A study in contrasts for inhibiting surface frost

Whether on airplane wings or old refrigerators, frost forms when water droplets nucleate on a surface, grow, coalesce, and finally freeze. Although the interactions between water and surfaces can seem simple, they are remarkably complex (see, for instance, “The first wetting layer on a solid” by Peter Feibelman, Physics Today, February 2010, page 34, and the Quick Study by Laurent Courbin and Howard Stone, Physics Today, February 2007, page 84 ). To slow down freezing and decrease freezing temperatures, scientists have explored nanopatterned or superhydrophobic surfaces that delay nucleation, for example, or increase droplet mobility. Now Amy Betz and colleagues at Kansas State University show that so-called biphilic surfaces that combine hydrophilic and hydrophobic regions can lower freezing temperatures even further, to as much as −6 °C. The team’s biphilic samples resemble slices of Swiss cheese on crackers: A hydrophilic substrate shows through regularly spaced holes (200 µm for some samples, 25 µm for others) in a self-assembled monolayer of a hydrophobic polymer. Placed inside a chamber kept at atmospheric pressure, 295 K, and 30%, 60%, or 75% relative humidity (RH), each sample was cooled in 0.5-K steps, with up to 3 hours between steps, until all the visible water droplets on it froze, like those seen here. Freezing was most inhibited at 60% RH. On all the surfaces, the initial droplets were about 5 µm across. But the researchers found significant differences in the surfaces’ average frozen-droplet size and density and in the time it took them to freeze; those differences must be due to how the droplets grow and merge. The researchers explain that the behavior is consistent with the energetics of coalescence. (A. S. Van Dyke et al., Appl. Phys. Lett. 107, 141602, 2015, doi:10.1063/1.4932050 .)