Freezing and melting of sessile droplet on micro- and hierarchically-structured silicon surfaces

Water droplet freezing and melting on three micro- or hierarchically-structured silicon surfaces and a smooth silicon wafer were studied experimentally. The surface temperature was kept at -13.8 degrees C during freezing and increased gradually to room temperature after turning off the bath circulator. The freezing behaviors including the freezing delay, solidification front movement, and the droplet profile were obtained through the photographic method. The results show that the tiny black silicon particles on hierarchical structures facilitate a longer freezing delay. The solidification stage lasted for 5-17 s depending on the surface structures, during which the freezing rate was larger in the initial seconds. Droplet on three micro-structured surfaces formed a tip with the angle of 122 +/- 2 degrees, smaller than that on the smooth silicon which was 136 +/- 3 degrees. The melting onset temperature for samples 1-3 was about 4.4 degrees C while melting happened on smooth silicon needed a smaller degree of superheat.