Formation Of Cavitation Bubbles Research Articles

Cavitation Behavior Observed in Three Monoleaflet Mechanical Heart Valves Under Accelerated Testing Conditions

Accelerated testing provides a substantial amount of data on mechanical heart valve durability in a short period of time, but such conditions may not accurately reflect in vivo performance. Cavitation, which occurs during mechanical heart valve closure when local flow field pressure decreases below vapor pressure, is thought to play a role in valve damage under accelerated conditions. The underlying flow dynamics and mechanisms behind cavitation bubble formation are poorly understood. Under physiologic conditions, random perivalvular cavitation is difficult to capture. We applied accelerated testing at a pulse rate of 600 bpm and transvalvular pressure of 120 mm Hg, with synchronized videographs and high-frequency pressure measurements, to study cavitation of the Medtronic Hall Standard (MHS), Medtronic Hall D-16 (MHD), and Omni Carbon (OC) valves. Results showed cavitation bubbles between 340 and 360 micros after leaflet/housing impact of the MHS, MHD, and OC valves, intensified by significant leaflet rebound. Squeeze flow, Venturi, and water hammer effects each contributed to cavitation, depending on valve design.

Examination of laser microbeam cell lysis in a PDMS microfluidic channel using time-resolved imaging.

We use time-resolved imaging to examine the lysis dynamics of non-adherent BAF-3 cells within a microfluidic channel produced by the delivery of single highly-focused 540 ps duration laser pulses at lambda = 532 nm. Time-resolved bright-field images reveal that the delivery of the pulsed laser microbeam results in the formation of a laser-induced plasma followed by shock wave emission and cavitation bubble formation. The confinement offered by the microfluidic channel constrains substantially the cavitation bubble expansion and results in significant deformation of the PDMS channel walls. To examine the cell lysis and dispersal of the cellular contents, we acquire time-resolved fluorescence images of the process in which the cells were loaded with a fluorescent dye. These fluorescence images reveal cell lysis to occur on the nanosecond to microsecond time scale by the plasma formation and cavitation bubble dynamics. Moreover, the time-resolved fluorescence images show that while the cellular contents are dispersed by the expansion of the laser-induced cavitation bubble, the flow associated with the bubble collapse subsequently re-localizes the cellular contents to a small region. This capacity of pulsed laser microbeam irradiation to achieve rapid cell lysis in microfluidic channels with minimal dilution of the cellular contents has important implications for their use in lab-on-a-chip applications.

Sub-epithelial gas breakthrough during femtosecond laser flap creation for LASIK

Introduction:The femtosecond laser produces photodisruption at the molecular level to generate plasma, displacing the surrounding tissue resulting in the formation of cavitation bubbles. We report a case of myopic LASIK...

Optimisation of sludge disruption by sonication

This study presents an examination on the correlation of sonication operating condition, sludge property, formation and behaviour of cavitation bubbles in sludge disruption under low-frequency ultrasound sonication. The influence of sonication time, sonication density, type of sludge and solids content on the disruption was evaluated. The most vigorous particle disruption was achieved in the initial period of sonication, which subsided subsequently. The explosive effect was likely due to the rapid cavitation arising from powerful transient bubbles generated in fractions of microseconds. A rating for the type of sludge was derived based on the finding that particles in secondary sludge were more readily disrupted than both primary sludge and mixed sludge. While sonication density exhibited the most significant role in cavitation bubble formation and behaviour, particle disruption could be optimised for energy input by sonicating at higher sonication density and shorter sonication time. Based on theoretical consideration, it was deduced that within an optimum sludge solids content ranging between 2.3% and 3.2%, superior particle disruption could be accomplished within a minute for secondary sludge sonicated at a density of 0.52 W/mL. Useful guidelines for sonication system installation, equipment protection and process reliability could be established from knowledge derived from a good understanding on the influence of solids content on sludge sonication.

Mechanism for cavitation of monoleaflet and bileaflet valves in an artificial heart

It is possible that mechanical heart valves mounted in an artificial heart close much faster than those used for clinical valve replacement, resulting in the formation of cavitation bubbles. In this study, the mechanism for mechanical heart cavitation was investigated using the Medtronic Hall monoleaflet valve and the Sorin Bicarbon bileaflet valve mounted at the mitral position in an electrohydraulic total artificial heart. The valve-closing velocity was measured with a charge-coupled device (CCD) laser displacement sensor, and images of mechanical heart valve cavitation were recorded using a high-speed video camera. The valve-closing velocity of the Sorin Bicarbon bileaflet valve was lower than that of the Medtronic Hall monoleaflet valve. Most of the cavitation bubbles generated by the monoleaflet valve were observed near the valve stop; with the Sorin Bicarbon bileaflet valve, cavitation bubbles were concentrated along the leaflet tip. The cavitation density increased as the valve-closing velocity and the valve stop area increased. These results strongly indicate that squeeze flow holds the key to cavitation in the mechanical heart valve. From the perspective of squeeze flow, bileaflet valves with a low valve-closing velocity and a small valve stop area may cause less blood cell damage than monoleaflet valves.

Acoustic bubble simulations for biomedical applications using boundary element method (BEM)

High intensity soundwaves, such as shockwaves and HIFU, are widely used in biomedical applications, for example, extracorporeal shockwaves lithotripsy (ESWL) and HIFU prostate cancer treatment. The high pressure in the tissue and nearby fluid often causes formation of cavitation bubbles. Our previous simulation results via BEM approach show complex bubble ultrasound interactions: in certain cases a jet is formed directed away from the nearby tissue while in others, towards it [Fong et al., Ultrasound Biol. Med., 32(6), 925–942, 2006]. In the present work, pulsed ultrasound microbubble interaction is studied using the same code to provide further understanding to the jetting behavior of the bubbles near (different) biomaterial surface which is critical to the success of tissue ablation and the minimization of auxiliary damages. Separately, we have also simulated shockwave bubble interaction using the BEM code with excellent concurrence to other compressible numerical schemes such as arbitrary Lagrangian-Eulerian and free Lagrange methods [Klaseboer et al., Comput. Methods Appl. Mech. Eng. 195, 4287–4302 (2006)] and experiments. It is suggested that the (primary) dynamic response of the bubble to soundwaves is mainly inertia controlled since compressibility of the fluid is not modeled.

Mechanisms of femtosecond laser nanoprocessing of biological cells and tissues

We investigated the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80 MHz repetition rate and with amplified pulses of 1 kHz repetition rate. Plasma formation in water, the evolution of the temperature distribution, thermoelastic stress generation, and stress-induced cavitation bubble formation were numerically simulated for NA = 1.3 and the outcome compared to experimental results. A comparison of the thresholds for the various physical effects with experimental parameters enables to assess the working mechanisms of both modalities for cell surgery.

High-density bump formation on a glass surface using femtosecond laser processing in water

Micrometer-sized bumps were formed on a glass surface using a focused femtosecond laser processing in water. The bumps were formed over a wide ranges of pulse irradiation parameters, including irradiation energy and focus position. The bumps exhibited a wide variety of morphologies and sizes depending on the parameters. The use of a liquid, namely heavy water, which returns after breakdown and cavitation bubble formation, enabled us to fabricate bumps with high spatial density, which is not possible using a solid coating that is ablated. A desired arrangement of bumps on a glass surface was fabricated by tuning the processing time interval to be more than the disappearance time of a bubble, generated by focusing a femtosecond laser pulse near the water/glass interface.

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.

Anthracene Crystallization Induced by Single-Shot Femtosecond Laser Irradiation: Experimental Evidence for the Important Role of Bubbles

A mechanism of femtosecond laser-induced crystallization was investigated using a supersaturated solution of anthracene. When a single-shot femtosecond laser pulse with a pulse energy above 3.1 μJ/pulse was shot into a sufficiently supersaturated solution, crystallization of anthracene was induced immediately after irradiation at the vicinity of the laser focal point. The threshold energy of the crystallization (3.1 μJ/pulse) was in agreement with that of laser-induced bubble formation, which was a sequential process after shockwave emission, cavitation bubble formation, and collapse. Sufficient supersaturation for crystallization decreased with an increase in the pulse energy. These results suggest that crystallization is triggered in the processes resulting in bubble formation. Furthermore, crystallization was enhanced at the surface of the bubble. The crystallization mechanism was completely different from that reported previously based on photochemical reactions or molecular alignment due to a strong optical field.

Mechanism for cavitation of monoleaflet and bileaflet valves in an artificial heart

It is possible that mechanical heart valves mounted in an artificial heart close much faster than those used for clinical valve replacement, resulting in the formation of cavitation bubbles. In this study, the mechanism for mechanical heart cavitation was investigated using the Medtronic Hall monoleaflet valve and the Sorin Bicarbon bileaflet valve mounted at the mitral position in an electrohydraulic total artificial heart. The valve-closing velocity was measured with a charge-coupled device (CCD) laser displacement sensor, and images of mechanical heart valve cavitation were recorded using a high-speed video camera. The valve-closing velocity of the Sorin Bicarbon bileaflet valve was lower than that of the Medtronic Hall monoleaflet valve. Most of the cavitation bubbles generated by the monoleaflet valve were observed near the valve stop; with the Sorin Bicarbon bileaflet valve, cavitation bubbles were concentrated along the leaflet tip. The cavitation density increased as the valve-closing velocity and the valve stop area increased. These results strongly indicate that squeeze flow holds the key to cavitation in the mechanical heart valve. From the perspective of squeeze flow, bileaflet valves with a low valve-closing velocity and a small valve stop area may cause less blood cell damage than monoleaflet valves.

Mass transfer enhancement produced by laser induced cavitation

A microelectrode is used to measure the mass transfer perturbation and characteristics during the growth and subsequent collapse of a single bubble (which, following its initial expansion, achieved a maximum radius, R m, of ∼500–1000 μm). This mass transfer enhancement was associated with the forced convection, driven by bubble motion, as the result of a single cavitation event generated by a laser pulse beneath a 25 μm diameter Au microelectrode. Evidence for bubble growth and rebound is gained from the electrochemical and acoustic measurements. This is supported with high-speed video footage of the events generated. A threshold for the formation of large cavitation bubbles in electrolyte solutions is suggested.

Mechanism for cavitation phenomenon in mechanical heart valves

Recently, cavitation on the surface of mechanical heart valve has been studied as a cause of fractures occurring in implanted Mechanical Heart Valves (MHVs). It has been conceived that the MHVs mounted in an artificial heart close much faster than in vivo sue, resulting in cavitation bubbles formation. In this study, six different kinds of monoleaflet and bileaflet valves were mounted in the mitral position in an Electro-Hydraulic Total Artificial Heart (EHTAH), and we investigated the mechanisms for MHV cavitation. The valve closing velocity and a high speed video camera were employed to investigate the mechanism for MHV cavitation. The closing velocity of the bileaflet valves was slower than that of the monoleaflet valves. Cavitation bubbles were concentrated on the edge of the valve stop and along the leaflet tip. It was established that squeeze flow holds the key to MHV cavitation in our study. Cavitation intensity increased with an increase in the valve closing velocity and the valve stop area. With regard to squeeze flow, the bileaflet valve with slow valve-closing velocity and small valve stop areas is better able to prevent blood cell damage than the monoleaflet valves.

QUANTITATIVE CHARACTERISTICS OF VORTICES IN THE REGURGITANT FLOW ACROSS MECHANICAL HEART VALVES

The initiation of bubble cavitation and subsequent stable bubble formation is thought to arise from low-pressure regions in vortices created by instantaneous valve closure and occluder rebound. We applied particle image velocimetry (PIV) in vitro to measure and reduce instantaneous plane flow fields into the vortex core radius, maximum tangential velocity, circulation strength, and pressure drop, to quantitatively analyze the role of vortices in cavitation. The Bjork-Shiley Monostrut 25 mm and the St. Jude Medical 25 mm were separately placed in the mitral position of a circulatory mock loop. Symmetric, vertical plane velocity fields in the center of each valve were measured via PIV. Time phases were set at 0.3, 2, 5, and 10 msec following impact of the occluder with the valve ring. The heart rate was set at 70, 90, and 120 bpm with respective cardiac outputs of 5, 6, and 7.5 L/min. The velocity profile of the vortex core identifies it as a Rankine vortex. The vortex strength rises at 0.3 to 2 msec, reflecting the rebound effect, and rapidly decreases at 10 msec, indicating viscous dissipation; vortex strength also intensifies with rising heart rate. Based on the Rankine vortex model, the maximum pressure drop at the center of the vortex is on the order of 40 mmHg. Cavitation and bubbles form when local flow field pressure drops below vapor pressure. Based on our results with a maximum pressure drop of merely 40 mmHg, this implies that the contribution of vortices in regurgitant flow is of little significance to cavitation formation.

Boiling Heat Transfer Enhancement Using a Submerged, Vibration-Induced Jet

In this paper we describe a new two-phase cooling cell based on channel boiling and a vibration-induced liquid jet whose collective purpose is to delay the onset of critical heat flux by forcibly dislodging the small vapor bubbles that form on the heated surface during nucleate boiling and propelling them into the cooler bulk liquid within the cell. The submerged turbulent vibration-induced jet is generated by a vibrating piezoelectric diaphragm operating at resonance. The piezoelectric driver induces pressure oscillations in the liquid near the surface of the diaphragm, resulting in the time-periodic formation and collapse of cavitation bubbles that entrain surrounding liquid and generate a strong liquid jet. The resultant jet is directed at the heated surface in the channel. The jet enhances boiling heat transfer by removing attached vapor bubbles that insulate the surface and provides additional forced convection heat transfer on the surface. A small cross flow maintained within the cell increases heat transfer even further by sweeping the bubbles downstream, where they condense. In addition, the cross flow keeps the temperature of the liquid within the cell regulated. In the present experiments, the cell dimensions were 51×25×76mm and water was the working liquid. Heat fluxes above 300W∕cm2 were obtained at surface temperatures near 150°C for a horizontal cell.

Cavitation in a Two-Dimensional Nozzle and Liquid Jet Atomization (1st Report, Ultra-High Speed Visualization)

Cavitation and internal flow in the nozzle of a liquid injector are known to affect the atomization of a discharged liquid jet. In the present study cavitation in a two-dimensional (2D) nozzle and the injected water jet were visualized using an ultra high-speed camera. As a result, the following conclusions were obtained : (1) The behavior of cavitation in the 2D nozzle and liquid jet agreed qualitatively with the previous observations of cylindrical nozzles. (2) When the cavitation number σ was1.18, the inception of small cavitation bubbles was observed near the nozzle inlet apart from the side wall. (3) The number of cavitation bubble clouds increased with decreasing of σ (0.75≤ σ<1.18), and the streamwise length of the cavitation region increased toward the middle of the nozzle (developing cavitation). The bubble clouds were clearly observed to recirculate in the separated boundary layer. In the case a wavy jet was observed with no significant effects of cavitation on liquid jet atomization. (4) With a slight decrease of σ from0.75, the cavitation region suddenly extended to the nozzle exit (super cavitation). The shape of the cavitation region near the inlet became smooth, which imply the formation of a long cavitation bubble near the inlet. At the downstream half of the nozzle, clouds of cavitation appeared and collapsed intermittently within the nozzle. In the case atomization was promoted to form spray with ligaments and droplets.

Summary of cavitation erosion investigations for the SNS mercury target

Intense proton beam-induced heating of the spallation neutron source mercury target will cause pressure spikes that lead to the formation of cavitation bubbles in the mercury. Erosion of the mercury container walls caused by violent collapse of bubbles could potentially limit its service lifetime. In-beam tests for a limited number of pulses (<1000 pulses for each test target) have demonstrated that pitting damage appears to be especially sensitive to beam intensity, surface treatment, and gas injection. Using results of off-line pressure pulse tests conducted for a million cycles or more to scale the results from limited in-beam tests, it is concluded that the mercury target will last at least two weeks at a time-averaged proton beam power level of 1 MW. However, because of remaining uncertainties, it is concluded that further research and development and target design efforts are needed to verify these conclusions and extend the target to higher operating powers and longer lifetimes.

Cavitation Phenomenon in Monoleaflet Mechanical Heart Valves with Electrohydraulic Total Artificial Heart

Recently, cavitation on the surface of mechanical heart valves has been studied as a cause of fractures occurring in implanted mechanical heart valves. In this study, to investigate the mechanism of cavitation bubbles associated with monoleaflet mitral valves in an electrohydraulic total artificial heart (EHTAH), and to select the best valves for our EHTAH system, we measured three parameters. First, an image was created of the cavitation bubbles using a high-speed camera. Second, pressure drop in the vicinity of the valve surface was measured using mini pressure sensor. Then, the closing of the valve was observed using a Laser displacement sensor. Most of the cavitation bubbles in the Medtronic Hall valve were observed at the edge of the valve stop. With the Omnicarbon valve, the cavitation bubbles were observed at the edge of the valve and on the inner side of the leaflet. On the other hand, cavitation bubbles were observed only on the inner side of the leaflet in Björk-Shiley valve. Cavitation bubbles concentrated on the edge of the valve stop; the major cause of these cavitation bubbles was determined to be the squeeze flow. The formation of cavitation bubbles depended on the valve closing velocity and the valve leaflet geometry. From a viewpoint of squeeze flow, a low closing velocity and a small size of the valve stop could minimize cavitation.

Observation of cavitation bubbles in monoleaflet mechanical heart valves

Recently, cavitation on the surface of mechanical heart valves (MHVs) has been studied as a cause of fractures occurring in implanted MHVs. In the present study, we investigated the mechanism of MHV cavitation associated with the Björk-Shiley valve and the Medtronic Hall valve in an electrohydraulic total artificial heart (EHTAH). The valves were mounted in the mitral position in the EHTAH. The valve closing motion, pressure drop measurements, and cavitation capture were employed to investigate the mechanisms for cavitation in the MHV. There are no differences in valve closing velocity between the two valves, and its value ranged from 0.53 to 1.96 m/s. The magnitude of negative pressure increased with an increase in the heart rate, and the negative pressure in the Medtronic Hall valve was greater than that in the Björk-Shiley valve. Cavitation bubbles were concentrated at the edge of the valve stop; the major cause of these cavitation bubbles was determined to be the squeeze flow. The formation of cavitation bubbles depended on the valve closing velocity and the valve leaflet geometry. From the viewpoint of squeeze flow, the Björk-Shiley valve was less likely to cause blood cell damage than the Medtronic Hall valve in our EHTAH.

Effects of Surface Active Properties on the Cavitational Degradation of Surfactant Contaminants

The influence of surfactant properties on the sonochemical degradation of organic compounds was investigated in a 20-kHz probe reactor and a dual-frequency 16- and 20-kHz near-field acoustical processor (NAP). The initial sonochemical degradation rates of 4-octylbenzene sulfonic acid (OBS) and t-octylphenoxy polyethoxyethanol (OPE) in the NAP as a function of concentration resulted in complicated but similar trends to catalysis. At concentrations above the critical micelle concentration, the formation of micelles created additional weak spots in solution that acted as additional nucleation sites for the formation of cavitation bubbles. In comparing the degradation of surfactants to that of nonsurfactants, enhanced degradation of surfactants in the NAP was attributed to the accumulation of surfactants on cavitation bubble surfaces, resulting in localization of contaminants at the site of highest temperature and hydroxyl radical concentration, [•OH]. The lack of enhanced degradation of surfactants in the probe was attributed to the higher power intensity, resulting in rapid bubble growth cycles with insufficient time for surfactants to diffuse to and accumulate on cavitation bubble surfaces compared to the low-power-intensity reactor.