Shape Oscillations of Microbubbles
When bubbles are exposed to ultrasound, their surfaces can destabilise through the Faraday instability, leading to complex shape oscillations and the formation of high-speed jets. We combine dual-view ultra-fast x-ray imaging and numerical modelling to reveal, for the first time, the full three-dimensional dynamics of wall-attached bubbles, uncovering how shape modes emerge, evolve, and drive jet formation through the appearance of singularities.
Contributors: Marco Cattaneo, Louan Presse
When bubbles adhere to a surface and are driven by ultrasound, their behaviour differs fundamentally from that of free bubbles. Using a novel dual-view imaging setup combining bright-field microscopy and phase-contrast x-ray imaging, we tracked the three-dimensional evolution of wall-attached bubbles in unprecedented detail.
We discovered that bubble shapes evolve through four dynamic regimes, from spherical oscillations to the emergence of Faraday waves and their superposition into complex patterns. Unlike free bubbles, which immediately display their final Faraday mode, wall-attached bubbles follow a stepwise transition.
Beyond visualising these shape modes, we identified the threshold interface acceleration at which Faraday lobes collapse into cyclic microjets. Two jetting modes were observed:
- Conical collapse, producing single jets directed towards the substrate
- Parabolic collapse, generating oppositely directed jet pairs
We derived scaling laws governing these collapse events, demonstrating a universal self-similar behaviour driven by inertia and capillarity. Moreover, we quantified how jet speeds depend on bubble size and Faraday wave amplitude, providing predictive insight into jet dynamics.
These findings illuminate the physical mechanisms behind Faraday-instability-driven jetting and establish how substrate effects alter shape mode selection and dynamics compared to free bubbles. Beyond fundamental physics, this understanding has direct implications for biomedical ultrasound safety, targeted drug delivery, and the design of low-power ultrasound systems for efficient biofilm removal. For more details see e.g. our external page journal publication.
