Browsing by Subject "Phase Change"

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    On A General Theory Of Phase Change, Nucleation, And Growth, And The Formation Of Ice In Cryopreserved Systems
    (2023-01) Kangas, Joseph
    In this work we address longstanding gaps in understanding in phase change theories linkingthe nucleation rate, growth rate, growth geometry, and transformed fraction of phase. We take a first principles approach whereby a fundamental understanding of the relationships between these properties can be derived without obfuscation by previous efforts. This is carried out by examining a growing region of space with some prescribed geometry which is transforming from one phase to another, tracking its volume as it grows and intersects with other transforming regions of space. Using this approach, we derive both ordinary and partial differential equations linking the nucleation rate, growth rate, fractal dimension, transformed fraction, phase size distribution, and initial distributions of phase for a system undergoing phase change. We then show that solutions to these equations under special conditions yield methods for extracting nucleation and growth rates for heat release curves, as well as more detailed descriptions of growth geometries. These nucleation and growth rates are important for understanding systems hindered by phase change, including cryobiology, metallurgy, pharmacology, and food science, among others. Extensions to gas phase allow for a deeper understanding of aerosol science and cavitation dynamics as well. Ice crystallization is studied in cryoprotectant agents (CPAs) in low concentrations via direct quenching and laser calorimetry. Critical cooling rates were measured by examining the temperature-time profiles during the direct quenching of droplets of CPA into liquid nitrogen. Critical warming rates were measured by examining ice crystallization in vitrified droplets of CPAs and plasmonic gold nanoparticles during high energy laser irradiation. High-speed imaging allowed for accurate measurements of the temperature rates necessary for avoiding ice formation on rewarming from a vitrified state. A model linking the critical cooling and warming rates in mixtures of CPA was also developed and verified. Additionally, the phase change theory we derived allow for corroboration of the rates necessary for the vitrification of pure water. The laser warming process was also studied numerically via Monte Carlo simulations of light transport in scattering media. The effect of system geometry, absorption coefficient, scattering coefficient, scattering anisotropy, and domain partitioning were studied for a variety of systems including the laser warming of spherical and hemispherical droplets laden with zebrafish embryos and coral nanofragments. Warming uniformity was the main focus of optimization as it is the driving factor in post-warming survival in laser warmed cryopreserved specimens. Laser warming in multi-laser systems is also briefly discussed.