Entry of a sphere into a water-surfactant mixture and the effect of a bubble layer

A rigid sphere entering a liquid bath does not always produce an entrained air cavity. Previous experimental work shows that cavity formation, or the lack thereof, is governed by fluid properties, wetting properties of the sphere and impact velocity. In this study, wetting steel spheres are dropped into a water-surfactant mixture with and without passing through a bubble layer first. Surprisingly, in the case of a water-surfactant mixture without a bubble layer, the critical velocity for cavity formation becomes radius dependent. This occurs due to dynamic surface tension effects, with the local surface tension in the splash increasing during surface expansion and decreasing as surfactant molecules adsorb to the newly formed interface. The larger sphere radii take longer to submerge and hence allow more time for the surface tension to decrease back to the equilibrium value and decrease the critical velocity for cavity formation. When a soap bubble layer is present subsurface cavities form at all impact velocities. Our analysis shows that the bubble layer wets the sphere prior to impact with a patchy coating of droplets and bubbles. The droplets alter the splash and create an aperture for air entrainment which leads to cavity formation at wetted locations on the sphere surface. The water-surfactant entry behavior of these partially wetted spheres results in a progression of cavity formation regimes with increasing Weber number, similar to the cavity regimes of hydrophobic spheres entering water. Nonuniform droplet coatings create cavity asymmetries altering transitions between these regimes.