Droplet impact on pillar-arrayed non-wetting surfaces

Droplet impact on pillar-arrayed polydimethylsiloxane (PDMS) surfaces with different solid fractions was studied. The lower and upper limits of Weber number, We, for complete rebound of impacting droplets decreased with decreasing solid fractions. Gaps were visible during the spreading and retraction processes of bouncing droplets on the surface with a solid fraction of 0.06 while no gaps were observed during the retraction process when We was greater than its upper limit, indicating that there existed a transition from the Cassie-Baxter wetting state to the Wenzel wetting state. Therefore, a novel model accounting for the penetration of a liquid into the cavities between the pillars was developed to predict the upper limit of the impact velocity of bouncing droplets. At high We, partial rebound was observed for surfaces with solid fractions of 0.50 and 0.20 while a sticky state was observed for the surface with a solid fraction of 0.06. Moreover, surface roughness has a great influence on the contact time of bouncing droplets. Besides, the maximum spreading parameter was found to follow a scaling law of We(1/4).

Automated summary

The study investigated the impact of droplets on polydimethylsiloxane (PDMS) surfaces with varying solid fractions, revealing changes in the upper and lower limits of the Weber number (We) as the solid fraction decreased. The behavior of droplets on surfaces with different solid fractions involved transitions between Cassie-Baxter and Wenzel wetting states, with the development of a novel model to predict the upper limit of impact velocity. Surface roughness was found to significantly influence the contact time of bouncing droplets, with the maximum spreading parameter following a We(1/4) scaling law.