Laser-induced cavitation bubble dynamics at different distances from a rigid boundary is investigated using high-speed synchrotron X-ray phase-contrast imaging. This is achieved through the design of a tailored experimental chamber specifically designed to reduce the X-ray absorption along the path length in water while mitigating boundary effects. The highly resolved undistorted radiographs are able to visualize a sharp bubble interface even upon complex shapes, which can serve as high-quality benchmarks for numerical simulations. Here, the measured bubble shapes are compared to simulations using the incompressible boundary integral method. The direct optical access to the high-speed liquid jet provides accurate measurements of the evolution of the jet speed which is contrasted to the simulated results. After the jet has impacted the opposite side of the cavitation bubble, the cavity assumes a toroidal shape, the volume of which can be accurately measured from the radiographs and its temporal evolution compared to the bubble ring model. Thanks to the clear optical access to the cavity lobes throughout the collapse, non-axisymmetric splashing within the bubble resulting from the jet impact, also known as Blake's splashing, is observed and characterized for stand- off parameters of γ < 1. Measurements extracted from the highly resolved visualizations provided herein have been validated against scaling laws for droplet impact on a thin liquid film, which contribute to confirm and elucidate the splashing phenomenon.

Gas-encapsulated droplets have recently been promoted as an effective technique for fluid transport. Shock waves are herein proposed as an instant release mechanism for the encapsulated fluid, which subsequently discharges into the surroundings. This release process relies on the intricate bubble dynamics and droplet response to the shock driving, which are discovered through numerical and theoretical investigations. The key factors involved in the process, such as the complex shock pattern, pressure amplification, and the generation of a sheet jet cascade, are characterized. These observations are further supported by analytical models derived to predict the water hammer pressure, sheet jet velocity, and droplet drift.

Cavitation has been extensively studied in Newtonian fluids, and to a lesser yet significant degree in shear-thinning fluids. However, cavitation has not been previously investigated in shear-thickening fluids, of which a water-cornstarch suspension is perhaps the best-known example. An interesting property of such fluids is that, when subjected to an increase in strain rate, their viscosity increases until they exhibit solid-like behavior and can even fracture. As cavitation bubbles are capable of generating extreme strain rates, they could be affected by shear-thickening fluid behaviour. As visual access is limited by opaque or non-index-matched particles present in such fluids, an experimental study of nominally cylindrical spark-induced cavitation bubbles is conducted in a 2-mm gap between two parallel flat and transparent plates, which allows visualization of the bubbles as they contact the boundary. They are theoretically studied through the cylindrical Keller-Miksis equation adapted to a shear-thickening fluid using a Cross model. For volume fractions starting from ϕ = 0.44, the limit between continuous and discontinuous shear thickening regime, cavitation bubbles deform increasingly until they are replaced by cavitation-induced fracture between ϕ = 0.46 and ϕ = 0.52. Fracture propagation speeds were found to be in the same range as fracture speeds previously reported for pressure-driven cavity expansion, albeit for estimated initial pressures that are now orders of magnitude higher.

No abstract

Upon interaction with underwater shock waves, bubbles can collapse and produce high-speed liquid jets in the direction of the wave propagation. This work experimentally investigates the impact of laser-induced underwater impulsive shock waves, i.e. shock waves with a short, finite width, of variable peak pressure on bubbles of radii in the range 10–500 $\mathrm {\mu }$ m. The high-speed visualisations provide new benchmarking of remarkable quality for the validation of numerical simulations and the derivation of scaling laws. The experimental results support scaling laws describing the collapse time and the jet speed of bubbles driven by impulsive shock waves as a function of the impulse provided by the wave. In particular, the collapse time and the jet speed are found to be, respectively, inversely and directly proportional to the time integral of the pressure waveform for bubbles with a collapse time longer than the duration of shock interaction and for shock amplitudes sufficient to trigger a nonlinear bubble collapse. These results provide a criterion for the shock parameters that delimits the jetting and non-jetting behaviour for bubbles having a shock width-to-bubble size ratio smaller than one. Jetting is, however, never observed below a peak pressure value of 14 MPa. This limit, where the pressure becomes insufficient to yield a nonlinear bubble collapse, is likely the result of the time scale of the shock wave passage over the bubble becoming very short with respect to the bubble collapse time scale, resulting in the bubble effectively feeling the shock wave as a spatially uniform change in pressure, and in an (almost) spherical bubble collapse.