Abstract
Abstract：Frost formation above and below the sub-freezing temperature in the presence and the absence of EHD is studied experimentally. In the present study, the mechanism of frost formation is investigated to exhibit a striking dependence on the environmental temperature. For the ambient temperature above the sub-freezing point, the water vapor deposits on the surface in the form of condensate and the ice columns develop with mutual coalescence; otherwise, the frost appears immediately as the moisture attaches to the cold surface and the ice columns develop with mutual squeezing at the early stage of frost formation. The number of droplets increases with the relative humidity but the size of the droplet decreases with it. A hexagonal structure is observed as the droplets grow over a critical size (d > 80 mm). However, for the ambient temperatures below the sub-freezing temperature, the hexagonal structure is not seen because of the lack of droplet coalescence, and the frost structure is comparatively uneven. In the presence of EHD, the ice column is pulled up towards the electrode and has a skinny and fragile structure. The weak structure can be easily broken up and fall down due to the influence of gravity. The electric polarity also affects the frost formation. For a negative polarity, the frost grows more quickly and the break-off frequency of the ice column is higher. It is likely that this phenomenon is related to the same direction of the dielectric force and the electrostrictive force at a negative polarity whereas the opposite direction of the dielectric force and the electrostrictive force at a positive polarity.