Dynamics of a single gas bubble under forced acoustic oscillations of very low frequency.*

Although the violent collapse of bubbles due to sufficiently strong variations of the ambient pressure in the surrounding liquid has been first theorized and described in 1917 by Lord Rayleigh, there is a lack of studies concerning this phenomenon when caused by acoustic fields of very low frequencies (<20 kHz). A very fundamental way to explore this topic is to study the dynamics of a single bubble surrounded by a liquid of infinite extent. Here, the single bubble dynamics are theoretically evaluated by means of several mathematical models of increasing complexity, from classical Rayleigh-Plesset-like equations to fully compressible flow models. It is revealed that, as the frequency is decreased, the assumption of incompressible flow for the liquid is increasingly less valid as very high Mach numbers are reached at sensibly lower acoustic pressures. However, the collapse presents the same features as the corresponding ultrasonic one, thus opening the possibility to engineering applications. A bespoke experimental set-up for the exploitation of the bubble dynamics at audible sound frequencies is presented.