Water droplet impact dynamics comparison on solid and hollow square micropillared substrates

Abstract

We experimentally investigate microliter-sized water droplet impact on solid and hollow square micropillared polydimethylsiloxane substrates. Micropillared substrates with different values of pitch (34, 47, and 62 μm) and hole sizes (0, 3, 6, and 10 μm) of pillars are fabricated using soft lithography following direct laser writer maskless photolithography. We observe that hollow micropillared substrates exhibit increased hydrophobicity as compared to the solid micropillared substrates. Interestingly, we find that hydrophobicity is further enhanced as the hole size is increased. To understand the impact dynamics, we perform high-speed visualization to acquire the transient evolution of the impacting droplets. Based on the impact velocity (0.22–0.62 m/s), pitch, and hole size, we identify various regimes, namely, non-bouncing, partial bouncing, and complete bouncing. At a given impact velocity and pitch value, non-bouncing and bouncing regimes are observed for solid and hollow micropillared substrates, respectively. We find that the hollow micropillared substrate exhibits higher values for capillary pressure, impalement pressure, and the energy barrier associated with the Cassie–Baxter to Wenzel transition toward the impacting droplets. This is due to a decrease in the solid fraction owing to the incorporation of circular holes in pillars. The analysis shows the energy loss due to viscous dissipation decreases with an increase in hole size, which enhances the bouncing fate possibility. The fundamental insights gained from this study can be effectively leveraged by modulating the surface morphology to realize the desired droplet impact characteristics for various potential applications such as self-cleaning and energy harvesting.