Cavitation upon impact: flow and bubble formation upon the underwater interaction of two objects*

Water may form bubbles when it is subject to a change in pressure (i.e., cavitation). It is known to cause rapid volume change, resulting in not only damage to the surface but also the possible application in beneficial ways. Cavitation may be introduced using acoustic pressure waves or a high-speed flow. Moreover, there are various simple ways to cavitate water without using expensive equipment. Bubbles can form when we strike a water-filled bottle (Pan et al., PNAS, 2017), or when a popper snaps to impact the underwater substrate (Kiyama et al., PRFluids, 2024), or when a sphere impacts the substrate (e.g., Seddon et al., EPL, 2012). We are interested in the detailed flow field and the cavitation process upon the underwater impact, where an object can be accelerated fast enough to cavitate water by simply striking it. We report our recent status on the attempt to understand how cavitation phenomena upon impact are related. We employ the particle tracking method to try to capture the overall flow patterns during this event to understand this transient process with the help of some simple theoretical considerations.