Abstract
The freezing of water drops on cold solid surfaces is ubiquitous in nature and has broad implications for industrial processes and applications, causing serious technological, engineering and economic issues. Understanding the physics of drop freezing is not only fundamental and practical but also a prerequisite and basis for developing ice protection and control strategies. Despite longstanding and numerous research efforts, existing knowledge on drop freezing is still limited, as evidenced by the continuous exploration and discovery of new icing phenomena or effects. One such is that, along with the freezing of a supercooled water drop in a dry or/and low-pressure environment, an explosive vapor is emitted to its surrounding space; this vapor can either generate a condensate halo consisting of small drops that further freeze into frost or directly desublimate into ice crystals, promoting ice propagation among the drop clusters deposited on the surface. Here, we extend previous carefully designed experimental studies on the freezing of supercooled drops on solid surfaces under low humidity and pressure to condensation frosting under standard laboratory conditions. Condensate halos were observed to form, grow and eventually disappear in a well-defined “fence” region around freezing drops during condensation frosting also at ambient humidity and pressure on sufficiently hydrophobic surfaces with low thermal conductivities. The evolution of the halo pattern involved multiphase transitions on timescales from milliseconds to seconds. By combining optical and thermal imaging techniques, we assessed the halo characteristics at each stage and elucidated the main underlying heat and mass transfer mechanisms. Our work further advances the physical understanding of complex dropwise freezing processes, and relevant findings can provide guidance for optimizing deicing and defrosting strategies.