The phenomena of frosting and defrosting are quite important in evaluation of efficiency of various heat equipments such as heat pump etc. In this paper, hydrodynamic phenomena in frosting and defrosting were studied experimentally. Cooling surfaces were flat or artificially grooved with the size of several tens of microns in pitch and depth. In frosting process, super cooled small water droplets were formed on the cooling surface. In the flat surface, droplets are formed around the cavities which are small holes or scars on the surface. In the artificially grooved surface, droplets are formed in the groove or flat part between grooves. Maximum size of droplets on the artificially grooved surface was controlled by pitch and depth of the grooves. In the defrosting, larger water droplets or water films were formed on the cooling surface. On the flat surface, water droplets or films remained on the surface for long time. On the other hand, on the grooved surface, water droplets or films were drained rapidly through microgroove due to the capillary driven flow.

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