Quantitative imaging of nonlinear interface oscillations in a micropit

Crevice-bubbles are used as cavitation nuclei in ultrasonic cleaning, sonochemistry, lithotripsy and targeted drug delivery. Controlled and safe operation of these techniques require a deeper physical understanding of the system and its dynamics. Here, we investigate analytically and experimentally the behavior of a crevice-bubble trapped in cylindrical 50–100 µm diameter micropits immersed in water. We force the system into vertical oscillations with a piezo-electric element, leading to surface waves at the bubble interface. We model the system using a potential flow approach where, similar to nonspherical bubble dynamics, the dominant nonlinearities are introduced through the boundary conditions. The order of the model is reduced by a Hankel projection of the interface shape on its Bessel modes. Driving the system with sufficiently large acceleration gives rise to nonlinear effects and, in particular, to non-axisymmetric parametric oscillations. Since the stochastic nature of parametric oscillations prohibits the use of stroboscopic methods, experimental validation is done using ultra-high-speed imaging. The 3-D shapes of the interface are then reconstructed using an optical model of the experiment. This enables study of oscillation dynamics, facilitating optimization of crevice-bubble applications. Combined with the analytical model, the system can also be used to acoustically measure the rheology of liquids and surfactants from 1 to 100 kHz, a range key for acoustics and not accessible using standard rheometry techniques.