Mechanism of delayed frost growth on superhydrophobic surfaces with jumping condensates: more than interdrop freezing.

Delayed frost growth on superhydrophobic surfaces (SHSs) with jumping condensates has been found by many researchers recently. However, the mechanism of this phenomenon has not been elucidated clearly. In this study, copper SHSs with or without jumping condensates were selected as the substrates for observing condensation icing at a relative humidity (RH) of 60%. The results showed that only SHS with jumping condensates showed delayed condensation icing. Moreover, when such SHSs were placed upward and the surface temperature was held at -10 °C, some discrete frozen drops first appeared on the SHSs. The following icing mainly occurred on these discrete global crystals and then expanded around them until covering the entire surface. Little macroscopic interdrop freezing phenomenon was found. The growth of the frost front is mainly dominated by jumping freezing (the condensed droplets jumped onto the ice crystals and were frozen) or direct vapor-ice deposition. Using microscopy, we found interdrop freezing occurred, in addition to the two mechanisms mentioned above. By placing the SHS downward at -10 °C and intentionally introducing or eliminating tiny dusts, we confirmed that there were no superhydrophobic defects on our SHSs. The discrete frozen drops first appearing on the SHSs were triggered by tiny dusts falling on the surface before or during condensation icing. The key approach in delaying or resisting frost growth on SHSs with jumping condensates is to retard initial ice crystal formation, e.g., eliminating the edge effect and keeping the SHSs clean.