Liquid Microjet Research Articles

Numerical Analysis of Toroidal Bubbles by the Boundary Element Method. Effects of Bubble-Bubble/Bubble-Wall Interactions.

This paper deals with the collapse and rebound of two bubbles with different initial radii and those of a bubble near a plane or curved rigid wall using the boundary element method. The dynamics of a toroidal bubble, which is formed after a liquid microjet passes through a bubble, is taken into consideration. It is shown that the maximum internal pressure of interacting bubbles can be higher than that of an isolated bubble due to the difference of the phase between the motion of two bubbles. It is also shown that the bubble motion is affected by the shape of a rigid wall. Although the maximum internal pressure of a bubble near a convex wall is higher than that near a concave wall, the pressure on the concave wall induced by the bubble collapse is higher than that on the convex wall. The wall pressure is dependent on the translational motion of a bubble toward the wall and the formation of a high pressure region due to the penetration of the microjet.

Analog experiments of tissue damage generated during ESWL treatments

Among the possible causes of ESWL tissue damage, the interaction of shock waves with cavitation bubbles is believed to be the most responsible. A series of analog experiments has been carried out for clarifying the mechanism of tissue damage during ESWL. Interactions of shock waves with single air bubbles in water were examined, including the formation of a rebound shock wave and the generation of liquid microjets. The collapse of air bubbles which were attached to gelatin walls and exposed to shock waves was quantitatively observed using double-exposure holographic interferometry and high-speed cinematography. In these analog experiments, shock waves were created via microexplosives: 10-mg silver azide pellets were pasted on the tip of an optical fiber and detonated with the radiation of a pulsed YAG laser beam. Details of the bubble collapse were well resolved. More complex materials analogous to human tissue have been tested and will be presented also.

Numerical analysis of a gas bubble near a rigid boundary in an oscillatory pressure field

The behavior of a gas bubble near a rigid boundary in an oscillatory pressure field is numerically investigated by means of two kinds of methods. The boundary integral method is utilized to simulate the bubble profiles at various times, and the image theory is also applied for solving the differential equations describing the nonlinear oscillations and migrations of a spherical gas bubble. Both calculations are performed, assuming that the liquid around a bubble is inviscid, incompressible, and irrotational and that the gas inside the bubble follows a polytropic gas law. The influence of the boundary proximity on the bubble motion is characterized by the bubble migration and the deformation of the bubble shape, especially by the liquid microjet formation. It is found that the period and the amplitude of the bubble oscillation are related not only to the nature of the time-dependent pressure in the liquid but also to the bubble location from a rigid wall. Furthermore, the influence of the boundary proximity on the frequency response curve associated with the bubble oscillation is examined on the supposition that the surrounding pressure oscillates with small amplitude and a rigid boundary is relatively far from a bubble.

Interaction of a Bubble Attached to a Gelatine Wall with a Shock Wave. A Study of Tissue Damage Caused by Bubble Collapse.

The interaction between a shock wave and an air bubble attached to a gelatine wall was experimentally and numerically investigated to elucidate the mechanism of tissue damage caused by bubble collapse. The gelatine concentration was varied between 10 and 30% by weight. It is shown by means of highspeed photography that a shock wave of strength 0.45 MPa causes 0.3∼1.8 mm diameter bubbles to produce high velocity liquid microjets with penetration depths of approximately 0.5 ∼1.0 times the initial bubble diameter. Our results show that bubbles collapse violently in response to sudden changes in the ambient pressure generating high pressure and high temperature. Tissue damage due to bubble collapse is caused by the combination of impact and impulsive pressure and thermal effects.

Ultrasonically Induced Cavitation Studies of Electrochemical Passivity and Transport Mechanisms: I . Theoretical

High intensity ultrasound causes the formation of cavitation bubbles which collapse in a manner that, in the presence of a solid surface, form high velocity fluid microjets directed toward the surface. The intense fluid agitation at surfaces in a focused ultrasound field enhances transport of momentum, heat, and mass and influences the behavior and integrity of surface films on metal electrodes. A mathematical model is proposed to determine the reaction and transport between liquid microjets and a reactive solid surface. The conditions were estimated under which oxide depassivation and repassivation occur as a function of ultrasonic intensity, surface film thickness, and fluid microjet surface coverage. The model was based on the concept that cavitation induces sufficient momentum and mass transfer rates (water hammer pressures) at a surface to create oxide film stresses leading to depassivation. The model was used in a companion paper (1) to evaluate experimental data on the corrosion behavior of iron in sulfuric acid.

An experiment on the interaction between a weak plane shock wave and bubbles. Behavior of liquid microjet produced by collapse of single and double bubbles.

This paper is concerned with the interaction between a weak-plane shock wave and bubbles. The behavior of a liquid microjet produced by the collapse of single and double bubbles was observed using a water shock tube, Ima-Con high speed camera and a PVDF pressure sensor with a high frequency response. The behavior of single collapsing bubble agrees qualitatively with previous experimental results, and it is shown that the present experimental method is effective in investigating the bubble collapse and generation of impulsive pressure. The speed of a microjet within the bubble is estimated to be about 100 m/s, or about ten times as much as that of a microjet penetrating the other side of the bubble surface. In the case of double collapsing bubbles the direction of microjet formation is different from the case of single bubble collapse.

Cavitation Microjets as a Contributory Mechanism for Renal Calculi Disintegration in Eswl

The rarefaction shock wave produced by an extracorporeal shockwave lithotripter can result in liquid failure at numerous discrete sites near the second focus. When the liquid fails, vapor-filled cavities can grow to relatively large sizes and subsequently collapse with enormous violence. This phenomenon, called acoustic cavitation, has been shown to cause severe erosion in materials exposed to cavitation fields. It is proposed in this paper that ESWL devices generate acoustic cavitation in vivo and that the high speed liquid microjets produced during cavitation bubble collapse play an important role in renal calculi disintegration.

Cavitation microjets as a contributing mechanism for stone disintegration in ESWL

The intense shock waves that are used for stone disintegration in ESWL can readily produce acoustic cavitation in a variety of liquids. It has been discovered that the damage to metal objects placed at the focus of an ESWL can be attributed to the action of liquid microjets produced by collapsing cavitation bubbles [A. J. Coleman et al., Ultrasound Med. Biol. 13, 69–76 (1987)]. Furthermore, Delius has shown that these shock waves can produce cavitation in vivo [Miller et al., J. Acoust. Soc. Am. Suppl. 1 83, S89 (1988)]. In this paper, a general review will be given of the mechanism of microjet formation in cavitation bubbles and arguments made that cavitation plays an important role in stone disintegration. A film will be shown of microjet formation in collapsing cavitation bubbles induced at low acoustic frequencies. [Work supported in part by NIH and ONR.]

A study on the impulsive pressure of a collapsing cavitation bubble. (An experiment by a local pressure sensor)

In order to distinguish the impulsive pressures of a liquid microjet and a shock wave, a local pressure sensor was manufactured for trial. The local pressure sensor was composed of four local pressure sensor elements, whose sensitive area was made as small as possible. Each pressure sensor element was made of a tip which was cut out of a barium titanate pressure transducer by a string saw to be 0.45 mm in width and 5 mm in length. The gap of each sensor element was filled with soft-type silicon resin to prevent their interaction of outer pressure action. By the local pressure sensor an impulsive pressure generated by the collapse of a spark-induced cavitation bubble was detected and the pressure of a water microjet was distinguished from that of a shock wave. The result was ascertained by high-speed photographs, which indicated water microjet generation in the collapsing cavitation bubble.

Mechanisms of impulsive pressure generation and damage pit formation by bubble collapse

A detailed experimental study has been made to clarify the mechanism of impulsive pressure generation from a single bubble collapsing in a static fluid – this is the most essential and important research task concerned with cavitation damage. First, the general feature of impulsive pressure generation is discussed, and then the impulsive pressure directly contributing to damage is investigated by various means. As a result, it is found that the impulsive pressure causing plastic deformation of material is closely related, directly or indirectly, to the behaviour of a liquid jet. Further more, it is demonstrated that the interaction of a tiny bubble with a shock wave or a pressure wave must be an important effect in producing a local high pressure which causes damage to material. The damage pit caused by the bubble-shock-wave interaction essentially results from the impact pressure from a liquid microjet.