Wetting transitions on superhydrophobic surfaces during recalescence

Supercooled droplet wetting and freezing on surfaces is important in nature and technology. The seemingly superb advantages of rapid supercooled liquid shedding and reduced ice adhesion are wholly reliant on the preservation of the fragile Cassie-Baxter composite wetting state on textured surfaces. Whilst condensation frosting and ice deposition are well documented failure modes for superhydrophobic solutions on the microscale, comparatively little research has investigated how the freezing of macroscopic droplets affect the stability of the non-wetting state. Here, we explore the recalescent freezing of a droplet, induced by rapid evacuation of the atmosphere, on a micropillar textured surface. Through this, we are able to determine the substrate characteristics necessary to induce so-called ice levitation behaviour and, in parallel, identify two distinct mechanisms through which repellency breaks down. These outcomes can then be rationalised through a theoretical model balancing the wetting properties of the surface with those resulting from the recalescence process. Finally, we present the complimentary ambient pressure freezing case, at reduced temperature, and link the two sets of findings together to facilitate the design of icephobic surfaces across the phase diagram.