Singular jetting triggered by drop impact on superhydrophobic surfaces and its applications

The jet induced by liquid droplets impacting nanostructured surfaces is a common phenomenon, which has numerous applications ranging from agriculture to advanced industry. Based on experimental methods, we investigated the jet evolution process induced by water droplets impacting nanostructured superhydrophobic surfaces, and systematically studied the mechanisms of jet and satellite droplets formation. After impacting superhydrophobic surfaces, liquid droplets typically undergo three stages: spreading, retraction, and jetting. Two different jetting mechanisms were observed, resulting in different relationships between the jet radius and jet velocity . At low Weber numbers (3≤We≤6), a cylindrical air cavity is formed in the middle of the droplet around the maximum spreading, which collapses and forms a bottle-shaped structure during retraction. Due to the rapid retraction of the top of the cavity, sub-millimeter bubble is trapped when it closes, which is accompanied by a high-speed singular jet with R_j~25μm and V_j∝R_j^-1. At We>6, the rapid spreading of the droplet causes the air cavity to form and collapse earlier, resulting in the generation of larger jets (R_j~100μm) with a relationship between jet velocity and jet size of V_j∝R_j^-0.5. In the further rising of the jet, Rayleigh-Plateau instability occurs, leading to the formation of satellite droplets. A universal formula describing the necking process of the jet was obtained through non-dimensional analysis, and the relationship between satellite droplets and Weber number was further derived. Finally, the potential applications of the singular jetting have been explored.