Cavitation occurs when a liquid experiences low pressure and vapor bubbles (cavitation bubbles) grow explosively and collapse implosively. Cavitation in a liquid containing particles exists in various technological fields, e.g., cavitation erosion of hydraulic machinery in silt-laden rivers, ultrasonic cleaning, kidney stone fragmentation, etc. These processes involve interactions of cavitation bubbles with free and/or surface attached particles, being attributed to the collapsing cavitation bubbles. Upon generation, a cavitation bubble expands with a tremendous velocity, during which period the influences on a nearby particle have not been fully investigated.

Entitled “Particulate Projectiles Driven by Cavitation Bubbles”, a recent work published in Physical Review Letters (128, 044501, 2022) and highlighted as an Editors’ Suggestion reveals an unexpected phenomenon that laser-induced cavitation bubbles provide a noninvasive way to lift millimeter-sized particles from an immersed surface (Figure 1). Shuhong Liu, Zhigang Zuo and colleagues of the Department of Energy and Power Engineering, Tsinghua University placed a spherical particle with diameter of 5 mm made of heavy metals (e.g., stainless steel) on a fixed horizontal substrate and generated a cavitation bubble of diameter about 5 mm by a pulsed laser in the neighborhood of the particle. Using high-speed photography, they observed that the particle was lifted from the immersed surface like a projectile with a controllable vertical displacement depending on the size and the position of the cavitation bubble.

Figure 1. The work highlighted as an Editors’ Suggestion (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.044501 )

Following Rayleigh-Plesset equation, the lifetime of a cavitation bubble with diameter of millimeters in water at 1 atm is shorter than 1 millisecond. The bubble dynamics and its interactions with the particle have to be studied with high-speed photography at a framerate larger than 75,000 fps. In the high-speed recordings, the particle is observed to detach the substrate before the cavitation bubble grows to its maximum size (Figure 2). This indicates that the particle is subjected to a strong lift force during the explosive growth of the cavitation bubble.

Figure 2. Image sequences showing the sudden acceleration of a stainless-steel particle (diameter 5 mm) from a substrate made of stainless steel by cavitation bubbles, with a maximum vertical displacement of the particle as high as 11.5 mm (case A).

To investigate the lift force, the authors formulated a theoretical model for the system of a cavitation bubble, a particle and a rigid substrate. Based on calculations, the maximum lift force exerted on the particle can reach more than 100 times the weight of the particle, resulting in a huge acceleration and thus leading to the projectile motion of the particle. In the experiments, the record of the maximum vertical displacement of the particle reaches as high as 10 times its radius. Besides, the authors identified two different dominant regimes (Figure 3a) and proposed the scaling laws for the maximum vertical displacements of the particles (Figure 3b).

Figure 3. Scaling laws controlling the projectile motion of the particle.

In conclusion, opening up a new mechanism of the particle removal from substrates by cavitation bubbles, this work indicates that cavitation bubbles with “soft” interfaces have huge power to lift particles made of heavy metals from substrates and throw them to a desired height. The findings of this work may be helpful to the laser-assisted noninvasive manipulation of particles, e.g., on-demand collection of samples without mechanical contact from ground, and may be inspiring for the related situation of removing calculi and their fragments from tissues with reduced harm from ablation.

The work selected as an Editors’ Suggestion is judged to be particularly important, interesting, and well written by the editors of Physical Review Letters, one of the most prestigious journals in the field of physics. A highlighted Letter has additional significance, because only about one Letter in seven is highlighted as a Suggestion due to its particular importance, innovation, and broad appeal.

This research was supported by the National Natural Science Foundation of China, the Deutsche Forschungsgemeinschaft, the State Key Laboratory of Hydroscience and Engineering, and the Creative Seed Fund of Shanxi Research Institute for Clean Energy, Tsinghua University. PhD student Zibo Ren is the first author of this work. Associate Professor Zhigang Zuo and Professor Shuhong Liu are the co-corresponding authors.

Citation:

Ren, Zibo, Zhigang Zuo*, Shengji Wu, and Shuhong Liu*. 2022. Particulate Projectiles Driven by Cavitation Bubbles. Phys. Rev. Lett. 128 (4):044501. doi: 10.1103/PhysRevLett.128.044501.

Original Article Link：

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.044501

Editor: Li Han