Influence of Electric Fields and Surface Chemistry on Ice Nucleation Kinetics

Abstract

The majority of studies on ice formation have attempted to prevent or reduce ice build-up; very few studies have focused on promoting ice nucleation which would have applications in appliances, cryopreservation, and pharmaceutical freeze-drying. Such studies are also relevant to the synthesis of methane hydrates for natural gas transportation. This paper details a fundamental study on the influence of interfacial electric fields on ice nucleation promotion. Electrofreezing, i.e. applying an electric field has been shown to electrically induce nucleation of supercooled water. The freezing temperatures of supercooled water can thus be increased via electrofreezing. However, the mechanisms responsible for elevating the freezing temperature are unclear. Typically, bare electrodes are submerged in water, which creates a volumetric electric field in water. With this type of electric field, the application of a voltage can result in multiple phenomena such as current flows, chemical reactions and gas bubble formation or growth. It is unclear whether electrofreezing is the result of the electric field or the current flow-related secondary phenomena. In the present work, the role of electric fields and surface charge on electrofreezing is isolated by studying electrofreezing of water droplets on a dielectric layer. This dielectric layer blocks current and creates an interfacial electric field with a build-up of electric charge at the solid-fluid interface. Ultra-high electric fields of up to 80 V/μm were applied, which is one order of magnitude higher than in previous studies. Infrared (IR) thermography was used to capture ice nucleation and determine the electrofreezing temperature. The results show that the electric fields alone can elevate the freezing temperature of water by as much as 15 °C; however, this effect saturates at electric fields of approximately 20–40 V/μm. Also, the electrofreezing effect was found to be polarity independent. Therefore, it is hypothesized that the mechanism underlying electrofreezing is a reduction in the Gibbs free energy for ice crystal nucleation. Furthermore, by intentionally creating pinholes in the dielectric layer, which creates current paths, the influence of electric current on electrofreezing was also studied. It was observed that electric currents and/or other secondary effects, such as bubble generation, further increased the electrofreezing temperatures. Overall, this work fills many existing gaps in the current understanding of electrofreezing. It is seen that both the electric field and electric current influence electrofreezing; however, the physical mechanisms are different.