Entry and penetration of a superhydrophobic sphere into a deep pool

Abstract

This study experimentally examines the entry and penetration of a superhydrophobic sphere into a quiescent deep pool, with special emphasis placed on the primary and secondary pinch-off of the air cavity existing in its wake. Two aspects are novel in this study. For one, the experiments are performed for a large range of dimensionless sphere densities, where lighter spheres, with their air cavity, exhibit a terminally ascending trajectory and heavier spheres a terminally descending trajectory. The second novel result is a strong correlation of primary and secondary pinch-off times with the Froude number at impact and the dimensionless density. A semi-empirical correlation for the air cavity volume following the primary pinch-off shows excellent agreement with measurements over all dimensionless densities. A scalar force balance predicts a drastic decrease of buoyancy upon pinch-off, reflected also in the abrupt change of deceleration, measured using two orthogonally placed high-speed cameras to capture the time resolved trajectory of the sphere in the pool. Comparisons are drawn between the trajectories of superhydrophobic spheres and those of hydrophilic spheres, measured in a previous study.