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Thesis (M.S.)--University of Wisconsin--Madison, 1984. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 127-131).

Thesis (M.S.)--University of Wisconsin--Madison, 1983. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 138-140).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1981. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 241-247).

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Studies of nucleation in freezing nanoparticles usually assume that the embryo of the solid phase is completely wet by the liquid and forms in the core of the droplet. However, recent experiments and computer simulations have suggested that some nanoparticles start nucleating at the liquid-vapor interface of the drop in a pseudoheterogeneous process. The goal of the present work is to propose phenomenological models suitable for the study of surface nucleation in nanoparticle systems that can be used to understand the contributions of the various surface phenomena, such as surface and line tensions, to the nucleation barrier.<p/> The nucleation barrier for the freezing of a 276 atom gold cluster is calculated using Monte Carlo simulation techniques while previous simulation studies of a 456 atom gold cluster are extended in order to find the probability that the embryo forms in the surface or core of the nanoparticle. These calculations confirm that the crystal embryo forms at the liquid-vapor interface. Geometric studies measuring the liquid-solid and solid-vapor surface areas of the embryo suggest that it changes shape as it becomes larger and grows in towards the core of the droplet.<p/> Three phenomenological models that are based on the capillarity approximation and can account for surface nucleation are proposed. These models highlight the importance of accounting for the surface curvature contributions related to the Tolman length and the presence of the three phase contact line in calculating the nucleation free energy barrier. In some cases, the models are able to reproduce the qualitative properties of the free energy barriers obtain from simulation but numerical fits of the models generally result in estimates of the solid-liquid surface tension that are lower than the values expected on the basis of partial wetting in the bulk.<p/> Finally, a semi-phenomenological model approach to nucleation is proposed where the usual phenomenological expression for the free energy barrier is retained, but where the geometric prefactors are obtained from molecular simulation of the embryo. This method is applied to nucleation in the gold cluster and to the freezing of a bulk Lennard-Jones liquid.<p/>

Studies of nucleation in freezing nanoparticles usually assume that the embryo of the solid phase is completely wet by the liquid and forms in the core of the droplet. However, recent experiments and computer simulations have suggested that some nanoparticles start nucleating at the liquid-vapor interface of the drop in a pseudoheterogeneous process. The goal of the present work is to propose phenomenological models suitable for the study of surface nucleation in nanoparticle systems that can be used to understand the contributions of the various surface phenomena, such as surface and line tensions, to the nucleation barrier.<p/> The nucleation barrier for the freezing of a 276 atom gold cluster is calculated using Monte Carlo simulation techniques while previous simulation studies of a 456 atom gold cluster are extended in order to find the probability that the embryo forms in the surface or core of the nanoparticle. These calculations confirm that the crystal embryo forms at the liquid-vapor interface. Geometric studies measuring the liquid-solid and solid-vapor surface areas of the embryo suggest that it changes shape as it becomes larger and grows in towards the core of the droplet.<p/> Three phenomenological models that are based on the capillarity approximation and can account for surface nucleation are proposed. These models highlight the importance of accounting for the surface curvature contributions related to the Tolman length and the presence of the three phase contact line in calculating the nucleation free energy barrier. In some cases, the models are able to reproduce the qualitative properties of the free energy barriers obtain from simulation but numerical fits of the models generally result in estimates of the solid-liquid surface tension that are lower than the values expected on the basis of partial wetting in the bulk.<p/> Finally, a semi-phenomenological model approach to nucleation is proposed where the usual phenomenological expression for the free energy barrier is retained, but where the geometric prefactors are obtained from molecular simulation of the embryo. This method is applied to nucleation in the gold cluster and to the freezing of a bulk Lennard-Jones liquid.<p/>

<p> A brief review of the theoretical and experimental aspects of liquid droplet nucleation from vapours and of crystal nucleation from aqueous solution is presented. In order to study the nucleation and crystal growth of several analytically important metal chelates, methods were developed to measure the size distribution of crystals growing in a supersaturated solution. These methods involved rapid mixing techniques followed by measurement of the size distribution of the precipitated particles using a Coulter counter and multichannel analyser. The size distributions were dumped from the analyser onto magnetic tape and recovered using computer methods. The mixing and counting techniques were calibrated and tested using barium sulphate and spheres of known size distribution. </p> <p> From the size distributions obtained for the metal chelates, conclusions were made regarding the nucleation step, and the parameters important in the classical Volmer-Weber-Becker-Doring theory of nucleation were calculated. The validity of the values were evaluated and comparisons made with values obtained by other workers. </p> <p> The laws controlling the crystal growth of the metal chelates and their importance in elucidating the type of nucleation process were also investigated. </p> / Thesis / Doctor of Philosophy (PhD)

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