Droplet impinging on sparse micropillar-arrayed non-wetting surfaces

Wettability of droplets and droplet impinging on sparse micropillar-arrayed polydimethylsiloxane (PDMS) surfaces were experimentally investigated. For droplets wetting on these surfaces, the contact line density model combining stability factor and droplet sagging depth was developed to predict whether the droplets were in the Wenzel or Cassie-Baxter wetting state. It was found that droplets on the sparser micropillar-arrayed PDMS surfaces were in the Wenzel wetting state, indicating that a complete rebound cannot happen for droplets impinging on these surfaces. For the case of droplets impinging on sparse micropillar-arrayed PDMS surfaces, it was found that there existed a range of impact velocity for bouncing droplets on the micropatterned surfaces with a solid fraction of 0.022. To predict the upper limit of impact velocity for bouncing droplets, a theoretical model considering the immersion depth of liquid into the micropillar structure was established to make the prediction, and the lower limit of impact velocity for bouncing droplets can be obtained by balancing kinetic energy with energy barrier due to contact angle hysteresis. In addition, the droplet maximum spreading parameter was fitted and found to follow the scale law of We1/4.