Proc. Natl Acad. Sci. USA 109, 16073–16078 (2012)

Striking though the formation of ice crystals may be, it is generally regarded as a hindrance in household and industrial settings alike. Extensive research has resulted in anti-ice surfaces that either repel or retard ice formation, but progress in their development is slowed by an incomplete understanding of the mechanism involved. Now, Stefan Jung and colleagues have determined a key role for evaporative cooling in the propagation of frost.

When a droplet attached to a surface freezes, it undergoes rapid crystal growth to form an ice-crystal scaffold. This structure is then filled out as the rest of the liquid freezes isothermally, releasing heat and inducing evaporation. It turns out that the resulting vapour condenses into a frost halo surrounding the droplet, as Jung et al. observed.

They found that the frost halo was capable of initiating freezing in neighbouring droplets — effectively propagating frost formation across a surface. Using heat-transfer analysis and a diffusive vapour transport model, Jung et al. computed the size of the halo and used it to predict subsequent local freezing events. They found that halo size depends on the thermal conductivity of the substrate, adding another piece to the puzzle of how ice propagates.