Numerical investigation of water entry of hydrophobic spheres

Abstract

We perform numerical simulations to study the dynamics of the entry of hydrophobic spheres in a pool of water. To track the air-water interface during the translation of the sphere in the pool of water, we use the volume of fluid model. The continuum surface force method computes the surface tension force. To represent the hydrophobic surface properties, we use wall adhesion in terms of a static contact angle. We perform simulations with different diameters and impact speeds of the sphere. Our simulations capture the formation of different types of air cavities, pinch-offs of these cavities, and other finer details similar to the experiments performed at the same parameters. Finally, we compare the drag force among the different hydrophobic cases. We further perform simulations of hydrophilic spheres impacting the pool of water and compare the drag force with the analogous hydrophobic cases. We conclude that the spheres with hydrophobic surfaces encounter a lower drag than their hydrophilic counterparts. This lower drag of the hydrophobic spheres is attributed to the formation of the air cavity by the hydrophobic surfaces while translating through the pool of water, which reduces the area of the sphere in contact with water. In contrast, no such air cavity forms in the case of hydrophilic spheres.