Capillary-scale solid rebounds: experiments, modelling and simulations

TL;DR

This study investigates the rebound behavior of superhydrophobic spheres impacting a fluid surface, combining experiments, simulations, and models to understand the physics across different impact velocities and Weber numbers.

Contribution

It introduces a comprehensive multi-approach analysis, including experiments, simulations, and asymptotic models, to explore capillary rebound phenomena at various impact conditions.

Findings

Rebound metrics depend on impact velocity and physical parameters.

New phenomena observed at extreme Weber numbers.

Models successfully predict experimental outcomes across regimes.

Abstract

A millimetre-size superhydrophobic sphere impacting on the free surface of a quiescent bath can be propelled back into the air by capillary effects and dynamic fluid forces, whilst transferring part of its energy to the fluid. We report the findings of a thorough investigation of this phenomenon, involving different approaches. Over the range from minimum impact velocities required to produce rebounds to impact velocities that cause the sinking of the solid sphere, we focus on the dependence of the coefficient of restitution, contact time and maximum surface deflection on the different physical parameters of the problem. Experiments, simulations and asymptotic analysis reveal trends in the rebound metrics, uncover new phenomena at both ends of the Weber number spectrum, and collapse the data. Direct numerical simulations using a pseudo-solid sphere successfully reproduce experimental data whilst also providing insight into flow quantities that are challenging to determine from experiments. A model based on matching the motion of a perfectly hydrophobic impactor to a linearised fluid free surface is validated against direct numerical simulations and used in the low Weber number regime. The hierarchical and cross-validated models in this study allow us to explore the entirety of our target parameter space within a challenging multi-scale system.