Dynamic Wetting Regimes in Droplet Impact on Micropatterned Surfaces

The main objective of this study is to explore the dynamic behavior of a droplet impact at low Weber ($We$) numbers on hydrophobic micropatterned surfaces and to obtain insight into all possible dynamic wetting regimes. A series of continuum simulations has been conducted for a 1 $\mu$L water droplet at $We<30$ on micropatterned surfaces, whose roughness size is on the order of 25 $\mu$m. We examined three surfaces with different solid area fractions of $\phi = 0.04, 0.0443, 0.0625$ but with a similar surface roughness ratio ($r\approx 1.75$), so that their static wetting states are all Cassie-dominant. In total, we find 6 different dynamic wetting regimes, i.e., Cassie, Cassie rebound, temporary penetration rebound, Wenzel, Wenzel to Cassie, and Wenzel rebound. For the surfaces with a smaller $\phi$, more possible wetting regimes have been observed, and hence, the final wetting state is dependent on the initial impact velocity. Moreover, the regime boundaries have been evaluated in terms of $We$ and $\phi$. Our results show that at a given impact velocity, the solid microstructure plays an important role in impact dynamics and determines the final wetting state.