High-Speed Microdroplet Impact on a Textured Rough Surface: A Numerical Investigation

In this paper, the investigation on dynamics of high-speed micron-sized droplet impact on textured surfaces is carried out through computational fluid dynamics (CFD) simulations. An open-source code Gerris has been adapted to address the challenging issues facing simulations of high-speed impact of microdroplets on the rough surface, such as thin spreading lamella, secondary droplet breakup, and small features of rough surfaces. Validation is first presented to evaluate the accuracy of the simulation code for modeling high-speed droplet impact on the microstructured surface. Then, we carry out 3D simulations of a 10 μm diameter water droplet impact on different textured surfaces with different impact velocities. We find that a large portion of the thin lamella actually surfs over the top of pillars during spreading with only center area of impact saturated with liquid. Our simulations indicate that both impact velocity and surface morphology play an important role in the splashing phenomenon. Increasing pillar spacing makes droplet impact more prone to splashing. Splashing on surfaces of larger pillar spacing is characterized by the breakup of high-speed jets. Larger impact velocity results in more intensified splashing. For a given impact velocity, densely packed pillars (i.e., smaller pillar spacing) can reduce or even suppress the splashing due to viscous drag effect from pillars in wetted region. The existing splashing threshold models that depend only on surface roughness fail in the prediction of the critical speed for splashing on textured surfaces.