Optic generation and perpetuation of acoustic bubble clusters.

Laser-induced cavitation bubbles offer precise control of the flow in space and time, but they are rarely used for the mechanical and chemical processing of liquids. Instead, strong acoustic fields are commonly used to nucleate and drive cavitation bubbles for liquid process applications. While acoustic field creates many more cavitation events, the resulting chaotic dynamics offers little control on the fluid mechanics, i.e., where and how bubbles deliver their energy. Here we present a method that utilizes a laser to nucleate a single cavitation bubble, which is then driven into violent oscillations by the ultrasound field, resulting in splitting of the bubble followed by formation of a cluster of cavitation bubbles. This combination offers means for cavitation control not available in conventional acoustic cavitation. Here, the cavitation bubble is generated with a custom build pulsed laser that is focused below a sonotrode driven at 20 kHz. In absence of the acoustic driving the bubble reaches a maximum diameter of 130 µm with a lifetime of approximately 10 µs. In the presence of the acoustic field the first few expansions and bubble collapses are strongly affected by the phase of nucleation. Over successive acoustic cycles a small bubble cluster develops that loses its connection with the phase of generation. We study the dynamics in the free field and constrained by a rigid boundary. For both geometries the cluster over many acoustic cycles dies off, yet through repetitive optical bubble seeding the cluster lifetime and its location can be controlled.