Fast particle ejection by a growing laser-induced bubble

Abstract: Cavitation involves the physical processes of the "explosive" growth, or expansion, and the "implosive" collapse of vapor-filled bubbles, driven by tension in the liquid, or a local deposition of energy. In hydrodynamic systems, the erosion of solid materials is among a number of undesirable effects promoted by cavitation, which can damage or impair the components of hydraulic machines. The causal mechanism of the damaging power of cavitation is believed to be a consequence of the effects of high-speed micro-jets resulting from the asymmetric collapse of cavitation bubbles near solid boundaries.

Nevertheless, a much greater erosion rate has been observed when cavitation occurs in silt-laden water as compared with the sum of both cavitation and silt erosion considered separately. Despite a few analytical attempts, the micro-mechanism of this enhancement of erosion, in terms of the interactions between the cavitation bubbles and the silt particles, remains poorly understood. In fact, the coexistence of cavitation bubbles and solid particles is also found in other scenarios, e.g., ultrasonic cleaning, kidney stone fragmentation, and drug delivery by ultrasonic cavitation.

To simulate the dynamics of a particle near a cavitation bubble, we designed and fabricated an experimental setup, and for the first time study the interactions between a laser-induced vapor bubble and a free-settling particle. Also, we establish a force balance model to account for the bubble and particle dynamics. Results show that four non-dimensional parameters influence the particle-bubble dynamics. The maximum particle velocity and the conditions of single particle-bubble bounce have been reasonably predicted.

In cases when the particle is initially in very close proximity to the bubble center, we also discover the high-speed ejection of the particle, and the formation of a cavity behind the particle. The maximum particle velocity reached over 60 m/s, which is enough to induce plastic deformation of a stainless steel plate. The fact that a similar phenomenon of high-speed particle ejections occurs with a system of tension-generated bubbles and suspended particles suggests that the dynamics described here may serve as one of the causal mechanisms for the enhanced cavitation erosion in silt-laden water.