Shape evolution of water and saline droplets during icing/melting cycles on superhydrophobic surface

In this study, bright nickel coating and superhydrophobic nickel film with micro-nano cones were prepared by electrodeposition. The topography and morphology of films were characterized by SEM and AFM. The evolution of shape parameters of water droplet including contact angle and contact diameter were measured by successive icing/melting cycles at −10/+25 and relative humidity of 30 ± 2%. In addition, the freezing delay time was measured as a function of the surface topography and morphology. In presence of micro-nano cones, longer freezing delay time (110 min) observed in comparison with bright nickel (34 min). Power spectral density of AFM images showed that the superhydrophobic film has higher roughness distribution in all frequencies than bright nickel film. Also, it was shown that saline droplet has similar contact angle with water droplet on superhydrophobic surface (slightly changes from 156 ± 2° to 154 ± 1.5°) in ambient temperature. Furthermore, it was observed that contact angle of pure water and saline droplets was decreased in single icing/melting cycle to 141 ± 2° and 138.6 ± 1°, respectively. The reason was attributed to formation of concave ice, capillary condensation and frosting. Finally, electrochemical measurements showed that after 40 icing/melting cycles (2 h icing at − 20 °C and melting at 25 °C for each cycle) by immersing in pure water and saline solutions, corrosion resistance of the superhydrophobic nickel film presented fairly less protection level, particularly in saline solution.