The collapse of a cavitation bubble near a rigid boundary induces a high-speed transient jet accelerating liquid onto the boundary. The shear flow produced by this event has many applications, examples of which are surface cleaning, cell membrane poration and enhanced cooling. Yet the magnitude and spatio-temporal distribution of the wall shear stress are not well understood, neither experimentally nor by simulations. Here we solve the flow in the boundary layer using an axisymmetric compressible volume-of-fluid solver from the OpenFOAM framework and discuss the resulting wall shear stress generated for a non-dimensional distance, $\unicode[STIX]{x1D6FE}=1.0$ ( $\unicode[STIX]{x1D6FE}=h/R_{max}$ , where $h$ is the distance of the initial bubble centre to the boundary, and $R_{max}$ is the maximum spherical equivalent radius of the bubble). The calculation of the wall shear stress is found to be reliable once the flow region with constant shear rate in the boundary layer is determined. Very high wall shear stresses of 100 kPa are found during the early spreading of the jet, followed by complex flows composed of annular stagnation rings and secondary vortices. Although the simulated bubble dynamics agrees very well with experiments, we obtain only qualitative agreement with experiments due to inherent experimental challenges.

Lab-on-a-chip devices are in strong demand as versatile and robust pumping techniques. Here, we present a cavitation based technique, which is able to pump a volume of 4000 μm³ within 75 μs against an estimated pressure head of 3 bar. The single cavitation event is created by focusing a laser pulse in a conventional PDMS microfluidic chip close to the channel opening. High-speed photography at 1 million frames s⁻¹ resolves the flow in the supply channel, pump channel, and close to the cavity. The elasticity of the material affects the overall fluid flow. Continuous pumping at repetition rates of up to 5 Hz through 6 mm long square channels of 20 μm width is shown. A parameter study reveals the key-parameters for operation: the distance between the laser focus and the channel, the maximum bubble size, and the chamber geometry.

The existence of bulk nanobubbles has long been regarded with scepticism, due to the limitations of experimental techniques and the widespread assumption that spherical bubbles cannot achieve stable equilibrium. We develop a model for the stability of bulk nanobubbles based on the experimental observation that the zeta potential of spherical bubbles abruptly diverges from the planar value below $10\mu \mathrm{m}$. Our calculations recover three persistently reported—but disputed—properties of bulk nanobubbles: that they stabilize at a typical radius of $\sim 100\mathrm{nm}$, that this radius is bounded below $1\mu \mathrm{m}$, and that it increases with ionic concentration.

We analyze systematically the acoustic transients emitted during the collapse of a laser-induced cavitation bubble for $0.4\le \gamma \le 5.2$ . γ is the standoff parameter, the ratio of the distance between the bubble's nucleation place and its maximum size. At the bubble collapse, the acoustic signals recorded contain several pressure peaks with rising times as fast as 18 ns. The time delay, $\Delta t$ , between these peaks is a few hundred nanoseconds apart for bubbles nucleated close to the boundary and decreases with γ. The pressure maxima correspond to shock fronts emitted around the time the bubble reaches its minimum volume and is correlated with the high-speed video recordings of the bubble dynamics. We also show that the amplitude ratio of the first to second acoustic transient is strongly dependent on γ. The experimental results are complemented with numerical simulations based on the Volume-of-Fluid method. The simulated results help clarify the physical mechanisms leading to the generation of acoustic transients and unveil in detail the morphology of the bubble approaching its minimum volume, a feature that cannot be resolved with the high-speed data. Furthermore, the numerical results reveal that the impact of the liquid jet on the rigid wall ensues a local increase in pressure over a significant time period, contrary to the shock wave formed during bubble collapse, which produces a sharp transient pressure peak that propagates radially outwards along the rigid wall. We also discuss the validity and shortcomings of the simulation and how to improve them in the future.

Insect-inspired surfaces repel bubble jets.