Binary Droplet Impact on Square-Micropillared Hydrophobic Substrate

The binary water droplet impact dynamics on square-micropillared hydrophobic surface is investigated experimentally. A primary droplet is first impacted on the substrate and allowed to achieve static condition, which is observed as in the Wenzel state. Thereafter, the secondary droplet is impacted as vertical head-on collision with the primary droplet. The subsequent dynamics behavior is studied considering the effect of the impact velocity (0.22 and 0.5 m/s) and the pitch of the pillars. The maskless photolithography is used to fabricate the square-micropillared surfaces of different pitches ranging from 30 to 76 μm, on which the equilibrium contact angles of water droplets fall within the range of 129° to 139°. High-speed photography and image analysis are performed to present the binary droplet impact dynamics qualitatively and quantitatively, respectively. In quantitative analysis, the temporal variation of the interfacial contact length between the droplets is analyzed. The results show that at lower impact velocity, the secondary droplet bounces off on lower-pitch substrates, while it merges with the primary droplet on higher-pitch substrates. Based on the variation of the impact velocities and pitch of the pillars, three different regimes, namely, no merging/bouncing, late merging, and gentle merging are investigated.