Thesis (M.S.)--Rutgers University, 2007. / "Graduate Program in Physics and Astronomy." Includes bibliographical references (p. 56-57).

Thesis (Ph. D.)--Chemical and Biomolecular Engineering, Georgia Institute of Technology, 2006. / Jones, Christopher, Committee Chair ; Schork, F. Joseph, Committee Co-Chair ; Weck, Marcus, Committee Member ; Meredith, Carson, Committee Member ; Agrawal, Pradeep, Committee Member.

Thesis (M. S.)--Materials Science and Engineering, Georgia Institute of Technology, 2006. / Gokhale, Arun, Committee Chair ; Gall, Ken, Committee Member ; Thadhani, Naresh, Committee Member.

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2006. / Nicholas V. Hud, Committee Chair ; Donald F. Doyle, Committee Member ; Rigoberto Hernandez, Committee Member ; Roger M. Wartell, Committee Member ; Loren D. Willliams, Committee Member.

Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2007. / Martha A. Gallivan, Committee Member ; Andrei G. Fedorov, Committee Member ; Christopher Summers, Committee Member ; Thomas H. Sanders, Jr., Committee Member ; Mo Li, Committee Chair.

La nucléation est l’étape est à l’origine de la cristallisation et c’est généralement elle qui va dicter les propriétés finales d’un cristal. Il est alors intéressant de comprendre les mécanismes qui gouvernent cette étape. Deux théories se confrontent aujourd’hui avec d’une part la théorie classique de la nucléation et d’autre part un modèle de nucléation en deux étapes. La théorie classique considère l’addition des monomères un par un pour former des clusters directement cristallins. Le modèle en deux étapes considère une première étape de concentration, au cours de laquelle les monomères vont former des agrégats denses, mais désordonnés, et une seconde étape de structuration qui permettra de réarranger les molécules au sein d’un cluster, pour obtenir une structure cristalline. Il semblerait que, dans le cas des protéines, le passage du mécanisme classique à celui en deux étapes se fasse par une augmentation des fluctuations de concentration en solution, ce qui se fait en se rapprochant de la spinodale de décomposition présente dans le diagramme de phase de certaines protéines. Ainsi, dans cette étude, nous développons des systèmes microfluidiques qui permettent d’étudier ces différents mécanismes. Dans un premier temps, nous avons conçus un dispositif utilisé pour déterminer des cinétiques de nucléation à l’aide de modèles probabilistes. Ensuite, nous avons adapté des systèmes microfluidiques à des grands équipements synchrotron, ce qui donne la possibilité de réaliser de la diffusion de rayons X aux petits angles. Ces deux approches complémentaires permettent alors d’obtenir des informations à l’échelle macroscopique et à l’échelle microscopique, et ainsi de participer à l’éclaircissement des zones d’ombre qui demeurent autour des mécanismes de nucléation. / Nucleation is the first step of crystallization and is the step that will give the final properties of crystals. It is therefore interesting to understand the mechanisms of this step. Nowadays, two theories exist : the classical nucleation theory and the two step theory. In the classical nucleation theory, monomers are added one by one to directly form crystalline clusters. In the two step theory, there is a first step of densification, in which monomers form dense amorphous aggregates and a second step of structuration in which aggregates arrange themselves to form crystalline clusters. It seems that for proteins, the two step mechanism is obtained by an increase of concentration fluctuations in solution, that happens when experimental conditions get closer to spinodal decomposition. In this study, experimental microfluidic devices are developped to study these mechanisms. Firstly, we developped microfluidic devices to determine nucleation kinetics with probabilistic models. Then, we coupled microfluidic devices with small angle X rays scattering from synchrotron sources. These two complementary studies give information on nucleation mechanisms on the macroscopic and microscopic scales.

Desde a apresentação do primeiro modelo da transição de fase de primeira ordem no início do século XX, acumulou-se muito conhecimento a respeito da cinética deste fenômeno. A Teoria Clássica da Nucleação (TCN) calcula o tamanho do núcleo crítico maximizando a diferença na energia livre de Gibbs entre as fases atual e nova, separadas por uma superfície de tensão bem definida. Técnicas experimentais modernas indicam que a TCN calcula a taxa de nucleação com acurácia razoável somente em estreita faixa de temperatura, indicando deficiências no modelo. Correções na teoria melhorou sua dependência com a temperatura, mas os resultados encontrados são superiores aos medidos experimentalmente em cerca de quatro ordens de grandeza. Diferentes abordagens têm surgido, como a Teoria Adiabática da Nucleação e a Teoria Entrópica da Nucleação (TEW). Ainda que aquela traga em seu escopo teórico parte do modelo clássico, como a tensão superficial, elege a maximização da entropia como a força motriz da transição de fase. O mesmo fez a TEN, porém sem especificar uma superfície de tensão. A primeira aplicação desta teoria foi na condensação do vapor de água. Mediante uma teoria da flutuação, no seu aspecto mais simplificado, foi determinado o tamanho do núcleo critico. Os resultados obtidos são menores do que os experimentais em cerca de três ordens de grandeza, devido a excessiva simplicidade do modelo empregado. Entretanto, verifica-se que a dependência do tamanho do núcleo com a supersaturação assemelha-se aos dados experimentais. Os resultados indicam que a abordagem entrópica é promissora. / In Classical Nucleation Theory (CNT) the size of the critical nucleous is obtained by maximizing the difference in bulk Gibbs free energy between the new and the evolving phase separated by a surface having a well defined tension, in relation to the nucleous size. CNT, albeit the introduction of numerous correction terms, did not compare well with experimental data by many orders of magnitude. Other theories have been proposed as the Adiabatic and Entropy Theones, ANT and ENT, respectively. While ANT rests on some CNT assumptions as the existence of a well defined surface separating the new and the old phase, considers the entropy as the driving force of phase transition instead of Gibbs Free Energy. On the other hand, ENT only rests on the assunption of the entropy as the driving force without considering a surface tension. This theory is applied to the study of water vapor condensation, by calculating the size of the critical nucleous with the aid small amplitude fluctuation theory. The results compare well with experimental values, being off by about 3 orders of magnitude but its dependence with supersaturation agrees well with experiment. The results are satisfactory and may be improved by refining the theory.

As três famílias de ligas de maior importância comercial para a indústria de fundição apresentam a formação de eutéticos do tipo facetado/não-facetado durante a solidificação: os ferros fundidos com grafita (eutético austenita/grafita), os ferros fundidos brancos (eutético austenita/carboneto) e as ligas Al-Si (eutético alfa/silício). No caso dos ferros fundidos brancos, a morfologia dos carbonetos eutéticos exerce influência decisiva sobre o comportamento mecânico e a resistência à degradação (em função de solicitações como abrasão ou ciclos térmicos e/ou mecânicos) das ligas resistentes a desgaste pertencentes ao sistema Fe-C-X (em que X representa um ou mais dos elementos Cr, V, Mo, W e, mais raramente, Nb). Investigou-se o efeito do potencial nucleante do banho metálico sobre o eutético de ligas do sistema Fe-C-V, e conseqüentemente sobre a morfologia do carboneto. As alterações de morfologia do eutético austentita/carboneto VC em função da adição de alumínio ou titânio foram avaliadas por meio de microscopias óptica e eletrônica de varredura. Observou-se que a adição de alumínio ou titânio, em ligas com teores residuais de oxigênio ou nitrogênio, promoveu alteração do eutético VC de irregular para fibroso (no caso da adição de alumínio) e para divorciado (no caso da adição de titânio). Conclui-se que a presença de partículas nucleantes induziu a nucleação seletiva da fase facetada (carboneto VC) a temperaturas mais elevadas, diminuindo o grau de cooperação durante o crescimento do eutético. / The three most commercially important families of alloys for the foundry industry show the formation of faceted/non-faceted eutectics type during solidification: cast irons with graphite (austenite/graphite eutectic), white cast irons (austenite/carbide eutectic) and Al-Si alloys (alfa/silicon eutectic). In the case of white cast irons, the eutectic carbide morphology has decisive influence on mechanical behavior and on degradation resistance (due to solicitation such as abrasion or thermal and/or mechanical cycling) of wearing resistant alloys of the Fe-C-X system (where X stands for one or more of the following elements: Cr, V, Mo, W and, seldom, Nb). The metallic bath nucleant potential on the eutectic of the Fe-C-V system and, thus, on the carbide morphology, were investigated. It was evaluated the nucleant potential for aluminum and titanium addition to the bath. The morphologies changes for the austenite/carbide eutectic were evaluated by optical and scanning electronic microscopy. It was observed that aluminum or titanium addition in alloys with residual content of oxygen or nitrogen promoted VC eutectic morphology change from irregular to fibrous (when aluminum is added) and to divorced (when titanium is added). It is concluded that the presence of nucleant agents in the metallic bath induced the preferred nucleation of the faceted phase at higher temperatures, decreasing the cooperation during the eutectic growth.

Homogeneous crystal nucleation in binary hard sphere mixtures is an active area of research for last two decades. Although Classical nucleation theory (CNT) gives a qualitative picture, it fails at high super saturations because of the following reasons. CNT assumes that the cluster formed is spherical in shape, its properties can be modeled using the bulk properties of the stable solid phase and the interfacial free energy γ between the nucleus and the surrounding metastable fluid is equal to the planar surface tension between two phases at coexistence. These assumptions get increasingly tenuous at higher degrees of super saturations where the critical nucleus formed is microscopic in size leading to breakdown in the predictions of CNT. In addition direct experimental observation of critical nucleus is very difficult because, 1. Critical nucleus is microscopic in size, consisting of few hundreds of particles. 2. Formation of critical cluster is very rare (typically of the order of 101– 106nuclei/cm3/s) 3. Its life time is very short (it either rapidly grows to form a solid phase or melts back to fluid) In these circumstances molecular simulations are an attractive tool to study the crystal nucleation, because in these simulations microscopic size critical nucleus properties can be calculated. However, brute force molecular dynamic (MD) simulation techniques to study the homogeneous crystal nucleation is currently not feasible due to long times involved. Hence, an indirect approach is needed. In this work, Monte Carlo Abstract v (MC) molecular simulation techniques are used to calculate free energy barrier height during the crystal nucleation. Phase behavior of Binary hard sphere mixtures with varying ratios of smaller diameter to larger diameter (α) is very similar to that of binary organic liquids. By studying the crystal nucleation in hard sphere system, the physics behind the nucleation for binary organic liquids can be understood. This is the key motivation to study the homogeneous crystal nucleation in binary hard sphere mixtures using MC simulations. Simulations were done using umbrella sampling in combination with local bond order analysis for the identification of crystal nuclei and to compute shape and height of nucleation barrier. Parallel tempering scheme of Geyer and Thomson was utilized to sample phase space more efficiently. Parallel tempering technique was implemented using Message passing interface (MPI) libraries. By using all the above Monte-Carlo simulation techniques, nucleation barrier was calculated during crystallization of binary hard sphere mixtures under the moderate degrees of super cooling in Isothermal-Isobaric semi grand ensembles. Crystal nucleation in binary hard sphere mixtures has been studied for size ratios α = 0.85, 0.42 and 0.43. For α=0.85, phase diagram contains eutectic point. In this system, the effect of eutectic composition on the nucleation barrier height was observed, by calculating nucleation barriers at various fluid mixture compositions keeping Laplace pressure constant. It is observed that as the fluid mixture composition move towards the eutectic point, free energy barrier height, surface tension and critical cluster sizes are increased and the nucleation rate is drastically decreased by a factor of 10-31. Thus the difficulty of homogenous crystal nucleation increases near the eutectic point. For α=0.42 and 0.43 in the hard sphere system, compound solids such as AB and AB2 solids are stable respectively. In these systems crystal nucleation study was done to observe the compound solid formation. It is observed that in these systems crystallization kinetics are very slow and more advanced simulation techniques need to be developed in order to study crystal nucleation.

The use of low-pressure membranes, mainly ultrafiltration (UF), has emerged in the last decade and began to show acceptance as a novel pretreatment process for seawater reverse osmosis (SWRO) desalination. This is mainly due to the superior water quality provided by these membranes, in addition to reduction in chemicals consumption compared to conventional methods. However, membrane fouling remains the main drawback of this technology. Therefore, frequent cleaning of these membranes is required to maintain water flux and its quality. Usually, after a series of backwash using UF permeate chemical cleaning is required under some conditions to fully recover the operating flux. Frequent chemical cleaning will probably decrease the life time of the membrane, increase costs, and will have some effects on the environment. The new cleaning method proposed in this study consists of using a solution saturated with carbon dioxide (CO2) to clean UF membranes. Under the drop in pressure, this solution will become in a supersaturated state and bubbles will start to nucleate on the surface of the membrane and its pores from this solution resulting in the removal of the fouling material deposited on the membrane. Different compositions of fouling solutions including the use of organic compounds such as sodium alginate and colloidal 5 silica with different concentrations were studied using synthetic seawater with different concentrations. This cleaning method was then compared to the backwash using Milli-Q water and showed an improved performance compared to it. An operational modification to this cleaning technique was then investigated which includs a series of sudden pressure drop during the backwash process. This enhanced technique showed an even better performance in cleaning the membrane, especially at severe fouling conditions. In most cases, the membrane permeability was fully recovered even at harsh conditions where conventional backwash failed to maintain a stable operation. Therefore, the new cleaning method might provide an efficient and environmentally friendly alternative cleaning technique to low-pressure membranes technology in the future.