Characteristics of entrapped jet and cavity instability following the water entry of a superhydrophobic sphere

Abstract

Following the impact of a superhydrophobic sphere on water, the formation of a stable-streamlined gas cavity undergoes several stages characterized by impact load slamming, cavity flow forming, cavity collapse, and unstable cavity. The former three stages have been extensively investigated. However, limited attention has been given to the post-collapse instability of the gas cavity. In the present study, a systematic experiment is conducted to investigate the evolution of the cavity subsequent to the water entry of a superhydrophobic sphere. The pattern of cavity collapse significantly influences the dynamic characteristics of the downward liquid jets entering the cavity. The liquid jets entrapped in the cavity behave as liquid films composed of tiny individual cavities generated with cavity collapse. Deep sealing with a relatively low Weber number consistently promotes the onset of jet penetration, whereas surface sealing results in droplet penetration. The repulsion of entrapped liquid films from the inner rim of the sphere plays a pivotal role in stimulating the periodic instability experienced by the descending gas cavity. The obtained conclusions shed light on the dynamics of the liquid films entrapped in a gas cavity and the cavity stability.