Experimental study on flow fields and bubble entrainments by the water droplet impact using 3D PIV tests

The phenomenon of a water droplet impact on a free surface is studied to understand the physics of free surface bubble entrainment. Particle image velocimetry (PIV) and high-speed image system are used to analyse the flow structure evolutions in the droplet impact cavity period and bubble entrainment cavity period, respectively. The photographic results show that the entrapped surface of the impact cavity remains intact and continuous, and individual bubble entrainment occurs during the secondary entrainment cavity evolution. The instantaneous distributions of velocity fields in the longitudinal and lateral directions are uniform and independent of the approaching impact velocity. For the entrainment cavity evolution, the transverse diffusion intensity is higher than that for the impact cavity period. An individual bubble forms during the transverse velocity penetration across the droplet impact area. Consequently, for the droplet impact on a water surface, the present study implies the presence of a mechanism by which the interior flow field plays a key role in the bubble entrainment.