Bubble Implosion Research Articles

Visualization of Irrigant Flow and Cavitation Induced by Er:YAG Laser within a Root Canal Model

Introduction Laser-activated irrigation (LAI) has recently been introduced as an innovative method for root canal irrigation. However, there is limited information about the cleaning mechanism of an Er:YAG laser. In this study, we visualized the action of laser-induced bubbles and fluid flow in vitro to better understand the physical mechanisms underlying LAI. Methods An Er:YAG laser was equipped with a novel cone-shaped tip with a lateral emission rate of approximately 80%. Laser light was emitted at a pulse energy of 30, 50, or 70 mJ (output energy: 11, 18, or 26 mJ) and a repetition rate of 1 or 20 pulses per second, without air or water spray. Fluid flow dynamics in a root canal model were observed by using glass-bead tracers under a high-speed camera. Moreover, laser-induced bubble patterns were visualized in both free water and the root canal model. Results Tracers revealed high-speed motion of the fluid. A full cycle of expansion and implosion of vapor and secondary cavitation bubbles were clearly observed. In free water, the vapor bubble expanded for 220 microseconds, and its shape resembled that of an apple. In the root canal model, the vapor bubble expanded in a vertical direction along the canal wall, and bubble expansion continued for ≥700 microseconds. Furthermore, cavitation bubbles were created much more frequently in the canal model than in free water. Conclusions These results suggest that the cleaning mechanism of an Er:YAG laser within the root canal might depend on rapid fluid motion caused by expansion and implosion of laser-induced bubbles.

Dynamics analysis of cavitation disintegration of microparticles during nanopowder preparation in a new Water Jet Mill (WJM) device

A physical analysis of cavitation-based implosive breakage of solid particles focusing on practical application during fine particle disintegration in a liquid suspension is submitted in the present paper. The physical source of the cavitation dynamics phenomena involved is an extreme velocity gradient induced by an ultrahigh-energy liquid jet mixing together with a slow liquid suspension of milled particles. Extreme tensile stresses occurring at velocity gradients over 1000 ms −1mm −1 at the operating temperature of 65 °C generates high-intensity pure vapor cavitation in the degassed water dispersion with extreme values of impact pressure in the final of bubble implosions on particle surfaces. Preparation of silicon nanoparticles with median diameter approximately 148 nm using a newly developed “Water Jet Mill” (WJM) device is demonstrated in the present article as an example of application of the aforementioned disintegration method as well as of theoretical analysis of this method. The disintegration method is characterized by a high potential for milling of submicron particles with high efficiency.

Ultrasonic coal-wash for de-sulfurization

Coal is the one of the world’s most abundant fossil fuel resources. It is not a clean fuel, as it contains ash and sulfur. SOx as a pollutant are a real threat to both the ecosystem and to human health. There are numerous de-sulfurization methods to control SO2 emissions. Nowadays, online flue gas de-sulfurization is being used as one such method to remove sulfur from coal during combustion. The biggest disadvantage associated with this method is formation of by-products (FGD gypsum). A way for effective usage of FGD gypsum has not yet been found. This will lead to acute and chronic effects to humans as well as plants. Power ultrasound can be used for the beneficiation of coal by the removal of sulfur from coal prior to coal combustion. The main effects of ultrasound in liquid medium are acoustic cavitation and acoustic streaming. The process of formation, growth and implosion of bubbles is called cavitation. Bulk fluid motion due to sound energy absorption is known as acoustic streaming. In addition, coupling of an acoustic field to water produces OH radicals, H2O2, O2, ozone and HO2 that are strong oxidizing agents. Oxidation that occurs due to ultrasound is called Advanced Oxidation Process (AOP). It converts sulfur from coal to water-soluble sulphates. Conventional chemical-based soaking and stirring methods are compared here to ultrasonic methods of de-sulfurization. The main advantages of ultrasonic de-sulfurization over conventional methods, the mechanism involved in ultrasonic de-sulfurization and the difference between aqueous-based and solvent-based (2N HNO3, 3-volume percentage H2O2) de-sulfurization are investigated experimentally.

Infrared Thermography of Cavitation Thermal Effects in Water

Although the thermal effects of cavitation are believed to be negligible for cavitation in water, they were successfully experimentally measured using infrared thermography. Cavitation was generated in a small container holding about 500 ml of water. It was oscillated with ultrasonic frequencies of 42 kHz to trigger growth and collapse of bubbles. For the temperature measurements a high-speed thermovision camera was used. It captures light in infrared spectrum with wavelength of 3-5 μm. The frequency of temperature field acquisition was set to 600 Hz. A silicon glass, which is transparent in the infrared light spectrum, was attached to a cylinder and partially submerged into water. Bubbles, which tend to appear in the vicinity of solid surface, appeared on the submerged side of the glass. The visual path for the thermovision camera was: air – silicon glass – water. In this way, the temperatures on the submerged side of the silicon glass where bubble growth and implosions occur could be measured. With the applied thermographic method small but distinctive local decreases of temperature (with magnitudes up to 0.3 K), caused by cavitation, were detected.

Influence of the accommodation coefficient on nonlinear bubble oscillations

This paper numerically investigates the effect of mass transfer processes on spherical single bubble dynamics using the Hertz-Langmuir-Knudsen approximation for the mass flux across the interface. Bubble behavior, with and without mass transfer, is studied for different values of pressure wave amplitude and frequency, as well as initial bubble radius. Whereas mass transfer processes do not seem to play a significant role on the bubble response for pressure amplitudes smaller than 0.9 atm, they appear to have an important effect when the amplitude is greater than or equal to 1 atm. For the later case, where the minimum liquid pressure reaches values around its vapor pressure, the importance of mass transfer depends on frequency. For frequencies in the 10(3)-10(5) Hz range and initial bubble radii of the order of tens of microns, bubble implosions with and with no mass transfer are significantly different; smaller radii display a lower sensitivity. In this regime, accurate model predictions must, therefore, carefully select the correct value of the accommodation coefficient. For frequencies greater than 10(5) Hz, as a first approximation mass transfer can be ignored.

Peering into Collapsing Bubbles

Chemistry A liquid excited by ultrasonic pressure waves emits a short burst of light caused by the implosion of either single bubbles or clouds of bubbles. Spectroscopy of this sonoluminescence has shown that temperatures of several thousand kelvin may be associated with bubble collapse, but it has

The effect of high power ultrasound on an aqueous suspension of graphite

Ultrasound treatment was used to study the decrease of the granulometry of graphite, due to the cavitation, which allows the erosion by separating grains. At a smaller scale, cavitation bubble implosion tears apart graphite sheets as shown by HRTEM, while HO and H radicals produced from water sonolysis, generate oxidative and reductive reactions on these sheet fragments. Such reactions form smaller species, e.g. dissolved organic matter. The methodology proposed is very sensitive to unambiguously identifying the in situ composition of organic compounds in water. The use of the atmospheric pressure chemical ionization (APCI) Fourier transform mass spectrometry (FTMS) technique minimizes the perturbation of the organic composition and does not require chemical treatment for analysis. The structural features observed in the narrow range ( m/ z < 300) were mainly aromatic compounds (phenol, benzene, toluene, xylene, benzenediazonium, etc.), C 4–C 6 alkenes and C 2–C 10 carboxylic acids. Synthesis of small compounds from graphite sonication has never been reported and will probably be helpful to understand the mechanisms involved in high energy radical reactions.

Negative real interest rate and housing bubble implosion – an empirical study in Hong Kong

PurposeThe purpose of this paper to identify the asymmetric effect of real interest rate on housing return.Design/methodology/approachIt tests empirically the impacts of positive and negative real interest rate on housing return in Hong Kong by time series regression analyses on series from 1984Q1 to 2009Q2, keeping other macroeconomic factors constant.FindingsIt shows that negative real interest rate imposes a much stronger, negative and significant effect on housing return than a positive one.Research limitations/implicationsThe results imply that the two housing bubbles in Hong Kong could be largely explained by the negative real interest rate. Although it is theoretically sound, empirical evidence on this asymmetric effect of real interest rate on housing return has seldom been found, because negative real interest rate is very rare in other countries in the past.Practical implicationsIt provides a good signal for housing bubbles in the future and helps understand the underlying causes of housing bubbles.Originality/valueThe currency board arrangement in Hong Kong enables the first empirical study on this issue.

Negative Real Interest Rate and Housing Bubble Implosion - An Empirical Study in Hong Kong

This paper tests empirically the relationship between real interest rate and housing return in Hong Kong. The time series date from the first quarter of 1984 to the first quarter of 2009. Employing multiple regression analysis with autoregressive and lagged independent variables, it shows that housing bubble implosions could be largely explained by the negative real interest rate. Furthermore, it shows an asymmetric effect of real interest rate on housing return. It can be a good predictor for housing bubbles in the future.

A Common Symptom of Asset Price Bubbles

By reviewing the previous three asset bubble implosions, viz. (1) the “Lost Decade” of Japan in the 1980s; (2) the Asian Financial Crisis in Hong Kong in 1997; and (3) the Financial Tsunami in the USA in 2008, a common symptom of negative real interest rate is very clearly revealed before the implosion of the three bubbles, although the bubbles might have been caused by very different reasons. Credit expansion causing negative interest rate has long been recognized, but many studies focused on one bubble and tried to explain it by the unique events of the bubble. In this study, a holistic framework incorporating money and credit supply and demand, risk premium and expected income growth is put forward. It provides a more complete picture of the causes of asset price bubbles. This paper found a strong and negative relationship between housing return and real interest rate in the three economies. The three bubble bursts were coincided with a period of negative interest rate. It aims to argue that, asset bubble implosion may not be prevented nor accurately predicted though, there is a clear symptom in the markets.

Development of a Novel Oil Cavitation Jet Peening System and Cavitation Jet Erosion in Aluminum Alloy, AA 6063-T6

Compressive residual stresses that improve fatigue strength of material are obtained by peening the surface. Unlike traditional processes, a novel process of oil cavitation jet peening was developed. The process is based on implosion generated by the oil cavitation jet that plastically deforms the surface, imparting compressive residual stresses. The process developed involves injection of a high-speed oil jet (∼230 m/s) through a suitably designed nozzle, into an oil-filled chamber containing the specimen to be peened. The region of cavitation generation, growth, and collapse, at the various cavitation numbers, was recorded using high-speed photography. To optimize the process parameters, a simple erosion test was performed in aluminum alloy, AA 6063-T6, specimens. The impact pressure generated during the implosion of cavitation bubbles causes plastic deformation and erosion of the surface. The surface deformation and cavitation jet erosion in the exposed specimens were characterized using optical and scanning electron microscopies. The standoff distance, which measures jet impact zone of the specimen from nozzle, was optimized at 15 mm in a cavitation number (which is a measure of pressure ratio across the nozzle) of 0.0017. The surface deformation produced by collapse of the oil bubble was similar to impact of oil droplet on the surface. The material removal mechanism during implosion of the bubble is predominately by ductile shear deformation.

Detection of cavitation in situ operation of kinetic pumps: Effect of cavitation on the characteristic discrete frequency component

Cavitation within a pump causes structural vibration with noise, among other by-products. Experiments have shown that there is a discrete frequency or broadband peak within the audible noise spectra, which is in strong correlation with the development of the cavitation process in the pump. Furthermore, the peak of the discrete frequency, or broadband peak, coincides with the net positive suction head (NPSH) critical value, which corresponds to a 3% drop in the total delivery head. Therefore, the discrete frequency tone can be used to detect the incipience of cavitation and its development as well as to determine the NPSH required or critical value with in situ operation of a pump. In this study, we wanted to clarify the mechanism of noise generation, which is responsible for the discrete frequency component and on which cavitation has an important effect. For this purpose, three different measurement methods were used: the first is based on measurement of the sound pressure level (SPL) in the surrounding air, the second is based on measurement of the underwater acoustics and the third is based on measurement of the structural vibration. Experiments have shown that the characteristic discrete frequency tone, which is in close correlation with the cavitation process, is a result of structural vibrations (modes) or resonances caused by implosion of bubbles and bombardment of the inner surfaces of the pump.

Use of audible sound for safe operation of kinetic pumps

Safe operation of kinetic pumps, as liquid movers, can be threatened by cavitation phenomenon, among others. Cavitation is the Achilles heel of kinetic pumps. It can deteriorate hydraulic performance, damage the pump by pitting and material erosion, and structure vibration and resulting noise. Cavitation can appear within the entire range of operating conditions; therefore it must, by all means, be prevented. To prevent cavitation in a pump, we have to know the beginning and development of cavitation in the pump. For this purpose, the emitted noise in the audible range can be used, among other possibilities. Experiments have shown that there is a discrete frequency tone within the audible noise spectra, which is in strong correlation with the development of the cavitation process in the pump, and so with the net positive suction head ( NPSH) critical value, which corresponds to the 3% drop in the total delivery head. The discrete frequency tone can thus be used to detect the incipient of cavitation and its development as well as to prevent the onset of the cavitation process in the pump and also in situ operation. Two different measurement methods were used to clarify the noise-generating mechanism, which is responsible for the discrete frequency component: by using a microphone and an accelerometer. Experiments have shown that the characteristic discrete frequency tone, which is in close correlation with the cavitation process, is a result of structural vibrations (modes) or resonances caused by implosion of bubbles and bombardment of the inner surfaces of the pump.

Parametric Analysis for a Single Collapsing Bubble

This work presents a sensitivity analysis for cavitation processes, studying in detail the effect of various model parameters on the bubble collapse. A complete model (Hauke et al. Phys Rev E 75:1–14, 2007) is used to obtain how different parameters influence the collapse in SBSL experiments, providing some clues on how to enhance the bubble implosion in real systems. The initial bubble radius, the frequency and the amplitude of the pressure wave are the most important parameters determining under which conditions cavitation occurs. The range of bubble sizes inducing strong implosions for different frequencies is computed; the initial radius is the most important parameter characterized the intensity of the cavitation processes. However, other parameters like the gas and liquid conductivity or the liquid viscosity can have an important effect under certain conditions. It is shown that mass transfer processes play an important role in order to correctly predict the trends related with the effect of the liquid temperature, which translates into the bubble dynamics. Moreover, under some particular circumstances, evaporation can be encountered during the bubble collapse; this can be profitably exploited in order to feed reactants when the most extreme conditions inside the bubbles are reached. Thus, this paper aims at providing a global assessment of the effect of the different parameters on the entire cycle of a single cavitating spherical bubble immersed in an ultrasonic field.

Cavitation erosion resistance of nanocrystalline TiN coating deposited on stainless steel

Nanocrystalline TiN coating was deposited on stainless steel X6CrNiTi18-10 by means of the cathodic arc method. The evaluation of nanocrystalline TiN coating resistance to cavitation erosion was based on the investigation performed in a cavitation tunnel with a slot cavitator and tap water as a medium. The investigations were performed at variable cavitation intensity. The estimated cavitation resistance parameters of coatings were the incubation period of damage and the instantaneous erosion rate after exposure of specified duration. It has been confirmed that the nanocrystalline TiN coating deposition on X6CrNiTi18-10 steel causes prolongation of the incubation period in comparison with the uncoated steel, the instantaneous erosion rates are close to each other, but after incubation period higher erosion rate was on the TiN coated specimen. The character of the TiN coating and uncovered substrate damage in multiple locations indicated that the hard coating micro-particles torn off during the cavitation bubbles implosion hit against the coating and the revealed areas of substrate. As a result, the TiN coating and especially the substrate of relatively low hardness beside cavitation erosion are subject to solid particle erosion with the hard torn off micro-particles of coating. The investigations and the analysis indicated that the main factors responsible for an elongated incubation period of the steel substrate/hard coating systems are the gained high hardness of substrate and high level of coating adhesion.

Transcranial Doppler and acoustic pressure fluctuations for the assessment of cavitation and thromboembolism in patients with mechanical heart valves

The formation and collapse of vapor-filled bubbles near a mechanical heart valve is called cavitation. Such microbubbles are suspected to have strong pro-coagulant effects. Therefore, cavitation may be a contributing factor to the pro-thrombotic effects of mechanical valves. Herein, we systematically review the available evidence linking cavitation and thrombosis. We also critically appraise the potential usefulness of transcranial Doppler and other new non-invasive diagnostic methods to study cavitation and cerebral embolism in mechanical valve patients. Experimental studies indicate that cavitation microbubbles cause platelet aggregation, complement-activation, fibrinolysis, release of tissue-factor, and endothelial damage. Administration of 100% oxygen to mechanical valve patients during transcranial Doppler examination can transiently decrease the counts of Doppler-detected cerebral microemboli compared with room air. This is associated with removal of most circulating gaseous emboli from cavitation. This method may therefore be applied to the study of cavitation and thromboembolism. Additionally, the analysis of high-frequency acoustic-pressure fluctuations detected from the implosion of cavitation bubbles is a promising method for assessment of cavitation in vivo; however, this requires further development. A better understanding of cavitation is important in order to adequately investigate its role in the overall pro-thrombotic effects in mechanical valve patients. Such studies may allow establishing guidelines for new valve designs.

Effect of Nozzle-Traveling Velocity on Oil Cavitation Jet Peening of Aluminum Alloy, AA 6063-T6

A new surface modification process was developed to introduce compressive residual stresses at the surface of components. In this process, instead of oil droplets a high-velocity cavitation jet (cloud of oil bubbles) impinges on the surface of the component to be peened. The impact pressure generated during implosion of cavitation bubbles causes severe plastic deformation at the surface. Consequently, beneficial compressive stresses are developed at the surface. In order to find the potential of this process, aluminum alloy AA6063-T6 specimens were peened at a constant cavitation number with various nozzle-traveling velocities. Residual stress induced by oil jet cavitation peening was measured using X-ray diffraction. Oil cavitation jet peening results in a smooth and hard surface. The developed compressive residual stresses at the peened surface are about 52%, 42%, and 35% of yield strength in samples for peened at nozzle traveling velocities of 0.05mm∕s, 0.10mm∕s, and 0.15mm∕s, respectively.

Influence of the substrate hardness on the cavitation erosion resistance of TiN coating

Nanocrystalline TiN coating was deposited by means of the cathodic arc method on stainless steels types X6CrNiTi18-10 and X39Cr13. Both steels were subjected to thermal treatment in order to obtain substrates of different hardness: 1.7 GPa, 2.8 GPa and 4.6 GPa. The TiN coating was 3.7 μm thick. The TiN coating has strong (1 1 1) crystallographic orientation and fine crystalline structure of δ-TiN phase. The TiN coating is characterized by high hardness (25.4 GPa) and good adhesion. The adhesion increases with the substrate hardness. The evaluation of TiN coating resistance to cavitation erosion is based on the investigation performed in a cavitation tunnel with a slot cavitator and tap water as a medium. The estimated cavitation resistance parameters of coating were the incubation period of damage and the total mass loss after the whole test. It has been confirmed that the incubation periods of the coated steels were from 2 to 4 times longer than that of the uncoated steels. The mass losses of the coated steels decrease approximately 2.5 times in comparison with the uncoated steels. The scanning microscope analysis indicates that the damage of TiN coating is mainly due to its delamination. The character of the coating and substrate damage in multiple locations indicates that the hard coating micro-particles torn off during the cavitation bubbles implosion hit against the coating and the revealed areas of substrate. As a result, the coating and especially the substrate of relatively low hardness beside cavitation erosion are subject to solid particle erosion with the hard torn off micro-particles of coating.

Effect of ultrasound on the heterogeneous nucleation of BaSO4 during reactive crystallization

The effect of ultrasound on reactive crystallization is studied by measuring the induction time when nucleation is predominantly heterogeneous. Barium sulphate is used as the working substance precipitated by mixing aqueous BaCl2 and Na2SO4 solutions with relatively low concentrations compared to experiments for homogeneous nucleation. It is observed that at a given supersaturation level, the induction time significantly decreases with increasing ultrasonic energy input. The mechanism of the ultrasonic effect on heterogeneous nucleation is analyzed. The effect of ultrasound is to increase the diffusion coefficient (DAB) while reducing the contact angle (θ) and geometric factor (f). This gives rise to an increase in the heterogeneous nucleation rate and shorter induction time. The ultrasonic effect on critical supersaturation ratio (Scrit) is also investigated to determine the mechanism of ultrasonic effect on f and θ. Both the force from the cavitational bubble implosion and the nucleation process occurring at bubble/solution/foreign solid interface are proposed as the predominant reasons why contact angle (θ) decreases with the increase of energy input.

Measurement of shock velocity from cavitation collapse in high static pressure liquids using Schlieren and shadowgraphy

High amplitude shock waves due to collapse of a cavitation bubble were photographed using a high power pulsed laser diode driven with an avalanche transistor pulser. The strong implosions of high pressure bubble cavitation travel initially at several times the local speed of sound. This method was used to measure shock wave velocity in water at static pressures up to 300 bars. The photographs were taken between 30 and 60 ns of the bubble implosion. Schlieren and shadowgraphy techniques were used and compared. The purpose is to characterize the pressure conditions that exist shortly after the implosion of a high pressure sonoluminescence bubble. [Work supported by SMDC Contract No. W9113M‐07‐C‐0178.]