Cavitation structures formed during the collision of a sphere with an ultra-viscous wetted surface

We investigate the inception of cavitation and its structures when a sphere collides with a solid surface covered with a layer of non-Newtonian liquid having kinematic viscosities of up to $\nu_{0}$ = 20,000,000 cSt. Liquids with high visco-elastic properties are shown to enable sphere rebound without any prior contact with the solid wall. Cavitation by depressurization (i.e. during rebound) in such non-contact cases is observed to onset after a noticeable delay from when the minimum gap distance is reached and originate from \textit{remnant} bubbles (remains of the obliterated primary bubble entrapped initially by the lubrication pressure of air during film entry). Contact-cases produced a cylindrical structure attached to the wall having undulations along the cavity interface which were further investigated using high-speed particle image velocimetry (PIV) techniques. We show the existence of shear-stress-induced cavitation during sphere approach towards the base wall (i.e. the pressurization stage) in ultra-viscous films. A theoretical model based on the lubrication assumption is solved for the squeeze flow in the regime identified for shear-induced cavity events to investigate the criterion for cavity inception in further detail.