Droplet Impact on Superhydrophobic Surfaces Under High Pressures.

Though numerous studies on droplet impact have been conducted, the maximum ambient pressure reported is limited to 100 bar, and our understanding of droplet behavior under higher pressures remains unexplored. This study presents the first experimental investigation of droplet impact under high ambient pressure (up to 200 bar) onto different superhydrophobic substrates under low Weber number conditions. Four different regimes are identified, i.e., no bouncing, droplet bouncing with both satellite droplet retention and gas entrapment, droplet bouncing with gas entrapment, and complete droplet bouncing. The transition among different regimes is highly dependent on the ambient pressure and substrate topology. The droplet bouncing capability increases with the increase of ambient pressure, and complete bouncing is achieved for all substrates at P ≥ 175 bar. A phenomenological mode is developed taking into the consideration of both enhanced cushioning effect and hydrodynamic impact dynamics at high pressure. With a modified water hammer coefficient, the hydrodynamic impact model can be used to explain the disappearance of satellite droplet. Such work advances droplet study into 200 bar domain, which is of high relevance to a few high-pressure applications such as deep sea oil/water separation.