Influence of surface chemistry and electric fields on the nucleation of ice and hydrates
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Understanding and controlling the formation of ice and hydrates has important energy-related applications including ice mitigation, methane harvesting from hydrates, and desalination by freezing. This dissertation describes multiple studies to explore the role of surface chemistry and electric fields on the nucleation of ice and hydrates. The first part of this dissertation describes a study on saltwater ice formation. A majority of the available literature on ice mitigation concerns freshwater icing, unlike this study. This work quantifies the influence of surface chemistry and texture on saltwater ice formation. Two kinds of experiments are conducted as part of this effort. The first set of experiments quantifies the influence of surface chemistry on ice nucleation of various saltwater solutions. It is noted that a large number of individual experiments were conducted, which makes the present results statistically meaningful, unlike most previous studies. The second set of experiments studies the dynamics associated with impact of saltwater droplets on supercooled superhydrophobic surfaces. It is seen that the saltwater droplets retract more than freshwater droplets (after impact). The greater bounciness of saltwater droplets is a result of slower ice nucleation propagation kinetics. These experiments indicate that superhydrophobic surfaces will offer greater resistance to impact icing with saltwater than pure water and can remain useful at temperatures as low as -40 °C. The second part of the dissertation includes a detailed study of electrofreezing, i.e., the electrically induced nucleation of ice from supercooled water. This work studies ice nucleation in electrowetted water droplets, wherein there is no electric field inside the droplet resting on a dielectric layer. Instead, there is an interfacial electric field and charge buildup at the solid-liquid interface. Through carefully planned experiments, the influence of electric fields and electric currents on the freezing temperature elevation is quantified. The results facilitate an in-depth understanding of various mechanisms underlying electrofreezing. Firstly, interfacial electric fields alone can significantly elevate freezing temperatures by more than 15 °C in the absence of current flow. Secondly, electrofreezing-induced temperature elevation saturates at high electric field strengths. Thirdly, the polarity of the interfacial charge does not significantly influence electrofreezing. Finally, current flow can further elevate the nucleation temperature by providing additional triggers for nucleation events. Overall, both the electric field and the electric current influence electrofreezing; however, the physical mechanisms are very different. The third part of the dissertation studies a novel concept to induce rapid formation of hydrates. The long induction times (hours to days) associated with hydrate nucleation is a significant barrier to many hydrate-based applications. The present work shows that electro-nucleation can promote rapid hydrate nucleation. Experiments were conducted with tetrahydrofuran (THF) hydrates, which is a model hydrate used in many studies as a precursor to studies with methane hydrates. The results show that hydrate nucleation can be triggered in less than ten minutes by applying electro-nucleation voltages. The voltage-induced current flow through the precursor solution leads to bubble generation on the electrodes. These bubbles act as nucleation sites and provide triggers to initiate rapid nucleation. Electro-nucleation is thus seen a powerful technique to induce rapid hydrate nucleation. In summary, the work conducted in this dissertation significantly advances our current understanding of nucleation and liquid-solid phase change. It also develops and characterizes new tools and concepts to stimulate and control nucleation. Therefore, in addition to the fundamental contributions in the field of phase change, this work can enable the development of new energy-relevant applications.