Review of experimental data associated with the solidification characteristics of water droplets on a cold plate surface at the early frosting stage

Frosting is widely seen in the surface of an evaporator extracting heat from air in heat pump or liquid natural gas fields. The solidification of a water droplet on a cold plate surface, a phase change from liquid status to solid status as reaching its freezing point, is the key process at the early frosting stage. To accurately control the frosting/icing process, it is necessary to well understand the heat and mass transfer mechanism during the water solidification. As a fundamental problem, the characteristics of water droplet's solidification on a cold horizontal flat plate surface is reviewed. The objectives and scopes of this paper are firstly introduced. This is followed by the detailed tips before and during freezing of droplet, including the temperature variation, nucleation rate, opaque phenomenon, singular shape, evolution of freezing front, trapped air bubbles, etc. The literatures mentioned take a long time span, from 1950s to 2020. To analyze some seemingly contradictory conclusions or ideas, some of them are experimentally investigated by authors. The current research obstacles are also discussed, with the meaning of this topic re-emphasized by its large time and space scales. This comprehensive and systematic review might provide an overview of the frontiers around water droplet's solidification, and shed new light on the following experimental design and numerical validation.