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physrevlett114214501.pdf

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PRL 114, 214501 (2015)

PHYSICAL REVIEW LETTERS week ending 29 MAY 2015

Particle Motion Induced by Bubble Cavitation

Stéphane Poulain,1 Gabriel Guenoun,2 Sean Gart,3 William Crowe,3 and Sunghwan Jung3,*

1Université de Toulouse, ISAE-Supaero, Département Aérodynamique, Énergétique et Propulsion, 10 avenue Edouard Belin, 31400 Toulouse, France

2Department of Physics, ENS Cachan, 61 Avenue du Président Wilson, 94230 Cachan, France

3Department of Biomedical Engineering and Mechanics, Virginia Tech, Blacksburg, Virginia 24061, USA (Received 28 February 2015; published 27 May 2015)

Cavitation bubbles induce impulsive forces on surrounding substrates, particles, or surfaces. Even though cavitation is a traditional topic in fluid mechanics, current understanding and studies do not capture the effect of cavitation on suspended objects in fluids. In the present work, the dynamics of a spherical particle due to a cavitation bubble is experimentally characterized and compared with an analytical model. Three phases are observed: the growth of the bubble where the particle is pushed away, its collapse where the particle approaches the bubble, and a longer time scale postcollapse where the particle continues to move toward the collapsed bubble. The particle motion in the longer time scale presumably results from the asymmetric cavitation evolution at an earlier time. Our theory considering the asymmetric bubble dynamics shows that the particle velocity strongly depends on the distance from the bubble as an inverse-fourth- power law, which is in good agreement with our experimentation. This study sheds light on how small free particles respond to cavitation bubbles in fluids.

DOI: 10.1103/PhysRevLett.114.214501

Cavitation is the physical process of bubble formation in a liquid medium by either decreasing pressure or increasing temperature. These cavitation bubbles are well known for causing undesirable damage in hydrodynamic systems [1] and offer advantages in many other systems, e.g., in sonocatalytic reactors [2], in noninvasive fracturing tools of kidney stones [3,4], and in drug delivery methods [5]. They are also present in natural systems: inside the human body [6], used as a hunting technique of some crustaceans [7,8], in plants [9], or in everyday life [10].

During the cavitation mechanism, a vapor bubble is nucleated and is rapidly turned back to its equilibrium liquid phase [11]. The detailed dynamics of spherical bubbles far from any boundaries, described by the Rayleigh-Plesset equation, has been extensively studied [12,13]. Most experimental configurations of interest are a cavitated bubble occurring near either solid or deformable boundaries [14–18] and a stationary bubble near moving particles [19,20]. For biomedical and engineering applica- tions, the effect of ultrasonic cavitation on deformations or fracture of large biotissue or bioagglomerate [3–5,21] and on collisions of micrometric particles [22] have been studied. However, cavitation in the vicinity of freely moving objects has received less attention, and little is known about how a particle responds to a cavitation bubble of similar size.

In this present work, we propose an experimental study of the interaction between a cavitation bubble and a freely moving particle whose radius is smaller than the maxi- mum bubble radius. We identify the response of the particle to the bubble dynamics, and also develop an

PACS numbers: 47.55.dp, 47.55.N−, 47.55.dd

analytical model for the particle behavior after the disappearance of the bubble that is compared with our experimental data.

There are two ways to initiate cavitation in water according to its phase diagram [23]: by lowering the pressure or by raising the temperature. In practice, pressure- based mechanisms are widely used due to their relatively simple setups [24]; however, with these methods it is difficult to precisely control the cavitation bubble’s loca- tion. As an alternative method, superheated cavitation bubbles are generated by laser pulses [25] or by electric sparks [26,27], and it is more convenient to control and study a single bubble and its effects [28–30]. We have chosen the spark-induced approach in this study. The electric spark is generated by the discharge of capacitors (the equivalent capacitance of the circuit is 23.5 mF) that can be charged up to 50 V. Two tinned copper electrodes linked to the circuit, approximately 0.17 mm in diameter, are touched together at the desired location of the nucle- ation. A trigger initiates the discharge of the capacitors, creating a short circuit and thereby a spark that nucleates a cavitation bubble. Experiments are performed in a Plexiglas tank filled with filtered water at room temperature. The bubble is nucleated far enough from the tank walls and from the air-water surface to neglect the effects of these boundaries. Three different voltages are used to charge the capacitors: 40, 45, and 50 V. Below 40 V no significant movement of the particles has been observed. This nucle- ates bubbles with respective maximal radii of Rb;max 1⁄4 2.6 ` 0.1, 3.3 ` 0.2, and 3.9 ` 0.2 mm; growing times tg 1⁄4 0.45 ` 0.03, 0.58 ` 0.06, and 0.68 ` 0.08 ms; and

0031-9007=15=114(21)=214501(5) 214501-1 © 2015 American Physical Society

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