Numerical study on splashing of high-speed microdroplet impact on dry microstructured surfaces

The high-speed impact of µm-sized droplets on solid surfaces is a key phenomenon in many industrial applications. In this work, we aim to investigate splashing of high-speed microdroplet impact on micropatterned surfaces through full three-dimensional numerical simulations. An adaptive mesh refinement method is employed 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 examples are first presented to evaluate the accuracy of the simulation code for modeling high-speed droplet impact on the textured rough surface. Then, we carried out 3D simulations of a 10 µm diameter water droplet impact on textured surfaces with different impact velocities. We find that during spreading a large portion of the thin lamella actually surfs over the top of pillars with only center area of impact saturated with liquid. When splashing does not occur, the maximum spreading factor for µm-sized droplet impact on smooth surface can be correctly predicted with existing simplified models, whereas these models need to be adjusted for predictions for textured surfaces. Both impact velocity and surface morphology play an important role in the splashing phenomenon. Increasing pillar spacing or reducing pillar height 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) or higher pillars 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.