Effects of Surface Wettability on the Dewetting Performance of Hydrophobic Surfaces

We studied the impact dynamics of a droplet on two types of surfaces, i.e., nanostructured/hierarchical (NS/HS) surfaces, with different extents of hydrophobicity. It was found that the contact time is related to wetting hysteresis. It can be concluded that wetting hysteresis plays a significant role in the contact process of bouncing drops based on the work done against resistance produced by contact angle hysteresis (CAH). For similar surface roughness, the work done by CAH dominates, and a lower CAH creates a smaller contact time. Compared with NS surfaces, the energy stored during the Cassie-Baxter/Wenzel state transition because of the more pronounced air pocket formation provides the upward kinetic energy, resulting in rapid detachment of a droplet from HS surfaces. Thus, HS-3 has a smaller contact/elongation time (∼8/2 ms) because of the enhanced air pocket formation and more favorable wettability (larger contact angle (CA) and smaller contact angle hysteresis (CAH)) than other surfaces. In addition, the results show that surface morphology affects the contact time of bouncing drops mainly by influencing the elongation stage. For different Weber numbers (We), the upward energy storage dominates and results in different varying trends of contact time with We for NS-3 and HS-3. For further study, the morphology evolution of bouncing drops with We was also investigated in detail. The results show that a satellite droplet is launched in a certain We range because of high adhesion resulting from the Cassie-Baxter/Wenzel state transition. These findings provide guidelines for the preparation of surfaces for both self-cleaning and anti-icing purposes.