Towards an effective capillarity in bubble cluster dynamics under ultrasonic excitation

Acoustically driven bubble clouds largely behave as single entities due to the interaction forces between neighbouring oscillating bubbles, known as the secondary Bjerknes forces. Here, we address the cohesion between oscillating bubbles resulting from mutual attraction by examining the analogy between secondary Bjerknes forces and capillary effects. Such an analogy is qualitatively supported by acoustically excited bubble cluster experiments, inspired from the classical wire-frame experiment designed to demonstrate surface tension effects. The results aim to determine whether the resultant of the secondary Bjerknes forces on a movable bar corresponds to a work done per unit surface, as for surface tension. We complement these experiments with theoretical modelling efforts in an attempt to demonstrate that a bubble cloud can be represented as an effective single bubble, which could greatly simplify bubble cluster modelling.