Paper detail

Shape modes and jet formation on ultrasound-driven wall-attached bubbles

Understanding how substrate-attached bubbles respond to ultrasound is important for applications from industrial cleaning to biomedical therapy. Under ultrasonic excitation, bubbles can deform through Faraday instability and periodically emit high-speed jets. Although this behavior is increasingly well understood for free bubbles, the dynamics of wall-attached bubbles remain largely unexplored. In particular, the three-dimensional selection and evolution of non-spherical modes and their relation to jetting have not been resolved. We investigate micrometric air bubbles in contact with a rigid substrate and driven by ultrasound, using a dual-view imaging setup combining top-view bright-field microscopy with side-view phase-contrast X-ray imaging. This approach reveals a stepwise evolution of bubble shape through four regimes: spherical oscillations, harmonic axisymmetric meniscus waves, half-harmonic axisymmetric Faraday waves, and the superposition of half-harmonic sectoral Faraday waves. This contrasts with free bubbles, which jump directly to their final Faraday pattern at instability onset. For the chosen substrate, the observed shape-mode spectrum is degenerate and spans a continuous range of mode degrees, consistent with theoretical predictions based on kinematic arguments. Free bubbles, although also degenerate, remain limited to discrete spherical harmonics. Measured ultrasound pressure thresholds for Faraday instability agree with classical interface-stability theory modified for a rigid boundary. Complementary 3D boundary-element simulations reproduce the observed shape evolution. Finally, we identify the acceleration threshold for cyclic jetting: unlike free bubbles, wall-attached bubbles always jet from the side not constrained by the substrate.