Experimental study on water entry of spheres with different surface wettability

The objective of this paper is to investigate the multiphase flow during water entry of spheres with different surface wettability for low Froude numbers. In the experiment, a synchronous trigger system based on a high-speed digital camera is established to obtain the impact images for both qualitative and quantitative analysis. Firstly, the influence of surface wettability and impact velocity on the hydrodynamics and kinematics of splash and cavity is investigated. The results show that the impact of the hydrophilic case creates a smaller splash and there are obvious differences in both the contact line position and the splash collapse rate due to the surface wettability. In addition, the hydrophilic and hydrophobic spheres experience different cavity regimes as the impact velocity increases. For deep seal cavity cases, two water jetting in opposite directions are formed at the point where cavity pinches off and boost the cavities collapse. After pinch-off, the lower cavity around the hydrophilic sphere is more stable, and the cavity ripples are not obvious compared to the hydrophobic one. Secondly, the effect of surface wettability on the trajectory, velocity and acceleration of the sphere as well as the hydrodynamic force it experiences is further investigated. Results show that the force coefficient in the no-cavity-forming case is dramatically high. In the cavity-forming cases, the hydrophobic sphere undergoes higher force coefficients. The difference in force coefficients is due to the combined effects of the surface wettability and impact velocity. •The multiphase flow during water entry of spheres with different surface wettability is studied by experimental method.•A synchronous trigger system based on a high-speed digital camera is established to obtain the impact images for analysis.•The effect of surface wettability on the splash and cavity dynamics during water entry of spheres is further investigated.•The trajectory and velocity of the sphere as well as the hydrodynamic force it experiences is quantitatively analyzed.