(a) Sketch of the experimental setup from the side, illustrating the electrowetting geometry. (b) Top view of a crystallizing droplet from the high speed camera.
Contact nucleation is often identified as one of the primary methods for ice formation in the atmosphere, and it is well established that contact nucleation is more efficient than immersion or deposition nucleation with the same substance. The reasons for the high efficiency of contact nucleation are not understood, however, and this implies that the treatment of contact nucleation in cloud models is highly uncertain. We have investigated the possible role of relative motion between an ice nucleating particle and supercooled water using electrowetting as a mechanism for controlling the relative motion. In the laboratory experiments, supercooled water is found to have a significantly enhanced freezing temperature during transient electrowetting with electric fields of order 1 volt per micrometer. High-speed imaging reveals that the nucleation occurs randomly at the three-phase contact line (droplet perimeter) and can occur at multiple points during one freezing event. Possible nucleation mechanisms are explored by testing various substrate geometries and materials. Results demonstrate that electric field alone has no detectable effect on ice nucleation, but the moving boundary of the droplet on the substrate due to electrowetting is associated with the triggering of nucleation at a much higher temperature.
