Stable-streamlined cavities following the impact of non-superhydrophobic spheres on water†
The formation of a stable-streamlined gas cavity following the impact of a heated Leidenfrost sphere on a liquid surface or a superhydrophobic sphere on water is a recently demonstrated phenomenon. A sphere encapsulated in a teardrop-shaped gas cavity was found to have near-zero hydrodynamic drag due to the self-adjusting streamlined shape and the free-slip boundary condition on the cavity interface. Here we show that such cavities can as well be formed following water impact from a sufficient height of non-superhydrophobic spheres with water contact angles between >30° and 120°. In this case the streamlined cavity is attached just above the sphere's equator, instead of entirely wrapping the sphere. Nevertheless, this sphere with attached cavity formation has near-zero drag and a predetermined free fall velocity in compliance with the Bernoulli law of potential flow. The effect of surfactant addition to the water solution is investigated. The shape and fall velocity of a sphere with streamlined cavity formation were unaffected by the addition of low surface modulus synthetic surfactants, but were destabilised when solutions containing high surface modulus surfactants, such as soaps, were used.