The phenomenon of cavity collapse has been the subject of much interest for over a century, due to the extraordinary states of pressure and temperature that may be achieved. Such conditions play an important role in cavitation damage, shock-wave lithotripsy, and ultrasonic cleaning, as examples. Much of our understanding of shock-cavity interaction has derived from experiments on fluids and gels, however strong refractive effects make direct observation of cavity interiors using visible light intractable. Existing experiments have therefore been limited to 2D cases, e.g. cylindrical cavities in transparent liquid and gel media, with extension to 3D and solid media only accessible through modelling.
In this work, we exploit the technique of ultra-high-speed phase-contrast X-ray imaging to study the shock-driven collapse of spherical cavities in a solid media. A combination of single- and two-stage gas guns were used to generate shock pressures between 0.49 and 16.6 GPa in PMMA to investigate the role of strength on the mechanisms of cavity collapse. The large, cm-scale X-ray beam size of beamline ID19 at the ESRF, in combination with a bespoke MHz-class multiplexed imaging system, enabled tracking of this rapid phenomenon from initial shock-wave interaction through to complete collapse, revealing new details of the time evolution of sub-surface damage and flow features. Strength-dominated behaviour, as evidenced by characteristic failure patterns and cavity kinematics, was observed to transition to fluid-dominated dynamics at approximate 2 GPa, consistent with earlier physical/electrical measurements in PMMA. Interestingly, we also find our observations of collapse time for this solid in close agreement with the power-law relationship first proposed by Rayleigh for fluids, with an offset we postulate to be linked to physical parameters. These results hint at a universal relationship which might be used to predict the conditions for collapse, and hence for ene