The removal of surface-attached particles with cavitation bubbles is usually attributed to the jetting or shear stresses when bubbles collapse. In this Letter, we report an unexpected phenomenon that millimeter-sized spherical particles made of heavy metals (e.g., stainless steel), when initially resting on a fixed rigid substrate, are suddenly accelerated like projectiles through the production of nearby laser-induced cavitation bubbles of similar sizes. We show experimentally and theoretically that the motion of a particle with radius $R_p$ is determined by the maximum bubble radius $R_{b,\max }$, the initial distance from the laser focus to the center of the particle $L_0$, and the initial azimuth angle ${\phi }_0$. We identify two dominant regimes for the particle’s sudden acceleration, namely, the unsteady liquid inertia dominated regime and the bubble contact dominated regime, determined by $R_{b,\max }{R}_p/L_0^2$. We find the nondimensional maximum vertical displacement of the particle follows the fourth power and the square power scaling laws for respective regimes, which is consistent with the experimental results. Our findings can be applied to nonintrusive particle manipulation from solid substrates in a liquid.

We investigate experimentally and theoretically the interactions between a cavitation bubble and a hemispherical pendant oil droplet immersed in water. In experiments, the cavitation bubble is generated by a focused laser pulse right below the pendant droplet with well-controlled bubble–wall distances and bubble–droplet size ratios. By high-speed imaging, four typical interactions are observed, namely: oil droplet rupture; water droplet entrapment; oil droplet large deformation; and oil droplet mild deformation. The bubble jetting at the end of collapse and the migration of the bubble centroid are particularly different in each bubble–droplet interaction. We propose theoretical models based on the method of images for calculating the Kelvin impulse and the anisotropy parameter which quantitatively reflects the migration of the bubble centroid at the end of the collapse. Finally, we explain that a combination of the Weber number and the anisotropy parameter determines the regimes of the bubble–droplet interactions.

In this paper, systematical experiments are carried out to study the distinct behavior of a buoyant cavitation bubble near a vertical rigid boundary. Under the combined influence of gravity and the rigid boundary, the bubble centroid migrates rapidly when the bubble completely collapses, and a microjet of intermediate strength subsequently penetrates the bubble in the same inclined direction. By recalling the nondimensional form of the Kelvin impulses, or the pressure anisotropy parameters, we analytically predict the angle between the jet and the horizontal direction at different experimental conditions. We further successfully establish the scaling law of the jet velocity by denoting the displacement of the bubble centroid as the characteristic length scale. The findings of this study contribute to our understanding of the nonspherical cavitation bubble dynamics under the simultaneous influence of multiple factors.

Control of cavitation in confining geometries is vital to the safety of hydraulic conduits and structures. Here, we investigate cavitation bubble dynamics in one typical type of open-ended tube with varying cross sections, i.e., a funnel-shaped tube, defined as a funnel-shaped section connected to a cylindrical section. In our experiments, single cavitation bubbles are generated by spark along the symmetric axis of the funnel-shaped tube at different positions, including in the funnel-shaped section (regime 1), near the throat (regime 2), and in the cylindrical section (regime 3). With high-speed photography, we observe that at the end of collapse, bubbles produce weak jets in regime 1, jets in a shape of an inverted cone in regime 2, and jets in a cylindrical shape with a rounded end in regime 3. With the help of OpenFOAM simulation, we better understand the surrounding flow fields during the bubble evolution. By capturing the main features of the flow fields, we derive dynamics equations for bubbles in each regime. Scaling laws are proposed for the bubble jet velocity, which depends on the position of bubble generation, the inner diameter of the cylindrical section of the tube, and the maximum distance where the bubble's upper surface can reach before the collapse. Our findings may be inspiring for understanding cavitation bubble dynamics in tubular conduits with complex geometries, e.g., cavitation in pipelines and draft tubes in hydrodynamic systems, and ultrasonic diagnosis in blood vessels.

Thermodynamic effects delay the growth of cavitation bubbles and may accumulate to a considerable level in a bubbly cloud. Under thermo-sensitive conditions, due to thermodynamic effects, a bubbly cloud is often believed to behave similarly to a single cavitation bubble with respect to its shape, oscillation, etc. Discrepancies in thermodynamic effects on cavitating flows in previous experimental studies may result from the lack of control of non-dimensional parameter groups under this special condition. In the present paper, we first derive the non-dimensional parameter groups from the dynamics of a single cavitation bubble traveling through a Venturi tube. Among them, three major non-dimensional parameters are proposed for similitude conditions of Venturi cavitation experiments between different liquids, namely, the thermodynamic parameter, the Reynolds number, and the relative cavitation number. Our theory is validated with systematic experiments of Venturi cavitation in water, Freon 113, and fluoroketone in a small-scale closed-circuit cavitation tunnel under well-controlled conditions. Simultaneous high-speed observations from top and front views provide improved measurement of the cavitation characteristics. By comparing the variations of the attached cavity lengths and their oscillation frequencies, we successfully achieve similarities between different working liquids. The results are of particular importance for surrogates, when the original working liquid is too costly or too hazardous, e.g., cryogenic liquid hydrogen LH2 or liquid oxygen LO2.

The unexpectedly low cavitation inception threshold of water is widely attributed to the presence of cavitation nuclei either in the bulk water or attached to immersed surfaces. Being compressible, such objects are expected to respond to modest tensile stresses. Surprisingly, a large corpus of experiments shows that micro- and nanobubbles attached to planar smooth surfaces are unresponsive to tensile stresses, seemingly ruling out their ability to serve as cavitation nuclei. Here, using optical microscopy we show that, although these surface micro- and nanobubbles are not directly responsive to tensile stresses induced by a nearby cavitation bubble, strong short-lived shear flows induced by cavitation jetting cause tethers to extend from the surface micro- and nanobubbles and lead to the pinch-off of daughter bubbles. Our results show a unique and previously unanticipated mechanism for the generation of cavitation nuclei.