Condensate Halos in Condensation Frosting.

The freezing of water drops on cold solid surfaces is ubiquitous in nature, and generally causes serious technological, engineering, and economic issues in industrial applications. Despite longstanding research efforts, existing knowledge on dropwise freezing is still limited, as this phase-change phenomenon is always accompanied by complex heat and mass transfer processes. Herein, drop-freezing phenomena in condensation frosting are investigated under standard laboratory conditions of humidity and pressure, highlighting their distinctions from those under some limiting conditions. Condensate halos consisting of massive tiny droplets are observed to form, grow, and eventually fade in a well-defined region around freezing supercooled drops on sufficiently hydrophobic surfaces with low thermal conductivities. The detailed halo evolution is very different from that reported previously in ultradry and low ambient pressure environments, and it shows no identifiable effect on the long-term frost propagation. By combining optical and thermal imaging techniques, this study scrutinizes the halo pattern evolution involving multiphase transitions on timescales from milliseconds to seconds, assesses the halo characteristics at each stage, and elucidates the underlying mechanisms. The work expands the fundamental understanding of complex dropwise freezing dynamics, and relevant findings can provide important guidance for developing anti-icing/frosting strategies.