Our work investigates theoretical approximations to the interface specific sum frequency generation (SFG) spectra at aqueous interfaces constructed using time correlation function (TCF) and instantaneous normal mode (INM) methods. Both approaches lead to signals in excellent agreement with experimental measurements. This work demonstrates how TCF and INM methods can be used in a complementary fashion to describe interfacial vibrational spectroscopy. Our approach is to compare TCF spectra with experiment to establish that our molecular dynamics (MD) methods can reliably describe the system of interest. We then employ INM methods to analyze the molecular and dynamical basis for the observed spectroscopy. We have been able to elucidate, on a molecularly detailed basis, a number of interfacial line shapes, most notably the origin of the complex O-H stretching SFG signal, and the identity of several intermolecular modes in the SFG spectra for the water/vapor interface. The success of both approaches suggests that theory can play crucial role in interpreting SFG spectroscopy at more complex interfaces.

Nonlinear optical processes probe systems in unique manners. The signals obtained from nonlinear spectroscopic experiments are often significantly different than more standard linear techniques, and their intricate nature can make it difficult to interpret the experimental results. Given the complexity of many nonlinear lineshapes, it is to the benefit of both the theoretical and experimental communities to have molecularly detailed computationally amenable theories of nonlinear spectroscopy. Development of such theories, bench marked by careful experimental investigations, have the ability to understand the origins of a given spectroscopic lineshape with atomistic resolution. With this goal in mind, this manuscript details the development of several novel theories of nonlinear surface specific spectroscopies. Spectroscopic responses are described by quantum mechanical quantities. This work shows how well defined classical limits of these expressions can be obtained, and unlike the formal quantum mechanical expressions, the derived expressions comprise a computationally tractable theory. Further, because the developed novel theories have a well defined classical limit, there is a quantum classical correspondence. Thus, semiclassical computational techniques can capture the true physics of the given nonlinear optical process. The semiclassical methodology presented in this manuscript consists of two primary components - classical molecular dynamics and a spectroscopic model. For each theory of nonlinear spectroscopy that is developed, a computational implementation methodology is discussed and/or tested.

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Les systèmes diphasiques à pompage capillaire sont couramment utilisés pour contrôler la température de l'électronique embarquée. Ces systèmes sont fiables et performants, mais ils présentent certaines limites associées essentiellement à la vaporisation dans le milieu poreux (limite capillaire, limite d'ébullition). Une façon d'étendre leurs performances en termes de longueur de transport de la chaleur et d'intensification des transferts serait de les coupler avec un système mécaniquement actif. Un des moyens pour réaliser cela est d'utiliser les forces électro-hydro-dynamiques (EHD) se développant dans le fluide lorsqu'on applique un champ électrique. Les travaux proposés sont une contribution à la compréhension de l'interaction entre une interface liquide-vapeur et un champ électrique afin de déterminer quels sont les mécanismes qui dans ces conditions contrôlent le pompage et le transfert de chaleur. La première partie se focalise sur l'étude expérimentale d'une interface liquide-vapeur sous un champ électrique avec ou sans flux de chaleur dans une configuration très académique (deux électrodes planes et verticales) tout en étant proche de ce qui se passe dans une cannelure de caloducs par exemple. L'objectif est d'observer, de quantifier et d'analyser les effets (forces, structures, instabilités) se développant sur l'interface. Une analyse a ensuite été menée à partir de modèles 1D et 2D. Nous avons ainsi pu vérifier que parmi l'ensemble des forces s'exerçant sur l'interface, la force diélectrophorétique est celle qui contrôle sa position et sa forme avec ou sans vaporisation. On a montré cependant qu'il existait des effets de couplage avec la conduction électrique dans le liquide pouvant sensiblement agir sur la courbure de l'interface. Enfin, les résultats en vaporisation ont confirmé que le champ électrique, en donnant des moyens de contrôle de la position et de la structure de l'interface de vaporisation, peut être effectivement mis à profit pour une intensification des transferts de chaleur proches d'une paroi chauffée. Dans un dernier volet, les travaux se sont concentrés sur la mise en mouvement d'un liquide diélectrique en mettant à profit le régime de conduction. Une étude expérimentale permettant d'étudier l'influence des différents paramètres (géométrie des électrodes, distance inter électrodes, nombre de modules) a été réalisée dans les fluides HFE-7000 et HFE-7100. Les résultats ont montré une faible reproductibilité des performances de la pompe pour ces fluides suggérant une forte sensibilité des phénomènes à l'état de surface des électrodes et aux régimes parasites d'injection de charge. / Two-phase systems based on the capillary pumping are widely used for electronics cooling. These systems are reliable and efficient, but the maximum heat load is given by the porous medium characteristics (pore size and conductivity) and the fluid properties. The use of an additional source of energy to actively control the heat transport and the heat transfers is a way to extend the performance. Electro-hydrodynamic forces (EHD) could fulfill this objective. This work proposed a contribution to the understanding of the interaction between a liquid-vapor interface and an electric field in order to determine which mechanisms control the pumping and heat transfer. The first part focused on the experimental study of a liquid-vapor interface under an electric field with or without heat flux in a very academic configuration while being close to the operating conditions of the vaporization in a groove of a heat pipe for example. The objective was to observe, quantify and analyze the effects (forces, structures, instabilities) developing on the interface. On top of that, analysis based on 1D and 2D models were made. We found that the dielectrophoretic force mainly controlled the position and the shape of the interface with or without vaporization. However, some effects of coupling with the electrical conduction in the liquid were proved to substantially modify the interface curvature. Finally, the results confirmed that the electric field can effectively be used to the heat transfer enhancement close to a heated wall. In a final section, the work is related to the pumping of a dielectric liquid based on the conduction regime. An experimental study was carried out in HFE-7000 and HFE-7100 fluids to investigate the role of different parameters (electrode geometry, inter-electrode distance, number of modules). For these fluids, the repeatability of results was not satisfactorily suggesting a high sensitivity of the phenomena according to the surface state of the electrodes and parasitic charge injection.

This dissertation includes a theoretical and numerical development to simulate liquid-vapor flows and the applications to microchannels. First, we obtain a consistent non-local pressure equation for simulating liquid-vapor interfacial flows using mean-field free-energy theory. This new pressure equation is shown to be the general form of the classical van der Waals" square-gradient theory. The new equation is implemented in two-dimensional (2D) D2Q7, D2Q9, and three-dimensional (3D) D3Q19 lattice Boltzmann method (LBM). The three LBM models are validated successfully in a number of analytical solutions of liquid-vapor interfacial flows. Second, we have shown that the common bounceback condition in the literature leads to an unphysical velocity at the wall in the presence of surface forces. A few new consistent mass and energy conserving velocity-boundary conditions are developed for D2Q7, D2Q9, and D3Q19 LBM models, respectively. The three LBM models are shown to have the capabilities to successfully simulate different wall wettabilities, the three typical theories or laws for moving contact lines, and liquid-vapor channel flows. Third, proper scaling laws are derived to represent the physical system in the framework of the LBM. For the first time, to the best of the author's knowledge, we obtain a flow regime map for liquid-vapor channel flows with a numerical method. Our flow map is the first flow regime map so far for submicrochannel flows, and also the first iso-thermal flow regime map for CO₂ mini- and micro-channel flows. Our results show that three major flow regimes occur, including dispersed, bubble/plug, and liquid strip flow. The vapor and liquid dispersed flows happen at the two extremities of vapor quality. When vapor quality increases beyond a threshold, bubble/plug patterns appear. The bubble/plug regimes include symmetric and distorted, submerged and non-wetting, single and train bubbles/plugs, and some combination of them. When the Weber number<10, the bubble/plug flow regime turns to a liquid strip pattern at the increased vapor quality of 0.5~0.6. When the Weber number>10, the regime transition occurs around a vapor quality of 0.10~0.20. In fact, when an inertia is large enough to destroy the initial flow pattern, the transition boundary between the bubble and strip regimes depends only on vapor quality and exists between x=0.10 and 0.20. The liquid strip flow regimes include stratified strip, wavy-stratified strip, intermittent strip, liquid lump, and wispy-strip flow. We also find that the liquid-vapor interfaces become distorted at the Weber number of 500~1000, independent of vapor quality. The comparisons of our flow maps with two typical experiments show that the simulations capture the basic and important flow mechanisms for the flow regime transition from the bubble/plug regimes to the strip regimes and from the non-distorted interfaces to the distorted interfaces. Last, our available results show that the flow regimes of both 2D and 3D fall in the same three broad categories with similar subdivisions of the flow regimes, even though the 3D duct produces some specific 3D corner flow patterns. The comparison between 2D and 3D flows shows that the flow map obtained from 2D flows can be generally applied to a 3D situation, with caution, when 3D information is not available. In addition, our 3D study shows that different wettabilities generate different flow regimes. With the complete wetting wall, the flow pattern is the most stable. / Ph. D.