Generation Of Cavitation Bubbles Research Articles

Bubble Structures Between Two Walls in Ultrasonic Cavitation Erosion

The cavitation bubble structures for the stationary specimen method were clarified for various distances, h, between the stationary specimen and the horn-tip surface. The generated cavitation bubbles constituted a huge number of tiny bubbles and bubble clusters of different sizes. The maximum cluster size was 1.4 mm. The observed cavitation patterns systematically changed during tests from the subcavitating state to the supercavitating state with respect to the separation distance, h. For h <4 mm, the bubbles have a definite trajectory, and the pressure patterns manifest a circular shape as a result of streaming induced by ultrasonic cavitation. The feature morphology of the eroded surfaces revealed that the predominant failure mode was fatigue. In the light of the material failure features and the cavitation patterns, it is also deduced that the important mechanism to transfer the cavitation energy to the solid is shock pressures accompanied by collapsing clusters.

132 水面近傍で発生する膨張波および気泡現象(流体工学IV,一般講演)

This paper reports a preliminary experimental result of the expansion wave and cavitation bubble generation near the water surface. Underwater spherical shock wave generated by detonating of micro explosive of a 10 mg silver azide (AgN_3)pellets propagated and reflected from the water surface. The reflected wave is an expansion wave. The process of expansion wave and cavitation bubble generation near the water surface were observed by shadowgraph method and recorded by a high-speed camera. The pressure distribution near the water surface was measured by a PVDF needle hydrophone.

3Pa5-8 Efficient Generation of Cavitation bubbles in Gel Phantom by Dual-Frequency Ultrasound Exposure(Poster Session)

3Pa5-8 Efficient Generation of Cavitation bubbles in Gel Phantom by Dual-Frequency Ultrasound Exposure(Poster Session)

Modeling the ultrasonic cavitation‐enhanced removal of nitrogen oxide in a bubble column reactor

AbstractA model study of the sonochemical removal of nitric oxide (NO) in a bubble column reactor is presented. The detailed model is developed to investigate the actual cavitation phenomena taking place during the absorption of NO. The expansion and subsequent collapse of cavitation bubble according to the theory of cavity collapse—initially developed by Lord Rayleigh and then improved on by coupling the energy balance equation of the bubble and the chemical reactions taking place inside the cavity to calculate the composition of different species formed during the collapse—are modeled. The model takes into consideration (1) cavitation bubble dynamics, (2) generation and transfer of oxidizing species from bubble collapse through reaction kinetics, (3) transfer of NO from gas to liquid, and (4) chemical reactions of oxidizing species with dissolved NO. The results of the simulations surprisingly indicate that the chemistry induced by ultrasonic cavitation cannot explain the absorption of NO beyond about 30% of the inlet concentration if the mass transfer is assumed to be the same as that in the bubble column without ultrasound. When experimental values of mass‐transfer coefficients, calculated in the studies by other researchers (which are in the range of about five times the physical mass‐transfer coefficient in a bubble column), are used, absorption up to 80% are calculated in the simulations consistent with experimental results obtained from the sonochemical bubble column reactor. The present model provides a framework on which more robust and rigorous models can be developed for the complex gas‐liquid sonochemical systems and reactors. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2397–2411, 2012

Observations of acoustically generated cavitation bubbles within typical fluids applied to a scroll expander lubrication system

An experimental study to evaluate the dynamic performance of three different types of cavitation bubbles is conducted. An ultrasonic transducer submerged into the working fluids of a scroll expander is utilised to produce cavitation bubbles and a high speed camera device is used to capture their behaviour. Three critical regions around the ultrasonic source, between the source and the solid boundary, and across the solid boundary were observed. Experimental results revealed that refrigerant bubbles sustain a continuous oscillatory movement, referenced as “wobbling effect”, without regularly collapsing. Analytical results indicate the influence of several factors such as surface tension/viscosity ratio, Reynolds number and Weber number which interpret that particular behaviour of the refrigerant bubbles. Within the refrigerant environment the bubbles obtain large Reynolds numbers and low Weber numbers. In contrast, within the lubricant and the water environment Weber number is significantly higher and Reynolds number substantially lower. The bubble radius and velocity alterations are accurately calculated during the cavitation process. Lubricant bubbles achieve the highest jet velocity while refrigerant bubbles having the lowest jet velocity are not considered as a destructive mean of cavitation for scroll expander systems.

Droplet printing through bubble contact in the laser forward transfer of liquids

The deposition process of the laser-induced forward transfer of liquids at high laser fluences is analyzed through time-resolved imaging. It has been found that, at these conditions, sessile droplets are deposited due to the contact of a generated cavitation bubble with the receptor substrate, in contrast to the jet contact mechanism observed at low and moderate laser fluences. The bubble contact results in droplets with a larger diameter, a smaller contact angle and a lower uniformity than those of the jet mechanism. Therefore, in order to attain a high degree of resolution this mechanism should be prevented.

Ablation efficiency of α-Al2O3 in liquid phase and ambient air by nanosecond laser irradiation

Ablation efficiency and influences of laser parameters on a material removal rate by a nanosecond laser irradiation of α-Al2O3 are studied in gas and liquid phases. The laser ablation in the air yields maximum material removal rate of 12 ng/pulse using a 4.6-mJ pulse energy at 4-kHz repetition rate, compared to 88 ng/pulse in the water flow. Using a specific interpulse distance and a laser repetition rate further increase material removal rate by factor of 3 and 65, respectively, owing to an optimized lattice temperature and laser pulse interactions with the generated cavitation bubble. For the ablation in the air, these parameters do not significantly affect the ablation efficiency.

Ionic physisorption on bubbles induced by pulsed ultra-sound

Ion flotation processes involve the use of bubbles in order to separate ionic species from a mixed solution. Due to bubble interfaces we may assume null curvature at the molecular scale, where selective ion adsorption might be more easily investigated than with liquid-liquid extraction. In contrast to a classical flotation set-up, where bubbles are introduced via a glass frit, we use here a controlled sono-device generating cavitation bubbles which are initially absolutely clean. Moreover we have a faster process with a smaller device. The liquid phase resulting from the coalescence of the overflowing foam is enriched in some ions versus the initial brine. We show here that this effect follows the Hofmeister series and can be attributed to a weak adsorption of hydrated ions at the surfactant-water interface. The selectivity of alkali metals physisorbed at interfaces is analysed through the concentrations of competing ions remaining in solution by inductively coupled plasma optical emission spectrometry. Cationic selectivity, which is independent of the method for obtaining a foam, is discussed via the Gibbs free energy difference for bulk to hydrated surfactant monolayer. Relative values of effective adsorption energies are determined versus sodium ions taken as reference and correspond to 1-3% of the total hydration free energy.

Single cavitation bubble generation and observation of the bubble collapse flow induced by a pressure wave

This study utilizes a U-shape platform device to generate a single cavitation bubble for a detailed analysis of the flow field characteristics and the cause of the counter jet during the process of bubble collapse caused by sending a pressure wave. A high speed camera is used to record the flow field of the bubble collapse at different distances from a solid boundary. It is found that a Kelvin–Helmholtz vortex is formed when a liquid jet penetrates the bubble surface after the bubble is compressed and deformed. If the bubble center to the solid boundary is within one to three times the bubble’s radius, a stagnation ring will form on the boundary when impinged by the liquid jet. The fluid inside the stagnation ring will be squeezed toward the center of the ring to form a counter jet after the bubble collapses. At the critical position, where the bubble center from the solid boundary is about three times the bubble’s radius, the bubble collapse flow will vary. Depending on the strengths of the pressure waves applied, the collapse can produce a Kelvin–Helmholtz vortex, the Richtmyer–Meshkov instability, or the generation of a counter jet flow. If the bubble surface is in contact with the solid boundary, the liquid jet can only move inside-out without producing the stagnation ring and the counter jet; thus, the bubble collapses along the radial direction. The complex phenomenon of cavitation bubble collapse flows is clearly manifested in this study.

The Effect of Ultrasonic Treatment on Microstructural and Mechanical Properties of Cast Magnesium Alloys

Magnesium alloys are important light metals. In recent years, they have been widely applied in the aerospace and automotive industries, and in the manufacture of communication devices, consumer-electronics appliances and computer products. However, cast magnesium and magnesium alloys are subject to problems due to gas pores, inclusion particles, oxide films, and so on. How to reduce the harm caused by these defects and refine the structure to improve casting quality has become an important topic. In this study, we evaluate the effect on casting quality of an ultrasonic method for treating the melt. The method is based on the generation of cavitation bubbles from ultrasonic treatment of the melt, which induces dispersion and degassing action. Analysis of the microstructure and determination of the mechanical properties of the resultant castings are the basis for identifying the quality of the magnesium and magnesium alloys. The microstructure was evaluated using an optical microscope and scanning electron microscopy (SEM). The elemental constituents of the inclusion particles and oxide films were identified using scanning electron microscopy in conjunction with an X-ray energy dispersive spectrometer and electron probe microanalyzer (EPMA). Finally, the mechanical properties of the magnesium and magnesium alloys, including the tensile strength, the elongation and the hardness, were also determined and discussed. In addition, variations in mechanical properties of cast aluminum and magnesium alloys by ultrasonic treatment are also discussed.

Laser energy dependence on femtosecond laser-induced nucleation of protein

We have investigated femtosecond laser-induced nucleation of hen egg-white lysozyme (HEWL) as a function of the laser pulse energy and pulse time width. This is the first recorded study to confirm that the femtosecond laser-induced nucleation of HEWL can be induced at a specific threshold laser energy. The threshold energy is comparable to that of cavitation bubble generation. The results strongly suggest that morphological changes in the solution are key factors for protein nucleation.

Generation of laser-induced cavitation bubbles with a digital hologram

We demonstrate a method using a spatial light modulator (SLM) to generate arbitrary 2-D spatial configurations of laser induced cavitation bubbles. The SLM acts as a phase hologram that controls the light distribution in the focal plane of a microscope objective. We generate cavitation bubbles over an area of 380 x 380 microm(2) with a 20x microscope objective through absorption of the pulsed laser light in a liquid ink solution. We demonstrate the ability to accurately position up to 34 micrometer sized bubbles using laser energies of 56 microJ.

The pressure field in the liquid column in the tube-arrest method

We have been using the method of tube-arrest as a means of producing transient single cavitation bubble. In the present paper we seek to comprehend the mechanism of production and inquire into the structure of the ab initio pressure field in the arrested liquid column. The generated pressure wave is shown by combining the theoretical analysis with the experimental observation to be a slightly varied version of water hammer. With relatively clean liquid, the magnitude of the tension peak generating the TSB is likely to reach of several millions Pa. It is also shown that the so generated cavitation bubble originating from the gas-containing bulk liquid is in ‘violent’ motion.

Laser-Induced Mixing in Microfluidic Channels

We demonstrate a novel strategy for mixing solutions and initiating chemical reactions in microfluidic systems. This method utilizes highly focused nanosecond laser pulses from a Q-switched Nd:YAG laser at lambda = 532 nm to generate cavitation bubbles within 100- and 200-microm-wide microfluidic channels containing the parallel laminar flow of two fluids. The bubble expansion and subsequent collapse within the channel disrupts the laminar flow of the parallel fluid streams and produces a localized region of mixed fluid. We use time-resolved imaging and fluorescence detection methods to visualize the mixing process and to estimate both the volume of mixed fluid and the time scale for the re-establishment of laminar flow. The results show that mixing is initiated by liquid jets that form upon cavitation bubble collapse and occurs approximately 20 micros following the delivery of the laser pulse. The images also reveal that mixing occurs on the millisecond time scale and that laminar flow is re-established on a 50-ms time scale. This process results in a locally mixed fluid volume in the range of 0.5-1.5 nL that is convected downstream with the main flow in the microchannel. We demonstrate the use of this mixing technique by initiating the horseradish peroxidase-catalyzed reaction between hydrogen peroxide and nonfluorescent N-acetyl-3,7-dihydroxyphenoxazine (Amplex Red) to yield fluorescent resorufin. This approach to generate the mixing of adjacent fluids may prove advantageous in many microfluidic applications as it requires neither tailored channel geometries nor the fabrication of specialized on-chip instrumentation.

The Effect of Frequency Doubled Double Pulse Nd:YAG Laser Fiber Proximity to the Target Stone on Transient Cavitation and Acoustic Emission

Scant information has been published describing the effect of laser fiber distance from the stone target on the mechanism of calculus fragmentation. Using high speed photography and acoustic emission measurements we characterized the impact of laser fiber proximity on stone comminution. We evaluated the effect of laser fiber distance from the stone target on resultant cavitation bubble formation and shock wave generation. Stone fragmentation was assessed using a FREDDY (frequency doubled double pulse Nd:YAG) (World of Medicine, Orlando, Florida) laser and a holmium laser. The FREDDY laser was operated using a 420 microm fiber at an output energy of 120 and 160 mJ in single and double pulse settings, and a pulse repetition rate of 1 Hz. The holmium laser was operated using a 200 microm fiber at an output energy of 1 to 3 J and a pulse repetition rate of 1 Hz. The surface of a 1 cm square BegoStone (Bego, Bremen, Germany) attached to an X-Y-Z translational stage was aligned perpendicular to the laser fiber, which was immersed in a Lucite tank filled with water at room temperature. An Imacon 200 high speed camera was used to capture transient cavitation bubbles at a framing rate of up to 1,000,000 frames per second. Acoustic emission signals associated with shock waves generated during the rapid expansion and collapse of the cavitation bubble were measured using a 1 MHz focused ultrasound transducer. At laser fiber distances of 3.0 mm or less cavitation bubbles and shock waves were observed with the FREDDY laser. In contrast to the holmium laser, the bubble size and shock wave intensity of the FREDDY laser was inversely related to the fiber-to-stone distance over the range tested (0.5 to 3.0 mm). While bubble size was noted to increase with a larger stone-to-fiber distance using the holmium laser, to consistently generate cavitation bubbles and shock waves using the FREDDY laser the laser fiber should be operated within 3.0 mm of the target stone. These findings have significant implications during clinical laser stone fragmentation.

Single Cell Control Based on Femtosecond Laser-induced Nonlinear Phenomena

When a near-infrared (NIR) femtosecond laser is focused in water, several kinds of nonlinear phenomena, such as shockwave propagation, cavitation bubble generation, and jet flow, are induced in the vicinity of the laser focal point. Such phenomena are also induced by near-infrared nanosecond and picosecond lasers, but femtosecond irradiation allows a better localization. Moreover NIR femtosecond laser focussing permits to perform a local processing of cell and tissue because of its efficient multi-photon absorption and photo-mechanical ablation mechanism. In this study, the impulsive force generated in cell culture medium by these nonlinear phenomena was applied to individual control of single animal cells. Mouse NIH-3T3 fibroblast cells cultured on a substrate were independently isolated by the impulsive force, and patterned by transferring them to another substrate. Furthermore nanoparticles and biological molecules are transfected to the cell by the local processing of the cell membrane. These demonstrations are promising for future femtosecond laser methodology for bioscience and biotechnology.

The inception of cavitation bubble clouds induced by high-intensity focused ultrasound

In many therapeutic applications of high-intensity focused ultrasound (HIFU) the appearance of cavitation bubbles is unavoidable, whereas the dynamics of the bubbles induced by HIFU have not been clarified. The objective of the present work is to observe the inception process of cavitation bubble clouds generated by HIFU transducer in water using high-speed photography. Sequential images captured within 600 μs after the onset of ultrasound transmission show the dynamics of cavitation bubbles’ generation, growth, deformation, expansion and collapse in the focal region. However, when the observation time is narrowed to the initial 145 μs, both the still and streak images reveal that the cavitation bubbles astonishingly stay stable in the focal region for at least 60 μs. The results imply that through adjusting the HIFU exposure time while other physical parameters are appropriately chosen, it might be possible to control the generation of stable cavitation bubbles locally in the focal region.

Cavitation selectively reduces the negative-pressure phase of lithotripter shock pulses

Measurements using a fiber-optic probe hydrophone, high-speed camera, and B-mode ultrasound showed attenuation of the trailing negative-pressure phase of a lithotripter shock pulse under conditions that favor generation of cavitation bubbles, such as in water with a high content of dissolved gas or at high pulse repetition rate where more cavitation nuclei persisted between pulses. This cavitation-mediated attenuation of the acoustic pulse was also observed to increase with increasing amplitude of source discharge potential, such that the negative-pressure phase of the pulse can remain fixed in amplitude even with increasing source discharge potential.

Comparison between an open-cage electrode lithotripter and an encapsulated electrode lithotripter

We compare the acoustic and cavitation field of two electrohydraulic lithotripters: one employed an open-cage electrode (OCE) and the other an encapsulated electrode (ECE). Acoustic pressure was measured with a fiberoptic probe hydrophone and cavitation using a passive detector consisting of two confocal transducers. The focused shock waves (FSW) of the two lithotripters were similar in shape and at 20 kV, peak positive pressure (P+) for the OCE was 23.2±4.4 MPa and peak negative pressure (P−) was 9.0±1.5 MPa. For the ECE the peak pressures were 30.8±9.4 and −8.2±1.3 MPa, respectively. However, the direct waves (DW) were different. The OCE-DW was triangular shaped and almost entirely positive pressure, whereas the ECE-DW consisted of numerous cycles and had a peak negative pressure of 1.3 MPa. For both electrodes the passive cavitation measurements exhibited two acoustic emissions characteristic of inertial collapses. For the OCE the first emission was weak (in comparison to the second emission) and was attributed to scattering of the FSW. For the ECE both emissions were of similar amplitude, indicating that the DW generated cavitation bubbles that were forced to a violent collapse by the FSW. The ECE produces two violent cavitation events for every shock wave. [Work supported by HMT and NIH.]

Laser-induced cavitation bubbles for cleaning of solid surfaces

When a high-power laser beam is focused into liquid, it results in a shock wave emission and cavitation bubble generation. Upon inserting a rigid substrate into the liquid, the bubbles migrate towards the substrate due to the Bjerknes attractive force. Due to bubble–substrate and/or bubble–free-surface interaction, a high-speed liquid jet is formed during bubble collapse, and a collapse shock wave is generated at the moment of bubble collapse near the substrate. These shock waves and liquid jet induce large forces acting on the substrate to remove particles from it. For a substrate several millimeters away from the laser focus point, the collapse shock wave and liquid jet play key roles in removal of particles. The cleaning efficiency increases with an increase of laser fluence and decreases with an increase of distance between substrate surface and laser beam focus point or depth below liquid surface. In a case of bubbles close to substrate and liquid-surface boundaries, implosion of the bubbles will give rise to shock waves and liquid jets oblique to the substrate surface with the parallel and perpendicular components of the forces onto the particles. These oblique liquid jets and shock waves result in high cleaning efficiency. A liquid, such as alcohol and commercial washing solution, as the surrounding medium, rather than air or vacuum, can reduce adhesion force and enhance cleaning efficiency.