Visual and Theoretical Analyses of the Early Stage of Frost Formation on Cold Surfaces

Meso-scale visual observations were conducted to investigate the process of frost formation on both bare and hydrophobic coated copper surfaces, which had contact angles of 56° and 110°. The experiments were carried out for −20−0°C surface temperatures, 19−22°C ambient air temperatures, and 15−85% relative humidities. The tests showed that the frost formation on cold surfaces was not a simple process of transition from steam directly to frost, but actually followed five steps: the formation of condensate droplets, droplet growth including coalescence of the supercooled droplets, freezing of the droplets, formation of initial frost crystals on the frozen droplets, and growth of frost crystals accompanied by collapse of some of the crystals. Compared to the bare copper surface, the hydrophobic surface had a sparser distribution of condensate droplets but larger droplet sizes, delayed droplet freezing and frost formation, and a smaller frost height, all of which support the observation that the hydrophobic surface retards frost formation and growth. The frosting phenomenon was also analyzed theoretically. The initial vapor condensation before frosting was explained based on the free energies for nucleation. For condensation of steam on cold surfaces below 0°C, the Gibbs free energy barrier for water nuclei is smaller than that for ice nuclei, so condensate droplets appear before frost on cold surfaces. Further, since the hydrophobic surface has a higher Gibbs free energy barrier for nucleation than the bare surface, the droplets form more readily on the bare surface.