Two-phase heat transfer techniques such a boiling make use of the high latent heat of fluids to enable dissipation of higher heat fluxes from surfaces compared to conventional single-phase cooling methods. To meet the increasing heat flux dissipation requirements of high-power electronic devices, modifications to the surface properties and roughness are often considered as a means to enhance two-phase heat transfer processes. Although surface roughness of varying length scales has been observed experimentally to enhance boiling heat transfer performance, the physical mechanisms that govern this improvement are not widely accepted. Correlations can be developed to map the behavior of specific surface structure geometries, but a broader investigation of the fundamental forces affecting evaporation at the three-phase contact line, which is critically important to the two-phase heat transfer process, may provide more widely applicable insights. In this thesis, an experimental setup was developed to investigate the effect of small scale surface roughness, with feature sizes below 1 micron, on the liquid film profile of a meniscus formed on a surface. This physical film profile can provide insight into how the surface roughness affects disjoining pressure, an important force that affects the phase change heat transfer process at the contact line. Using an interferometry technique to measure the liquid film profile for a model system of octane on silicon substrate with varying roughness, the change in disjoining pressure in the liquid film was observed. We found that the strength of disjoining pressure in the liquid film increases with increasing surface roughness feature depth.<br>

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Flotation is regarded as the best available separation method for the recovery of valuable minerals such as chalcopyrite (CuFeS2), sphalerite (ZnS), etc., from mined ores. Practically all metals humans use today are produced by flotation. The process relies on controlling the stability of the thin liquid films (TLFs) of water formed between minerals and air bubbles (wetting film), air bubbles (foam film), and mineral particles (colloid films). In flotation, a desired mineral is rendered hydrophobic by surfactant coating as a means to destabilize the TLFs, so that they can be attached to the hydrophobic air bubbles. A TLF ruptures when the disjoining pressure (or surface forces per unit area) of the film becomes negative, i.e., Π < 0. Thermodynamically, a wetting film can rupture when the contact angle (θ) of a mineral surface is larger than zero. It would, therefore, be reasonable to consider the roles of the surface forces to better understand the fundamental mechanisms involved in flotation. The surface forces considered in the present work included the electric double layer (EDL), van der Waals (vdW), and attractive hydrophobic (HP) forces. A flotation model has been developed by using the hydrophobic force as a kinetic parameter, which made it possible to track the fates of mineral particles of different of size, surface liberation, and contact angle to predict both recovery and grades for the first time. The model has been validated against the plant survey data obtained from an operating copper flotation plant. The simulation results obtained using the first principles model have been utilized to address the limitations of current flotation practices. One such limitation is the presence of slow-floating target minerals present in the cleaner-scavenger tails (CST) that are routinely recycled back to the rougher flotation bank as circulating loads (CLs) to allow longer retention times for the slow-floating particles for additional recovery. The simulation results show also that opening a flotation circuit by treating the CST streams separately in an advanced circuit can substantially improve the plant performance. One of the major limitations of flotation is that the coarse particles in a feed stream are difficult to recover due to the low hydrophobicity associated with poor surface liberation. A new flotation model developed in the present work suggests various ways to address the problem. One is to increase the hydrophobicity of the composite (poorly liberated) particles using the Super Collectors that can increase the contact angles to 150 -170o. Simulation results obtained using the model developed in the present work show significant financial benefits of using Super Collectors. Flotation is controlled by surface forces as noted above. As particle size becomes larger than 150 µm, however, the gravitational force comes into the picture and can override the surface forces. A new flotation cell has been developed to mitigate the effects of the extraneous force by decreasing the effective specific gravity (SG) by attaching air bubbles to facilitate levitation and by creating a pulsation to allow particles to move according to SGs independent of particle size, which should help increase the upper particle size limit of flotation. Surface forces in foam and oil-in-water emulsion films have been measured at different temperatures to determine the changes in thermodynamic properties of the thin liquid films (TLFs) of water confined between two bubbles and two oil drops. The results show that the films are destabilized by the attractive hydrophobic forces created during the course of building H-bonded structures in confined spaces, which entails decreases in enthalpy (H < 0) and entropy (TS < 0), the second term representing the thermodynamic cost of building the structures. / Doctor of Philosophy / Flotation is a kinetic process designed to separate valuable minerals from mined ores. This process depends on several hydrodynamic and surface chemistry parameters making it hard to model. A U.S. patent was awarded to Sulman and Picard in 1905 for using air bubbles to selectively collect hydrophobic particles from the aqueous phase, leaving hydrophilic particles behind. Since then, the separation process known as flotation has been used to produce practically all metals humans use. Many investigators developed flotation models using hydrodynamic parameters, e.g., particle size, bubble size, energy dissipation rate, etc., but without a reference to particle hydrophobicity. Therefore, the models were successful in predicting recoveries but not product grades. Derjaguin and Dukhin (1961) were the first to model flotation using surface forces but without due consideration of the role of hydrophobic force in flotation. Therefore, it also failed to predict product grades. In the current work, a new flotation model has been developed using the hydrophobic force as a kinetic parameter. This approach made it possible to predict both recoveries and grades for the first time. The model has been reduced to a simple form mimicking the Arrhenius equation so that it can be used to delineate the different conditions required for optimizing coarse and fine particle flotation. The model has been derived by considering the surface forces in the thin liquid films (TLFs) of water confined between bubbles, and bubbles and particles. It has been found that the hydrophobic force plays a decisive role in destabilizing a wetting film and inducing bubble-particle attachment. The surface forces measured in the present work show that the hydrophobic interactions in macroscopic scales are controlled by enthalpy rather than entropy, which is contrary to the nanoscale hydrophobic interactions. The model has been validated against a full-scale plant operation and demonstrated predictive capabilities. The simulation results have been analyzed to determine the limitations of the current flotation practices. It was found that coarse particle flotation is difficult either due to the presence of composite particles reducing the particle contact angle or due to their poor hydrodynamic properties. Utilizing the insights from the model, various methods of alleviating these limitations have been developed and presented in the current work. References Derjaguin, B.V., Dukhin, S.S., 1961. Theory of flotation of small and medium-size particles. Inst. Min. Metall. 241–267. Sulman, H.L., and Kirkpatrick-Picard (1905). U.S. Patent No. 793,808.

Cette thèse traite de la modélisation de la croissance et de la dissolution de cristaux confinés. Nous nous concentrons sur la dynamique dans les contacts lubrifiés (ou hydrophiles) et dérivons un modèle de continu de couche mince prenant en compte la diffusion, la cinétique de surface, l’hydrodynamique, la tension de surface et les interactions avec le substrat (pression de disjonction). Premièrement, nous étudions la dissolution induite par une charge extérieure (dissolution sous contrainte). Nous trouvons que la forme fonctionnelle de la pression de disjonction -finie ou divergente au contact- est cruciale dans la détermination des taux de dissolution et des morphologies stationnaires. Ces formes conduisent respectivement à des taux de dissolution dépendant ou indépendants de la charge, et à des profils de surface plats ou pointus. Deuxièmement, nous avons considéré la croissance des cristaux à proximité d’un mur plat. Nous avons constaté qu’une cavité apparaît sur la surface cristalline confinée. Nous obtenons un diagramme de morphologie hors équilibre en accord avec les observations expérimentales. En traversant la ligne de transition, une cavité peut apparaître de manière continue ou discontinue en fonction de la forme de la pression de disjonction (répulsive ou attractive). Pour les épaisseurs de film nanométriques, la viscosité peut entraver la formation de la cavité. Enfin, nous étudions la force de cristallisation exercée par un cristal croissant entre deux parois planes. Nous soulignons l’importance d’une définition précise de l’aire de contact pour définir la pression d’équilibre thermodynamique. Pendant la croissance, la ligne triple subit une transition cinétique dépendant uniquement du rapport entre: la constante de diffusion, et le produit de la constante cinétique de surface et de la distance entre les murs. Après cette transition, la force de cristallisation diminue jusqu’à s’annuler, et un film macroscopique se forme / This thesis discusses the modeling of growth and dissolution of confined crystals. We focus on the dynamics within lubricated (or hydrophilic) contacts and derive a thin film continuum model accounting for diffusion, surface kinetics, hydrodynamics, surface tension and interactions with the substrate (disjoinining pressure). First, we study dissolution induced by an external load (pressure solution). We find the functional form of the disjoining pressure -finite or diverging at contact- to be crucial in determining steady state dissolution rates and morphologies. These forms respectively lead to load-dependent or load-independent dissolution rates, and to flat or pointy surface profiles.Second, we considered crystal growth in the vicinity of a flat wall. We found that a cavity appears on the confined crystal surface. We obtain a non-equilibrium morphology diagram in agreement with experimental observations. When crossing the transition line, a cavity can appear continuously or discontinuously depending on the form of the disjoining pressure (repulsive or attractive). For nanometric film thicknesses, viscosity can hinder the formation of the cavity.Finally, we study the force of crystallization exerted by a crystal growing between two flat walls. We point out the importance of a precise definition of the contact area to define the thermodynamic equilibrium pressure. During growth, the triple-line undergoes a kinetic pinning transition depending solely on the ratio between the diffusion constant and the product of the surface kinetic constant and distance between the walls. After this transition, the crystallization force decreases to zero, and a macroscopic film forms

Model-H is used to describe structures found in the phase separation in films of binary liquid mixture that have a surface that is free to deform and also may energetically prefer one of the components. The film rests on a solid smooth substrate that has no preference for any component. On the one hand the study focuses on static aspects by investigating steady states that are characterised by their concentration and film height profiles. A large variety of such states are systematically analysed by numerically constructing bifurcation diagrams in dependence of a number of control parameters. The numerical method used is based on minimising the free energy functional at given constraints within a finite element method for a variable domain shape. The structure of the bifurcation diagrams is related to the symmetry properties of the individual solutions on the various branches. On the other hand the full time dependent model-H is linearised about selected steady states, in particular, the laterally invariant, i.e.\ layered states. The resulting dispersion relations are discussed and related to the corresponding bifurcation points of the steady states. In general, the results do well agree and confirm each other. The described analysis is performed for a number of important cases whose comparison allows us to gain an advanced understanding of the system behaviour: We distinguish the critical and off-critical case that correspond to zero and non-zero mean concentration, respectively. In the critical case the investigation focuses on (i) flat films without surface bias, (ii) flat films with surface bias, (iii) height-modulated films without surface bias, and (iv) height-modulated films with surface bias. Each case is analysed for several mean film heights and (if applicable) energetic bias at the free surface using the lateral domain size as main control parameter. Linear stability analyses of layered films and symmetry considerations are used to understand the structures of the determined bifurcation diagrams. For off-critical mixtures our study is more restricted. There we consider height-modulated films without and with surface bias for several mean film heights and (if applicable) energetic bias employing the mean concentration as main control parameter.

An experimental and numerical investigation has been conducted to examine steady, internal, nozzle-generated, gas/liquid mist cooling in vertical channels with ultra-thin, evaporating subcooled liquid films. Interest in this research has been motivated by the need for a highly efficient cooling mechanism in high-power lasers for inertial fusion reactor applications. The aim is to quantify the effects of various operating and design parameters, viz. liquid atomization nozzle design (i.e. spray geometry, droplet size distribution, etc.), heat flux, liquid mass fraction, film thickness, carrier gas velocity, temperature, and humidity, injected liquid temperature, gas/liquid combinations, channel geometry, length, and wettability, and flow direction, on mist cooling effectiveness. A fully-instrumented experimental test facility has been designed and constructed. The facility includes three cylindrical and two rectangular electrically-heated test sections with different unheated entry lengths. Water is used as the mist liquid with air, or helium, as the carrier gas. Three types of mist generating nozzles with significantly different spray characteristics are used. Numerous experiments have been conducted; local heat transfer coefficients along the channels are obtained for a wide range of operating conditions. The data indicate that mist cooling can increase the heat transfer coefficient by more than an order of magnitude compared to forced convection using only the carrier gas. The data obtained in this investigation will allow designers of mist-cooled high heat flux engineering systems to predict their performance over a wide range of design and operating parameters. Comparison has been made between the data and predictions of a modified version of the KIVA-3V code, a mechanistic, three-dimensional computer program for internal, transient, dispersed two-phase flow applications. Good agreement has been obtained for downward mist flow at moderate heat fluxes; at high heat fluxes, the code underpredicts the local heat transfer coefficients and does not predict the onset of film rupture. For upward mist flow, the code underpredicts the local heat transfer coefficients and, contrary to experimental observations, predicts early dryout at the test section exit.