Laser ablation of a solid target immersed in liquid (such as water) has many important applications such as laser synthesis of nanoparticles, laser micromachining in water, and laser shock peening. Laser ablation of a solid target in water involves complicated physical processes. One important process often involved is the generation and evolution of a bubble in water and attached to the target surface, which may have significant effects on the target and the ambient water, and hence may greatly affect the relevant practical applications. Some experimental studies were reported in the literature on bubble evolutions induced by laser ablation of a solid target in water. However, the reported previous relevant physics-based modeling work is not sufficient. A physics-based model may help improve the process fundamental understanding and generate valuable information to related applications. In this paper, physics-based modeling work has been performed on the shrinking process of a bubble induced by laser metal ablation in water, together with time-resolved shadowgraph imaging experiments to verify the model. The model-predicted bubble evolution agrees reasonably well with the experimental measurement shown in the paper. Under the studied conditions, it has been found that near the bubble collapse moment (i.e., the moment when the bubble shrinks to a minimum size): (1) the bubble shrinks very fast, and the peak fluid velocity magnitude occurs inside the bubble and can exceed ∼550 m/s; (2) the temperature inside the bubble increases very quickly and approaches ∼2000 K; and (3) the pressure inside the bubble becomes very high, and can reach a peak magnitude of ∼380 MPa at the collapse moment at the bubble center. During the shrinking process, a high-pressure region outside and near the bubble wall is generated near the collapse moment, but the temperature of the region outside the bubble mostly remains low.

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The evolution of bubbles in water may be a critical process in many technologies or applications, including several manufacturing processes. Despite the previous work on cavitations and bubbles, the prior investigations in the literature are not sufficient about the effect of a micro-scale structure (such as a microhole sidewall) confinement on the bubble evolution and on the bubble-generated shock waves. In this paper, this effect has been studied using a physics-based model, which has been verified by comparing its predictions with experimental measurements in the literature on bubble evolutions in water without a micro-scale structure confinement. Under the investigated conditions, it has been found that due to the reflection of the shock waves by the microhole sidewall and the interactions among the bubble and the reflected waves, the peak pressure on the hole bottom wall surface has been significantly enhanced. It is good work in the future to study the implications of the discovery on related manufacturing processes and other applications, and how to intentionally utilize the pressure enhancement effect to benefit the related applications and manufacturing processes.

Microholes with varying diameters at different depths are very desirable in various important applications. However, it is very challenging to produce microholes with varying diameters when the variation is in a complicated way and/or when the hole diameter is very small. This paper presents physics-based modeling work on the interactions among a picosecond (ps) laser pulse, a pre-existing plasma plume inside a microhole, and the hole sidewall. The modeling work implies the potential feasibility of a novel dual-pulse laser ablation and plasma amplification (LAPA) process for drilling microholes with varying diameters at different depths.

A numerical model has been developed for focused ultrasound propagation in water towards a solid target, for which the previous modeling work in the literature has been limited. The model predications are reasonably consistent with experimental observations on the cavitation region size at the target surface in the given ultrasound transducer power range, which has provided a verification of the model. The model calculations show that the ultrasound-induced peak negative pressure is highly spatially non-uniform on the side surface of a studied cylindrical target under the investigated conditions. This implies that to uniformly peen the target with focused ultrasonic cavitation peening, a special process design and/or parameter selection may be needed, for which the developed model may provide a useful guiding tool.