Contact-time reduction of viscous droplets impacting a grooved superhydrophobic surface

Adding a macroscale groove structure to the superhydrophobic surface makes the water droplet to bounce in a petal shape and dramatically reduces the contact time of the water droplet. Most studies on petal bouncing have been conducted on water droplets without considering the effects of viscosity. In this study, the bouncing dynamics of glycerol/water droplets impacting a grooved hydrophobic surface were investigated by changing the viscosity and impact speed of the droplets. As the viscosity of the droplets increased, the Weber number range in which petal bouncing occurred decreased. Petal bouncing was observed in up to 50 wt. % glycerol/water droplets with a viscosity approximately six times that of water. In the low Weber number region (We < 25), as the viscosity of the droplet increased, a sufficient amount of capillary energy was not stored in the fluid penetrating the grooved structure, owing to the viscous dissipation of the fluid. In contrast, in the moderate-Weber-number region (25 < We < 40), the impact energy of the droplet became sufficiently large to overcome the viscous force of the fluid, enabling spreading and retraction along the bottom of the structure. This caused a discrepancy between the time at which the retraction of the fluid above the structure started and the time for the fluid to penetrate and empty the structure, resulting in a transition from petal bouncing to conventional rebound. The critical Weber number for petal bouncing was calculated using the energy-balance approach, and the results were similar to the experimentally observed values.