Electrofreezing of Water Droplets under Electrowetting Fields

Electrofreezing is 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. This situation is in contrast to most previous electrofreezing studies, which have used bare electrodes, involve current flow, and have a volumetric electric field inside the liquid. Infrared and high-speed visualizations of static water droplets are used to analyze surface electrofreezing. Ultrahigh electric fields of up to 80 V/mu m are applied, which is one order of magnitude higher than in previous studies. The results facilitate an in-depth understanding of various mechanisms underlying electrofreezing. First, it is seen that interfacial electric fields alone can significantly elevate freezing temperatures by more than 15 degrees C, in the absence of current flow. Second, the magnitude of electrofreezing induced temperature elevation saturates at high electric field strengths. Third, the polarity of the interfacial charge does not significantly influence electrofreezing. Overall, it is seen that electrofreezing nucleation kinetics is primarily influenced by the three-phase boundary and not the solid-liquid interface. Through careful electrofreezing measurements on dielectric layers with pinholes to allow current flow, the individual role of electric fields and electric currents on electrofreezing is isolated. It is seen that both the electric field and the electric current influence electrofreezing; however, the physical mechanisms are very different.