Droplet impact on sparse hydrophobic pillar surface: Impact phenomena, spreading mode, and droplet breakup

Functional surfaces with controllable droplet spreading and breakup dynamics have received widespread attention in self-cleaning, spraying cooling, 3D printing, etc. The arrangement of a microstructure is of great value for the design of functional surfaces. Here, we numerically investigated the droplet impact dynamics on the sparse hydrophobic pillar surface with OpenFOAM. We investigated the effect of Weber number, impact locations, and pillar spacing. Outcomes are most strongly influenced by impact locations, pillar pitch, Weber number, and eight spreading patterns were registered, including circle, square, cross-shaped, Chinese knot, octopus, ellipse, dumbbell, and hexagram. Furthermore, a set of theoretical models were developed for the spreading pattern transition to predict the critical Weber number for different droplet spreading patterns. The breakup dynamics of droplets strongly depend on the spreading patterns and the impact location, which can emit secondary droplets in specific directions. The cross pattern significantly reduces the threshold for secondary droplet generation. The results obtained some essential characteristics for droplet impinging sparse hydrophobic pillar surface, which could provide valuable insights into functional surface design, fluidic-based systems and applications.