelt, welches es ermöglicht, das Elastizitätsmodul von Grenzflächen in einem Frequenzbereich von 1-500 Hz zu bestimmen. Dies bedeutet eine Erweiterung um zwei Dekaden gegenüber herkömmlichen Methoden. Die Idee ist denkbar einfach: In einer mit Flüssigkeit gefüllten Kammer wird über die Bewegung eines Piezos eine Luftblase in Schwingung versetzt und mit einem in der Kammer befindlichen Drucksensor die Schwingungsantwort der Blase aufgezeichnet. Unsere Untersuchungen zeigen, dass die Voraussetzung für die Ausbildung einer stabilen Schaumlamelle das Vorkommen einer intrinsischen Oberflächenviskosität ist. Eine anschauliche Erklärung verdeutlicht dies: Eine viskose Oberfläche ist in der Lage, eine eingehende Störung lokal zu dämpfen, im Gegensatz zu einer komplett elastischen Oberfläche, wo sich die Störung über die gesamte Schaumlamelle verbreiten kann.

Untersuchungen mittels der IR-VIS SFG Spektroskopie ergaben, dass die Struktur des Wassers bei der Beschreibung der Schaumstabilität auf molekularer Ebene eine entscheidende Rolle spielt: Die Oberflächenviskosität ist mit einem dissipativen Vorgang innerhalb der Grenzschicht verbunden. Dieser dissipative Vorgang konnte auf molekularer Ebene durch das Aufbrechen von Wasserstoffbrückenbindungen identifiziert werden. Ausschlaggebend war dabei der Austausch der adsorbierten Amphiphile in der Grenzfläche und der angrenzenden Sublage.

Dans cette thèse, des simulations DFT-MD couplées à des techniques inno-vantes de métadynamique, sont appliquées pour acquérir une compréhensionglobale des interfaces aqueuses d'oxyde de cobalt Co3O4 et CoO(OH) dansla catalyse de la réaction d'évolution de l'oxygène (OER), et ainsi éventuellement aider à la conception de nouveaux catalyseurs basés sur des matériaux non précieux, un domaine clé de la recherche scientifique et technologique, particulièrement important pour l'économie de l'hydrogène, pour les technologies vertes dans une période de temps avec une demande toujours plus croissanteen énergie verte. Dans cette thèse, nous révélons étape par étape les mécanismes de l'OER sur les électrocatalyseurs aqueux d'oxyde de cobalt Co3O4 etCoO(OH) via de nouvelles techniques de métadynamique.Jusqu'à présent, la littérature n'a jamais pris en compte les modificationsau niveau atomique de la structure des électrodes ainsi que de l'eau interfaciale dans leur modélisation des processus OER. Ce manque de connaissances représente clairement un obstacle important au développement de catalyseurs améliorés, qui pourrait être surmonté en utilisant des méthodes capables de suivre les caractéristiques catalytiques de l'OER à l'échelle atomique. Pour la première fois, nous montrons combien il est important de prendre en considération la présence de l'environnement aqueux dans la caractérisation structurale des surfaces du catalyseur, c'est-à-dire (110)-Co3O4 et (0001)-CoO(OH) dans ce travail. Une caractérisation détaillée des propriétés chimiques et physiques des interfaces aqueuses est fournie (la structure, la dynamique, la spectroscopie, le champ électrique), pour les surfaces (110)-Co3O4 et (0001)-CoO(OH) en contact avec l'eau liquide.Une étude détaillée de l'OER est présentée non seulement du point de vue descatalyseurs, mais aussi en abordant le rôle de l'environnement de l'eau dans leprocessus catalytique, ce qui n'a pas été fait auparavant dans la littérature. En conséquence, l'OER en phase gazeuse et en phase liquide sont étudiés ici auxinterfaces aqueuses (110)-Co3O4 et (0001)-CoO(OH) en adoptant une nouvelleapproche de métadynamique d'échantillonnage amélioré, capable d'identifieret caractériser les mécanismes de réaction chimique et d'intégrer pleinement lerôle des degrés de liberté du solvant, permettant ainsi de dévoiler des réactivités chimiques d'une complexité remarquable. L'énergétique, la cinétique et la thermodynamique derrière l'OER sont donc trouvées à ces surfaces d'oxyde de cobalt à l'interface avec l'eau
In this thesis, DFT-MD simulations, coupled with state-of-the-art metadynamics techniques, are applied to gain a global understanding of Co3O4 and CoO(OH) cobalt oxide aqueous interfaces in catalyzing the oxygen evolution reaction (OER), and hence possibly help in the design of novel catalysts basedon non-precious materials, a current key field of research in science and technology, especially of importance for the hydrogen economy, for green technology in a period of time with an ever more growing demand in green-energy. In this thesis, we step-by-step reveal the OER mechanisms on spinel Co3O4 andCoO(OH) cobalt aqueous electrocatalysts carefully and rationally via novelmetadynamics techniques.Up to now, the literature has never taken into account the atomistic modifications on the electrode structure as well as on the interfacial water into their modeling of OER processes. Such lack of knowledge clearly represents a significant hurdle toward the development of improved catalysts, which couldbe overcome by employing methods able to track the catalytic features of theOER at the atomistic scale. For the first time, we show how important itis to take into consideration the presence of the liquid water environment inthe structural characterization of catalyst surfaces, i.e. for (110)-Co3O4 and(0001)-CoO(OH) in this work. A detailed characterization of chemical andphysical properties of the aqueous interfaces is provided (i.e. structure, dynamics, spectroscopy, electric field), for the (110)-Co3O4 and (0001)-CoO(OH)aqueous surfaces.A study of the OER is presented not only by looking at the catalysts, butalso by addressing the role of the water environment in the catalytic process,not done before in literature. Accordingly, both gas-phase and liquid-phaseOER are here investigated at the (110)-Co3O4 and (0001)-CoO(OH) adoptinga novel enhanced sampling metadynamics approach able to address a widerange of chemical reaction mechanisms and to fully include the role of thesolvent degrees of freedom, allowing to unveil reaction networks of remarkablecomplexity. The energetics, kinetics and thermodynamics behind the OER aretherefore found at these cobalt oxide surfaces

Thesis (Ph. D.)--University of Wisconsin--Madison, 1995.
Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 295-305).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1996.
Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 303-315).

Thesis (Ph. D.)--University of Georgia, 2005.
Directed by Richard A. Dluhy. Includes articles published in Biophysical journal, Journal of physical chemistry, Vibrational spectroscopy, and Biophysical chemistry, and an article submitted to Advanced physics letters. Includes bibliographical references.

Yes
Understanding the dynamic free surface of geophysical flows has the potential to enable direct inference of the flow properties based on measurements of the free surface. An important step is to understand the inherent response of free surfaces in depth-limited flows. Here a model is presented to demonstrate that free surface oscillatory spatial correlation patterns result from individual surface features oscillating vertically as they advect over space and time. Comparison with laboratory observations shows that these oscillating surface features can be unambiguously explained by simple harmonic motion, whereby the oscillation frequency is controlled by the root-mean-square water surface fluctuation, and to a lesser extent the surface tension. This demonstrates that the observed “complex” wave pattern can be simply described as an ensemble of spatially and temporally distributed oscillons. Similarities between the oscillon frequency and estimated frequency of near-bed bursting events suggest that oscillon behaviour is linked with the creation of coherent flow structures.

This work explores impact phenomena experimentally and theoretically for various interfacial systems ranging from medical diagnostics to drop impacts on solids and immiscible liquids. We study a hitherto unexplored impact phenomenon during an ophthalmology procedure called Non-Contact Tonometry. Using high fidelity experiments and theoretical modeling, we show that noninvasive ocular diagnostics demonstrate a propensity for droplet generation and present a potential pathway for pathogen transmission. The air puff-induced corneal deformation and subsequent capillary waves lead to flow instabilities (Rayleigh–Taylor, Rayleigh–Plateau) that lead to tear film ejection, expansion, stretching, and subsequent droplet formation. Effective cooling is one of the significant application areas of impact systems. In the context of cooling problems, we provide new insights using ab initio scaling and boundary layer analysis of the integral and differential forms of the conservation equations. We have probed the limiting scaling regimes by incorporating the evap- orative effects at the liquid-vapor interface. The dependence of liquid film thickness and Nusselt number on various non-dimensional numbers has been explored. We then investigate the class of drop impact problems where we study impacts on solids, bio-inspired substrates, and immiscible liquid pools at low to moderate impact en- ergies. We explore impacts on glass, PDMS, and soft lithographically fabricated replicas of the lotus leaf and rose petals. Surprisingly, the rose petal and lotus leaf replicas manifest similar impact dynamics. The observation is extremely intriguing and counter-intuitive, as rose petals and their replicas are sticky in contrast to lotus leaves. Air entrapment in the micrometer features of bio-inspired surfaces prevents frictional dissipation of droplet kinetic energy, leading to contact edge recession and subsequent break-up modes of the droplet. We explore the air entrapment dynamics beneath an impacting droplet on an immiscible viscous liquid pool using high-speed reflection interferometry imaging and linear stability analysis. We have detected unique hydrodynamic topology in thin air film surrounding the central air dimple (peripheral disc). The pattern resembles spinodal and finger-like structures typi- cally found in various thin condensed matter systems. We attribute the formation of multi-scale thickness perturbations, associated ruptures, and finger-like protru- sions in the draining air film as a combined artifact of thin-film and Saffman–Taylor instabilities. We also investigate the air craters formed on the surface of the impact- ing droplet and attribute its formation to the rapid deceleration of the droplet due to viscous drag force. The droplet response to the external impulsive decelerating force induces oscillatory modes on the surface exposed to the air forming capillary waves that superimpose to form air craters of various shapes and sizes. We introduce a non-dimensional parameter, the ratio of the drag force to the capillary force acting on the droplet to characterize the air craters. Further, we generalize the local droplet response with a global response model for low-impact energies based on an eigenvalue problem. We represent the penetrating drop as a constrained Rayleigh drop problem with a dynamic contact line. The air-water interface dynamics are analyzed using an in-viscid droplet deformation model for small deformation amplitudes. The local and global droplet response theories conform and depict that the deformation profiles could be represented as a linear superposition of eigenmodes in a Legendre polynomial basis. We further study air layer dynamics beneath an impacting droplet on a heated surface at various surface temperatures at low impact energy. The air layer thickness profile consisting of the dimple and the peripheral disc has been measured using high-speed reflection interferometric imaging. We decipher that a Gaussian profile can approximate the dimple height profile characteristics. The dim- ple thickness profile has a weak dependence on the substrate temperature and is a function of impact energy in general. The air layer rupture time scale and rupture radius increase with an increase in the substrate temperature. We characterize the air layer profile as a Knudsen field and show that a unified treatment, including continuum and non-continuum mechanics, is required to understand the air layer dynamics. The asymmetric wetting of the substrate by the impacting droplet initi- ates in the peripheral disc region. In the non-continuum regimes in the peripheral air disc, we discover intriguing asymmetric interface perturbations. These perturbative structures cause asymmetric wetting/contact between the droplet and the substrate. The sub-micron length scales of the structures exist due to the asymptotic effects of capillary and Van-der Waals interaction in the disc region.

碩士
國立陽明大學
醫學工程研究所
96
A triblock copolymers compose of poly(ethylene oxide)(PEO) and poly(propylene oxide) (PPO). Its characters are block sequence of PEO-PPO-PEO and have excellent surface activity. The commercially available trade names are Pluronics or Poloxamers. PEO-PPO-PEO block copolymers are used widespread in industrial and biomedical fields. Pluronics take the surface modification materials rule by simple adsorption or desorption. They have many advantages for investigating the adsorption and desorption behavior at the surface. In order to observe the mechanism of Pluronics’s adsorption/desorption behavior, we use a video-enhanced pendant bubble tensiometry to detect the surface tension change at the air-water interface in Pluronics solutions. From the dynamic surface tension curves, the surface tension decreases as solution concentration increases. The higher the concentration of solution, the more the surface tension change. When the concentration of solution is lower than 1×10-3 mg/ml, induction period of dynamic surface tension curves is observed, and the period decreases as the concentration of solution increases. Besides, some slope change of solution with different concentrations which takes place at the same surface pressure is observed. Some of these slope change can be observed from the equilibrium isotherm(Wilhelmy Plate method), some can only be observed from the dynamic surface tension curves. Effective coefficient (αN)2D and D is calculated from the dynamic surface tension and dynamic surface concentration curves. It shows that the diffusion controlled mechanism may play a role in low surface pressure region.

Structure, dynamics and reactivity of the hydrated electron Frank Uhlig In this work, one of the products of ionization of water, namely the hydrated electron, has been investigated. The hydrated electron is a key-intermediate in aqueous radiation chemistry. Although known to exist for over 50 years, its structure remained elusive and under discussion up to the present day. We show in this work, that we can obtain a faithful picture of the hydrated electron, its equilibrium structure, dynamics after attachment to water, and its reactivity, using ab initio methods. To this end, small cluster models and extended bulk and slab geometries of water including an excess electron have been investigated.