The coupled impact-freezing mechanism of supercooled droplet on superhydrophobic surface

The coupled effect of dynamics and nucleation during supercooled droplet’s collision on superhydrophobic surface plays an important role in the anti-icing capability of different superhydrophobic surface, however, without any method to evaluate it. In this work, the impact-freezing behaviors of supercooled droplets on surfaces with different wettability, including two typical hydrophobic surfaces, were investigated experimentally. The morphology, size, velocity, and nucleation rate of freezing on each surface at different temperatures were extracted, based on which emphasis was put on discussing the discrepancy of freezing processes and the formation mechanism of freezing morphologies on different superhydrophobic surfaces. The main findings are: (1) The freezing morphology on superhydrophobic surface was independent of contact angle and supercooling degree, but depended on the surface roughness; (2) the interaction between the fast motion of unfrozen water and the generation of ice nucleus dominates in the formation of freezing morphology, while the ice growth process has less influence. On smooth surface, multiple ice nucleus generating before bounce impeded the fast retraction of droplet, forming irregular-hill freezing shape whose size enlarged with decreasing temperature. On rough surface, because of the later nucleation after retraction process finished, the freezing morphology showed convergent sphere shape with supercooling-independent freezing size; (3) considering more complicated impact dynamics, including breaking and bouncing, on different superhydrophobic surfaces, an impact-freezing model was established and could be used to estimate the average frozen spreading ratio.