Study on a vertically falling droplet toward a liquid pool with an inclined bottom wall*

We investigate the evolution of liquid interface deformation after a vertically falling droplet impacts to a liquid pool with an inclined bottom wall. We observed that initially an almost hemispherical cavity was formed, and then asymmetric cavity reversal was observed. Eventually, a tilting jet was measured. For the systematic experiments, substrate angle, depth, droplet diameter, impact velocity, surface tension, and viscosity were varied. We found out that the hemispheric cavity development, which is driven by inertia, is analogous to the impact problems in a deep bath. Next, when the cavity retracts, it shows an oblique conical shape, which is due to the different wave propagation mechanism depending on the bath depth, i.e. a relatively shallow and deep bath. This purely geometry-induced effect causes the tilting jet. Finally, we provided a theoretic model to predict the jet inclination angle by assuming that two different wave propagation competed. Furthermore, we controlled the jet direction by changing the boundary condition of the inclined substrate.