Freezing of supercooled water drops on cold solid substrates: initiation and mechanism

Drops of liquid are placed on substrates having temperatures below the melting point of the liquids (supercooling). All well-known stages of cooling and freezing (supercooling–recalescence–main solidification–ice cooling) of water drops on both hydrophilic and hydrophobic surfaces were observed using a k-type thermocouple and an IR camera. Duration of the main solidification and the ice cooling stages were found to be dependent on the drop size. However, no conclusive relationship can be made regarding the duration of the supercooling stage with the drop size and the surface hydrophobicity. Using an IR camera, we observed that the solidification initiation of the main drop, which corresponds to the supercooling stage duration, is triggered by the erratic wave-like recalescence front of the surrounding condensed microdrops. The nucleation always initiates from the trijunction and propagates into the drop volume with different structural morphologies determined by temperatures and surface hydrophobicity of the substrate. The recalescence front speed of the main drop varies from 50 to 150 mm/s with supercooling temperatures.