Using megahertz x-ray free electron laser imaging with x-ray pulses of ~25 femtoseconds and a machine-learning strategy, we have conducted comprehensive in situ imaging studies on the dynamics of cavitation bubble clouds in ultrasound fields at the SPB/SFX beamline of the European XFEL. The research unambiguously revealed the quasi-simultaneous implosion of multiple bubbles and simultaneous collapse of bubble cloud in nanosecond scale and their dynamic impacts onto two-dimensional (2D) materials for layer exfoliation. We have also performed multiphysics modeling to simulate the shock wave emission, propagation, impact, and stresses produced. We elucidated the critical conditions for producing instant or fatigue exfoliation and the effects of bonding strengths and structural defects on the exfoliation rate. The discoveries have filled the long-standing missing knowledge gaps in the underlying physics of exfoliating 2D materials in ultrasound fields, providing a solid theoretical foundation for optimizing and scaling-up operation to produce 2D materials in a much more cost-effective and sustainable way.
