High-speed observation of acoustic cavitation erosion in multibubble systems

High-speed imaging of supersaturated cavitation clouds and the vibration modes of the radiation surface of high-power transducers

The vibration mode of the radiation surface of transducer (or structure of supersaturated cavitation cloud in thin liquid) is investigated experimentally by high-speed photography. The classification of saturated, supersaturated and undersaturated cavitation clouds was proposed, and a comparison was made between saturated and supersaturated cavitation cloud structures in liquid thin layers. The characteristics and formation mechanism of supersaturated cavitation cloud structure were investigated. Based on the close correspondence and rapid response between the distribution of supersaturated cavitation clouds and vibration modes of radiation surface, a new approach is proposed to measure the vibration mode of transducer operating at high power and large amplitude in real time.

Mapping of 20 L capacity ultrasonic reactor using cavitation activity meter and dye degradation

Mapping of a novel 20 L capacity ultrasonic (US) reactor having a total of 44 transducers was done by measuring the local cavitation intensity using a cavitation activity meter at different horizontal planes and subsequent validation based on dye degradation. A fixed frequency of 33 kHz and temperature of 30 °C was used during the mapping performed at two different power levels of 250 W and 400 W. In addition, the mapping of specific plane 2 was also performed with transducers operating on walls 1 and 3, while switching the transducers on walls 2 and 4 off and vice versa so as to establish the role of using multiple transducers. Degradation of RO4 dye was also measured at the plane 2 at various powers as 250 W, 400 W, and 1000 W. The degradation of the RO4 dye directly correlated to the cavitation intensity measured at the various location inside the US reactor. The average cavitation intensity was 265.38, 317.25, 185, and 300.5 Cavins for power dissipations of 250 W, 400 W, 250 W (wall 1 and 3 transducers in operation), and 400 W (wall 2 and 4 transducers in operation), respectively. Correspondingly, the average degradation was 10.35 %, 13.03 %, 5.52 %, and 8.9 % for same sequence of operational power and transducers. The investigation amply illustrated dependency of the cavitational activity on the location, power dissipation, and operating mode elucidating important design related information useful for scale up of sonochemical reactors.

Numerical investigation on the influence of dual-frequency coupling parameters on acoustic cavitation and its analysis of the enhancement and attenuation effect

To understand the effect of coupling parameters between two ultrasonic waves on acoustic cavitation, in this work, Keller-Miksis equation was introduced to built a bubble dynamics model that was used to describe the dynamic evolution of bubble and to discuss the effect of dual-frequency coupling parameters, such as frequency difference f (5 ∼ 280 kHz), phase difference φ (0 ∼ 7π/4 rad), and power allocation ratio β (0 ∼ 9), on acoustic cavitation in the presence of two ultrasonic waves irradiation. The enhancement and attenuation effect of cavitation have also been analyzed in detail by comparing the different dual-frequency combinations with single-frequency mode. It was found that all coupling parameters have a significant impact on acoustic cavitation, where the smaller values of f and φ were employed when β = 1, the stronger cavitation intensity was observed. Nevertheless, as the power allocation ratio is increased from 1 to 9 at φ = 0 for different frequency differences, the acoustic cavitation exhibits an attenuation trend. When the total acoustic power is evenly distributed, namely β = 1, the largest maximum expansion ratio (i.e. 12.96) was obtained at φ = 0 and f = 5 kHz, which represents a strongest cavitation effect. In addition, for different frequency combinations, the enhancement effect is found under the mixture of low and low frequency, whereas attenuation effect is generated easily by the combination of high and low frequency. Moreover, the effect become more pronounced as the proportion of high frequency component increases.

Simulations of a full sonoreactor accounting for cavitation

In spite of the increasing interest in ultrasound processing applications, industrial scale-up remains limited, in particular by the unavailability of predictive computer tools. In this study, using a previously published model of cavitating liquids implementable as a non-linear Helmholtz equation, it is shown that a full sonoreactor can be modelled and simulated. The model includes the full transducer and the vibrations of the vessel walls, using the physics of elastic solids and piezo-electricity. The control-loop used by the generator to set the optimal frequency is also accounted for. Apart from the geometry, the unique input of the model is the current feeding the transducer whereas the dissipated electrical power, transducer complex impedance and working frequency are available as outputs. The model is put to the test against experiments realized in different geometries, varying either the input current or the transducer immersion depth. Despite the overestimation of the power dissipated in the liquid, the evolution of the acoustic load in both cases is reasonably well reproduced by simulation, which partially validates the method used.

Experimental investigation on the ultrasonic impregnation of wood through measurements of the intensity of sonoluminescence

Ultrasonic impregnation is thought to be an effective way of permeation of liquid into material through the material-surface reforming with the attack by an ultrasonic cavitation jet or by the shock wave emitted from a collapsing bubble, or through dynamic transformation of material like a sponge. The action of a cavitation bubble can also provide penetration of liquid into the interior of the material. This paper investigates whether there is a correlation between the intensity of sonoluminescence (SL) measured at different positions and the increment in the mass of the wood material (cedar) after sonication with immersion into water in order to clarify the role of cavitation bubbles for ultrasonic impregnation. It was found that a high mass change was obtained for the material located at the position for high (the maximum) SL intensity. The number density of ultrasonic cavitation bubbles that are able to collapse leading to the emission of SL is correlated with the degree of ultrasonic impregnation.

Particulate Projectiles Driven by Cavitation Bubbles

The removal of surface-attached particles with cavitation bubbles is usually attributed to the jetting or shear stresses when bubbles collapse. In this Letter, we report an unexpected phenomenon that millimeter-sized spherical particles made of heavy metals (e.g., stainless steel), when initially resting on a fixed rigid substrate, are suddenly accelerated like projectiles through the production of nearby laser-induced cavitation bubbles of similar sizes. We show experimentally and theoretically that the motion of a particle with radius R_{p} is determined by the maximum bubble radius R_{b,max}, the initial distance from the laser focus to the center of the particle L_{0}, and the initial azimuth angle φ_{0}. We identify two dominant regimes for the particle's sudden acceleration, namely, the unsteady liquid inertia dominated regime and the bubble contact dominated regime, determined by R_{b,max}R_{p}/L_{0}^{2}. We find the nondimensional maximum vertical displacement of the particle follows the fourth power and the square power scaling laws for respective regimes, which is consistent with the experimental results. Our findings can be applied to nonintrusive particle manipulation from solid substrates in a liquid.

Experimental Study on Two-Dimensional Rotatory Ultrasonic Combined Electrochemical Generating Machining of Ceramic-Reinforced Metal Matrix Materials

According to the machining characteristics of ceramic-particle-reinforced metal matrix composites, an experimental study on difficult-to-machine materials was carried out by two-dimensional (2D) rotatory ultrasonic combined electrolytic generating machining (RUCEGM), which organically combined an ultrasonic effect with a high-speed rotating tool electrode and electrolysis. After building the one-dimensional (1D) and 2D-RUCEGM systems, the factors influencing the combined machining process were analyzed and the experiments on RUCEGM were conducted to explore the feasibility and advantages of 2D-RUCEGM. The experimental results showed that, compared with 1D-RUCEGM, 2D-RUCEGM had higher accuracy, which increased about 21% and also reduced the machining time. Under certain conditions, the efficiency of 2D-RUCEGM was proportional to the voltage, and the machining efficiency could be enhanced by increasing the feed rate. The inter-electrode voltage detection module used in the experiment could improve the machining stability of the system.

Quantification of sonochemical and sonophysical effects in a 20 kHz probe-type sonoreactor: Enhancing sonophysical effects in heterogeneous systems with milli-sized particles

Even though acoustic cavitation has been widely investigated, only few researchers focused on the relationship between sonochemical and sonophysical activities and on the enhancement of sonophysical activity. In this study, sonochemical and sonophysical activities were investigated in a heterogeneous system to understand the relationship between these two activities and to suggest optimal conditions for ultrasonic desorption/extraction processes comprising milli-sized glass beads. The sonochemical activity was quantitatively analyzed using potassium iodide dosimetry in homogeneous and heterogeneous systems. Sonophysical activity was quantitatively and qualitatively analyzed using paint-coated bead desorption tests and aluminum foil erosion tests under three probe positions of "T" (1 cm below the liquid surface), "B" (1 cm above the vessel bottom), and "M" (midpoint between "T" and "B"). Three different sizes of glass beads (diameter: 0.2, 1.0, and 4.0 mm) were used in this study. The highest sonochemical activity was obtained at "B" in both homogeneous and heterogeneous systems. However, three times lower sonochemical activity was observed in the heterogeneous system than in the homogeneous system because significant attenuation and unstable reflection of ultrasound occurred in the bead layer and suspension. Higher sonophysical activity was observed, when the bead size decreased and the probe approached the bottom. However, no significant sonophysical activity was detected when the beads were attached to the bottom. Therefore, the sonophysically active region was the zone around the probe body, opposite to the ultrasound irradiation tip, and only suspended beads could undergo severe cavitational actions. This was confirmed via aluminum foil tests. Several erosion marks on the foil were observed in the area around the probe body, whereas no severe damage was observed at the bottom. Moreover, the degree of sonophysical activity did not change for various saturating gases. This might be due to the different thresholds of sonochemical and sonophysical activities.

Numerical Study of the Interaction between a Collapsing Bubble and a Movable Particle in a Free Field

This study numerically investigates the interactions between a collapsing bubble and a movable particle with a comparable size in a free field, which is associated with the microscopic mechanisms of the synergetic effects of cavitation erosion and particle abrasion on the damages of materials in fluid machineries. A new solver on OpenFOAM based on direct numerical simulations with the volume of fluid (VOF) method capturing the interface of a bubble and with the overset grid method handling the motion of the particle was developed to achieve the fluid–structure interaction (FSI). The results show that bubbles in cases with stand-off parameter χ (defined as (d0−Rp)/R0), where d0 is the initial distance between the centers of the bubble and particle, and Rp,R0 are the particle’s radius and the initial radius of the bubble respectively >1, experience spherical-shaped collapse under the influence of the approaching particle, which is attracted by the collapsing bubble. The bubbles in these cases no longer present non-spherical collapse. Additionally, a force balance model to account for the particle dynamics was established, in which the particle velocity inversely depends on the size of the particle, and approximately on the second power of the initial distance from the bubble. This analytical result accords with the numerical results and is valid for cases with χ>1 only, since it is based on the theory of spherical bubbles. These conclusions are important for further study of the interactions between a bubble and a movable particle near a rigid wall.

Sanitization of Oak Barrels for Wine-A Review

Oak barrels form an integral part of wine production, especially that of high-quality wines where they are implemented as fermentation and aging vessels. Insufficient cleaning and sanitization of barrels can result in microbial spoilage which may have a detrimental impact on wine quality. To date, no review has been published on the various sanitization methods for wine barrels. The objective of this review is to provide an overview of the sanitization methods used in wineries from conventional techniques like the use of sulfur dioxide and steam to alternative and new approaches using ozone and high-power ultrasound. The methods' efficacies are outlined in terms of their ability to eradicate spoilage microorganisms such as Brettanomyces and acetic or lactic acid bacteria. Furthermore, their advantages and drawbacks are described together with their influence on physicochemical properties of the wood. Finally, limitations in existing knowledge are discussed and areas that merit further research are identified.

Low-intensity ultrasound induced cavitation and streaming in oxygen-supersaturated water: Role of cavitation bubbles as physical cleaning agents

A number of acoustic and fluid-dynamic phenomena appear in ultrasonic cleaning baths and contribute to physical cleaning of immersed surfaces. Propagation and repeated reflection of ultrasound within cleaning baths build standing-wave-like acoustic fields; when an ultrasound intensity gradient appears in the acoustic fields, it can in principle induce steady streaming flow. When the ultrasound intensity is sufficiently large, cavitation occurs and oscillating cavitation bubbles are either trapped in the acoustic fields or advected in the flow. These phenomena are believed to produce mechanical action to remove contaminant particles attached at material surfaces. Recent studies suggest that the mechanical action of cavitation bubbles is the dominant factor of particle removal in ultrasonic cleaning, but the bubble collapse resulting from high-intensity ultrasound may be violent enough to give rise to surface erosion. In this paper, we aim to carefully examine the role of cavitation bubbles from ultrasonic cleaning tests with varying dissolved gas concentration in water. In our cleaning tests using 28-kHz ultrasound, oxygen-supersaturated water is produced by oxygen-microbubble aeration and used as a cleaning solution, and glass slides spin-coated with silica particles of micron/submicron sizes are used to define cleaning efficiency. High-speed camera recordings and Particle Image Velocimetry analysis with a pressure oscillation amplitude of 1.4 atm at the pressure antinode show that the population of cavitation bubbles increases and streaming flow inside the bath is promoted, as the dissolved oxygen supersaturation increases. The particle removal is found to be achieved mainly by the action of cavitation bubbles, but there exists optimal gas supersaturation to maximize the removal efficiency. Our finding suggests that low-intensity ultrasound irradiation under the optimal gas supersaturation in cleaning solutions allows for having mild bubble dynamics without violent collapse and thus cleaning surfaces without cavitation erosion. Finally, observations of individual bubble dynamics and the resulting particle removal are reported to further support the role of cavitation bubbles as cleaning agents.

Study of High Power Ultrasound for Oak Wood Barrel Regeneration: Impact on Wood Properties and Sanitation Effect

This study aims to investigate the ability of high power ultrasound (HPU) to ensure oak barrel sterilization and wood structure preservation. Optimization was performed in terms of temperature and time and the impact of the HPU process on the porous material was also characterized. In this research, several wood characteristics were considered, such as the specific surface area, hydrophobicity, oxygen desorption and spoilage microorganisms after treatment. The study showed that the microbial stabilization could be obtained with HPU 60 °C/6 min. The results obtained show that microorganisms are impacted up to a depth of 9 mm, with a Brettanomyces bruxellensis population < 1 log CFU/g. The operating parameters used during the HPU treatment can also impact on wood exchange surface and oxygen desorption kinetics indicating that tartrate is removed. Indeed, the total oxygen desorption rate was recovered after HPU treatment, close to a new oak barrel, and thus may indicate that there is no impact on the ultrastructure (vessel, pore size or rays). Finally, wood wettability can also be impacted, depending on the temperature and the duration of exposure.

Cavitation at filler metal/substrate interface during ultrasonic-assisted soldering. Part I: Cavitation characteristics

The cavitation characteristics at filler metal/substrate interface during ultrasonic-assisted soldering were first recorded by high-speed photography in this work. Two kinds of bubbles, steady cavitation bubbles and transient cavitation bubbles were observed. Steady cavitation bubbles did not collapse within one acoustic period and could last longer than 50 acoustic periods. Transient cavitation bubbles formed and collapsed within one acoustic period. The cavitation process was divided into two stages based on the cavitation characteristics. The first violent cavitation stage was in fact the degassing process, which lasted approximately 2700 acoustic periods and was affected by the gas content trapped inside the filler metal and the stronger vibration at the initiation stage of ultrasonic-assisted soldering. The second steady cavitation stage had obvious low bubble density and accounted for the most of the soldering process. Higher cavitation densities were observed when small channel width and large ultrasonic power were used because of larger sound pressures inside the filler metal.

Depth effect on the inertial collapse of cavitation bubble under ultrasound: Special emphasis on the role of the wave attenuation

Acoustic cavitation concentrates and releases a very large amount of energy in localized areas, which can be used for many physical and chemical processes. Even though acoustic cavitation has been studied widely for decades in lab-scale sonoreactors, only few studies have been devoted to characterize this event in big-scale sonoreactors, where the liquid depth may have a critical influence on the bubble collapse. The present computational study furnished numerical data about the effect of depth (z = 0-10 m) on acoustic cavitation with special focus on the role of attenuation of the ultrasound wave on the dramatic conditions developed within bubbles at collapse. The used mathematical model takes into account the liquid compressibility, surface tension and viscosity, depth as well as the attenuation of the ultrasound wave with depth. It was found that the maximum bubble temperature (T_max) and pressure (p_max) at the collapse diminished considerably with deepening into water up to 10 m with a considerable contribution of the ultrasound wave attenuation in the overall reduction event. The reduction in T_max and p_max with depth was more pronounced at higher frequency (1000 kHz) and lower temperature (10 °C) in which losses of about up to 72% in T_max and till 94% in p_max (as compared with values at z = 0) were obtained at z = 10 m. Depending on operating conditions, i.e. frequency, acoustic intensity or liquid temperature, the ultrasound wave attenuation may contribute with up to 47% and 79% in the overall reductive effect of depth toward T_max and p_max, respectively. These results were discussed, interpreted and used to support some available experimental observations. Finally, the results of the present study may help in designing large-scale sonoreactors through providing data about the effect of one of the missing links between lab-scale sonoreactors and industrial large-scale sonoreactors.

Rod-shaped cavitation bubble structure in ultrasonic field

Rod-shaped cavitation bubble structure in thin liquid layers in ultrasonic field is investigated experimentally. It is found that cavitation structure successively experiences several stages with the change of the thickness of the thin liquid layer. Rod-shaped structure is a stable structure of the boundary between the cavitation cloud region and the non-cavitation liquid region, which can be formed in two different ways. Cavitation bubbles in a thin liquid layer have a distribution in the thickness direction. The rod-shaped structures tend to crosslink with each other to form stable Y-branch structures. The angle of the Y-branch structure is Gauss distribution with mathematical expectation μ = 119.93. A special rod-shaped cavitation structure with source is also investigated in detail. Due to the pressure gradient in the normal direction, the primary Bjerknes force causes the bubbles in the rod-shaped structure on both sides to converge to the axis. The secondary Bjerknes forces between the bubbles also make the cluster converge, so the large bubbles which are attached to the radiating surface tend to align themselves along the central line. According to the formula deduced in this paper, the variation of curvature of curved rod-shaped structure is qualitatively analyzed. The Y-branch structure of cavitation cloud and Plateau boundary of soap bubbles are compared.

Sono-precipitation of Ag2CrO4-C composite enhanced by carbon-based materials (AC, GO, CNT and C3N4) and its activity in photocatalytic degradation of acid orange 7 in water

Enhancing the photocatalytic activity of Ag₂CrO₄ with coupled carbon-based materials like activated carbon, graphene oxide, carbon nanotubes and carbon nitride has been investigated in removal of Acid Orange 7 from wastewater. Sono precipitated Ag₂CrO₄-C composite based photocatalysts were characterized by XRD, BET, FESEM, FTIR and UV-vis DRS and the photocatalytic activity of theses samples was evaluated in terms of degradation amount of acid orange 7 under visible light irradiations. BET analysis showed that with addition of carbon based materials, the specific surface area of the Ag₂CrO₄-C composite increased. XRD analysis indicated that the crystallinity of Ag₂CrO₄ peaks decreased after addition of all studied carbon-based materials and C₃N₄ has lowered the crystallinity of Ag₂CrO₄ less than others. Higher crystallinity has the positive effect of higher photocatalytic activity because among above mentioned composites, Ag₂CrO₄-C₃N₄ photocatalyst exhibited higher photocatalytic activity and stability under visible light irradiations. DRS analysis confirmed good match of electronic structures of Ag₂CrO₄ and C₃N₄. On the other hand Ag₂CrO₄ and C₃N₄ formed heterojunction which separates photo-generated electron-hole pairs effectively. Also evaluation of photocatalytic reaction in various operating parameters showed Ag₂CrO₄-C₃N₄ had the highest photocatalytic activity in neutral pH and 1g/L of catalyst loading.

High-speed photographic observation of the sonication of a rabbit carotid artery filled with microbubbles by 20-kHz low frequency ultrasound

The aim of this study is to assess the physical damage of cavitation effects induced by low frequency ultrasound and microbubbles (MBs) to an in vitro vessel. A rabbit carotid artery filled with SonoVue MBs and methylene blue was irradiated with 20-kHz ultrasound, and the results were recorded by high-speed photography at 3000 frames per second. The carotid artery filled with MBs experienced a slight tremor during ultrasonication. Six intermittent blue flow events occurred in two places on the artery wall during the 5-s process. The duration of each leakage event was 90-360ms with an average of 200ms. Hematoxylin-eosin (H-E) staining demonstrated the separation of the carotid artery elastic membrane, local blood vessel wall defects and hole formation, and the surface of the ruptured area was rough and irregular. Another carotid artery was filled with a 0.9% NaCl solution and methylene blue as a control and irradiated with 20-kHz ultrasound. No blue liquid flow was seen, and no holes in the vessel were observed. H-E staining revealed intact vascular endothelial cells and smooth muscles with no vascular wall defects. Low-frequency ultrasound combined with MBs can cause a vessel to rupture and holes to form in a short time. High-speed photography is useful for observing transient changes caused by the effects of ultrasound cavitation on an in vitro vessel.

Numerical study of acoustically driven bubble cloud dynamics near a rigid wall

The dynamics of a bubble cloud excited by a sinusoidal pressure field near a rigid wall is studied using a novel Eulerian/Lagrangian two-phase flow model. The effects of key parameters such as the amplitude and frequency of the excitation pressure, the cloud and bubble sizes, the void fraction, and the initial standoff distance on the bubbles' collective behavior and the resulting pressure loads on the nearby wall are investigated. The study shows that nonlinear bubble cloud dynamics becomes more pronounced and results in higher pressure loading at the wall as the excitation pressure amplitude increases. The strongest collective bubble behavior occurs at a preferred resonance frequency. At this resonance frequency, pressure peaks orders of magnitudes higher than the excitation pressure result from the bubble interaction when the amplitude of the pressure excitation is high. The numerically obtained resonance frequency is significantly different from the reported natural frequency of a spherical cloud derived from linear theory, which assumes small amplitude oscillations in an unbounded medium. At high amplitudes of the excitation, the resonance frequency decreases almost linearly with the ratio of excitation pressure amplitude to ambient pressure until the ratio is larger than one.

Motion of a Free-Settling Spherical Particle Driven by a Laser-Induced Bubble

We document experimentally four different interactions of a laser-induced bubble and a free-settling particle, with different combinations of the geometric and physical parameters of the system. Our force balance model shows that four nondimensional factors involving the particle radius a, the maximum bubble radius R_{max}, the initial separation distance l_{0} between the particle center and the bubble center, the fluid viscosity μ_{f}, and the particle and fluid densities ρ_{p} and ρ_{f}, respectively, in detail l_{0}/R_{max}, a/R_{max}, ρ_{p}/ρ_{f}, and μ^{*}=μ_{f}T_{c}/ρ_{f}R_{max}^{2}, where T_{c}=0.915R_{max}sqrt[ρ_{f}/(p_{∞}-p_{v})], influence the particle-bubble dynamics, and reasonably predict the maximum particle velocity and the limiting condition when the particle starts to "bounce off" the bubble during bubble growth. In particular, we also discover the high-speed ejection of the particle, and a cavity behind the particle, in cases when initially the particle is in very close proximity to the bubble. These observations offer new insights into the causal mechanism for the enhanced cavitation erosion in silt-laden water.

Membrane cleaning with ultrasonically driven bubbles

A laboratory filtration plant for drinking water treatment is constructed to study the conditions for purely mechanical in situ cleaning of fouled polymeric membranes by the application of ultrasound. The filtration is done by suction of water with defined constant contamination through a membrane module, a stack of five pairs of flat-sheet ultrafiltration membranes. The short cleaning cycle to remove the cake layer from the membranes includes backwashing, the application of ultrasound and air flushing. A special geometry for sound irradiation of the membranes parallel to their surfaces is chosen. Two frequencies, 35kHz and 130kHz, and different driving powers are tested for their cleaning effectiveness. No cleaning is found for 35kHz, whereas good cleaning results are obtained for 130kHz, with an optimum cleaning effectiveness at moderate driving powers. Acoustic and optic measurements in space and time as well as analytical considerations and numerical calculations reveal the reasons and confirm the experimental results. The sound field is measured in high resolution and bubble structures are high-speed imaged on their nucleation sites as well as during their cleaning work at the membrane surface. The microscopic inspection of the membrane surface after cleaning shows distinct cleaning types in the cake layer that are related to specific bubble behaviour on the membrane. The membrane integrity and permeate quality are checked on-line by particle counting and turbidity measurement of the permeate. No signs of membrane damage or irreversible membrane degradation in permeability are detected and an excellent water permeate quality is retained.

Experiment and modeling of translational dynamics of an oscillating bubble cluster in a stationary sound field

Translational motion of an oscillating bubble cluster under sound irradiation is studied experimentally and is modeled in the framework of the classical approach of Bjerknes. An experimental technique is proposed to observe bubble cluster formation and its translational dynamics interacting with wall boundaries due to the secondary Bjerknes force. The translational motion observed in the experiment is modeled by extending the classical theory of Bjerknes on a single bubble; a bubble cluster is treated as a single bubble. The extended Bjerknes theory is shown to allow us to predict the overall trajectory of the cluster translating toward a wall of finite acoustic impedance by tuning acoustic energy loss at the wall. The drag force turns out to be unimportant for the translation of a millimeter-sized cluster that we observed.

CO2-enhanced dehydrogenation of ethane over sonochemically synthesized Cr/clinoptilolite-ZrO2 nanocatalyst: Effects of ultrasound irradiation and ZrO2 loading on catalytic activity and stability

CO2-enhanced oxidative dehydrogenation of ethane was investigated over sonochemically synthesized Cr/clinoptilolite-ZrO2 nanocatalyst with the aim of assessing the effect of composite support and ultrasonic irradiation on the nanocatalyst reactivity and stability. To this aim, ZrO2 promoted clinoptilolite supports varying in zirconia content (0, 25, 50wt%) were synthesized by hydrothermally precipitation method and impregnated with chromium nitrate under ultrasound irradiation. The samples were characterized by XRD, FESEM, EDX, TEM, ICP, BET, FTIR, TPR-H2 and TPD-NH3 techniques. The characterization results indicated that ultrasound irradiation could not only reduce the formation of Cr2O3 and decrease submicron particle size of chromium oxide to nanometer scale, but also promote the distribution of metallic particles and strengthen the chromium-support interaction. As a result, utilizing ultrasound irradiation in the synthesis of Cr/Clinoptilolite helped to maintain a high and stable catalytic activity. These features were more prominent in the presence of zirconia. It was found that the metal oxide nanoparticles with about 4-8nm are dispersed uniformly on the surface of composite support containing 25wt% ZrO2 (CLT-Z25). Moreover, the addition of ZrO2 resulted in the formation of new strong acid sites and a significant modification in the reducibility of chromium species, which alongside homogenous and small Cr nanoparticles account for the superior catalytic performance of ZrO2 containing samples. However, excessive loading of ZrO2 (50wt%) severely covered the surface of clinoptilolite, afforded the aggregations of metallic particles and thereupon, weakened the contact between clinoptilolite and ZrO2, which together with more acid strength seriously resulted in the deactivation of catalyst. In spite of superior initial activity of ZrO2-rich sample among the catalysts tested, ultrasonic synthesized Cr/CLT-Z25 nanocatalyst showed the best catalytic performance after 5h-catalytic reaction.

Vortex dynamics of collapsing bubbles: Impact on the boundary layer measured by chronoamperometry

Cavitation bubbles collapsing in the vicinity to a solid substrate induce intense micro-convection at the solid. Here we study the transient near-wall flows generated by single collapsing bubbles by chronoamperometric measurements synchronously coupled with high-speed imaging. The individual bubbles are created at confined positions by a focused laser pulse. They reach a maximum expansion radius of approximately 425μm. Several stand-off distances to the flat solid boundary are investigated and all distances are chosen sufficiently large that no gas phase of the expanding and collapsing bubble touches the solid directly. With a microelectrode embedded into the substrate, the time-resolved perturbations in the liquid shear layer are probed by means of a chronoamperometric technique. The measurements of electric current are synchronized with high-speed imaging of the bubble dynamics. The perturbations of the near-wall layer are found to result mainly from ring vortices created by the jetting bubble. Other bubble induced flows, such as the jet and flows following the radial bubble oscillations are perceptible with this technique, but show a minor influence at the stand-off distances investigated.

Effect of Cavitation Bubble Collapse on the Modification of Solids: Crystallization Aspects

This review examines the concepts how cavitation bubble collapse affects crystalline structure, the crystallization of newly formed structures, and recrystallization. Although this subject can be discussed in a broad sense across the area of metastable crystallization, our main focus is discussing specific examples of the inorganic solids: metal, intermetallics, metal oxides, and silicon. First, the temperature up to which ultrasound heats solids is discussed. Cavitation-induced changes in the crystal size of intermetallic phases in binary AlNi (50 wt % of Ni) alloys allow an estimation of local temperatures on surfaces and in bulk material. The interplay between atomic solid-state diffusion and recrystallization during bubble collapses in heterogeneous systems is revealed. Furthermore, cavitation triggered red/ox processes at solid/liquid interfaces and their influence on recrystallization are discussed for copper aluminum nanocomposites, zinc, titanium, magnesium-based materials, and silicon. Cavitation-driven highly nonequilibrium conditions can affect the thermodynamics and kinetics of mesoscopic phase formation in heterogeneous systems and in many cases boost the macroscopic performance of composite materials, notably in catalytic alloy and photocatalytic semiconductor oxide properties, corrosion resistance, nanostructured surface biocompatibility, and optical properties.

Numerical simulation of the nonlinear ultrasonic pressure wave propagation in a cavitating bubbly liquid inside a sonochemical reactor

We investigate the acoustic wave propagation in bubbly liquid inside a pilot sonochemical reactor which aims to produce antibacterial medical textile fabrics by coating the textile with ZnO or CuO nanoparticles. Computational models on acoustic propagation are developed in order to aid the design procedures. The acoustic pressure wave propagation in the sonoreactor is simulated by solving the Helmholtz equation using a meshless numerical method. The paper implements both the state-of-the-art linear model and a nonlinear wave propagation model recently introduced by Louisnard (2012), and presents a novel iterative solution procedure for the nonlinear propagation model which can be implemented using any numerical method and/or programming tool. Comparative results regarding both the linear and the nonlinear wave propagation are shown. Effects of bubble size distribution and bubble volume fraction on the acoustic wave propagation are discussed in detail. The simulations demonstrate that the nonlinear model successfully captures the realistic spatial distribution of the cavitation zones and the associated acoustic pressure amplitudes.

Statistical optimization of cell disruption techniques for releasing intracellular X-prolyl dipeptidyl aminopeptidase from Lactococcus lactis spp. lactis

X-prolyl dipeptidyl aminopeptidase (PepX) is an intracellular enzyme from the Gram-positive bacterium Lactococcus lactis spp. lactis NRRL B-1821, and it has commercial importance. The objective of this study was to compare the effects of several cell disruption methods on the activity of PepX. Statistical optimization methods were performed for two cavitation methods, hydrodynamic (high-pressure homogenization) and acoustic (sonication), to determine the more appropriate disruption method. Two level factorial design (2FI), with the parameters of number of cycles and pressure, and Box-Behnken design (BBD), with the parameters of cycle, sonication time, and power, were used for the optimization of the high-pressure homogenization and sonication methods, respectively. In addition, disruption methods, consisting of lysozyme, bead milling, heat treatment, freeze-thawing, liquid nitrogen, ethylenediaminetetraacetic acid (EDTA), Triton-X, sodium dodecyl sulfate (SDS), chloroform, and antibiotics, were performed and compared with the high-pressure homogenization and sonication methods. The optimized values of high-pressure homogenization were one cycle at 130 MPa providing activity of 114.47 mU ml(-1), while sonication afforded an activity of 145.09 mU ml(-1) at 28 min with 91% power and three cycles. In conclusion, sonication was the more effective disruption method, and its optimal operation parameters were manifested for the release of intracellular enzyme from a L. lactis spp. lactis strain, which is a Gram-positive bacterium.

Measuring cavitation and its cleaning effect

The advantages and limitations of techniques for measuring the presence and amount of cavitation, and for quantifying the removal of contaminants, are provided. After reviewing chemical, physical, and biological studies, a universal cause for the cleaning effects of bubbles cannot yet be concluded. An "ideal sensor" with high spatial and temporal resolution is proposed. Such sensor could be used to investigate bubble jetting, shockwaves, streaming, and even chemical effects, by correlating cleaning processes with cavitation effects, generated by hydrodynamics, lasers or ultrasound.

Sound field measurement in a double layer cavitation cluster by rugged miniature needle hydrophones

During multi-bubble cavitation the bubbles tend to organize themselves into clusters and thus the understanding of properties and dynamics of clustering is essential for controlling technical applications of cavitation. Sound field measurements are a potential technique to provide valuable experimental information about the status of cavitation clouds. Using purpose-made, rugged, wide band, and small-sized needle hydrophones, sound field measurements in bubble clusters were performed and time-dependent sound pressure waveforms were acquired and analyzed in the frequency domain up to 20 MHz. The cavitation clusters were synchronously observed by an electron multiplying charge-coupled device (EMCCD) camera and the relation between the sound field measurements and cluster behaviour was investigated. Depending on the driving power, three ranges could be identified and characteristic properties were assigned. At low power settings no transient and no or very low stable cavitation activity can be observed. The medium range is characterized by strong pressure peaks and various bubble cluster forms. At high power a stable double layer was observed which grew with further increasing power and became quite dynamic. The sound field was irregular and the fundamental at driving frequency decreased. Between the bubble clouds completely different sound field properties were found in comparison to those in the cloud where the cavitation activity is high. In between the sound field pressure amplitude was quite small and no collapses were detected.

Mechanisms of single bubble cleaning

The dynamics of collapsing bubbles close to a flat solid is investigated with respect to its potential for removal of surface attached particles. Individual bubbles are created by nanosecond Nd:YAG laser pulses focused into water close to glass plates contaminated with melamine resin micro-particles. The bubble dynamics is analysed by means of synchronous high-speed recordings. Due to the close solid boundary, the bubble collapses with the well-known liquid jet phenomenon. Subsequent microscopic inspection of the substrates reveals circular areas clean of particles after a single bubble generation and collapse event. The detailed bubble dynamics, as well as the cleaned area size, is characterised by the non-dimensional bubble stand-off γ=d/Rmax, with d: laser focus distance to the solid boundary, and Rmax: maximum bubble radius before collapse. We observe a maximum of clean area at γ≈0.7, a roughly linear decay of the cleaned circle radius for increasing γ, and no cleaning for γ>3.5. As the main mechanism for particle removal, rapid flows at the boundary are identified. Three different cleaning regimes are discussed in relation to γ: (I) For large stand-off, 1.8<γ<3.5, bubble collapse induced vortex flows touch down onto the substrate and remove particles without significant contact of the gas phase. (II) For small distances, γ<1.1, the bubble is in direct contact with the solid. Fast liquid flows at the substrate are driven by the jet impact with its subsequent radial spreading, and by the liquid following the motion of the collapsing and rebounding bubble wall. Both flows remove particles. Their relative timing, which depends sensitively on the exact γ, appears to determine the extension of the area with forces large enough to cause particle detachment. (III) At intermediate stand-off, 1.1<γ<1.8, only the second bubble collapse touches the substrate, but acts with cleaning mechanisms similar to an effective small γ collapse: particles are removed by the jet flow and the flow induced by the bubble wall oscillation. Furthermore, the observations reveal that the extent of direct bubble gas phase contact to the solid is partially smaller than the cleaned area, and it is concluded that three-phase contact line motion is not a major cause of particle removal. Finally, we find a relation of cleaning area vs. stand-off γ that deviates from literature data on surface erosion. This indicates that different effects are responsible for particle removal and for substrate damage. It is suggested that a trade-off of cleaning potential and damage risk for sensible surfaces might be achieved by optimising γ.

Micropatterning for the control of surface cavitation: visualization through high-speed imaging

For the first time, we apply a high-speed imaging technique to record the activity of acoustically driven cavitation bubbles (86 kHz) on micropatterned surfaces with hydrophobic and hydrophilic stripes. The width of the hydrophobic stripes lies between 3.5 and 115 μm. This work provides the first direct visualization of the preferential location of bubbles on the hydrophobic areas of the patterns. The results confirm our previous prediction that surface cavitation strongly depends on the surface energy of the irradiated substrate. The observations show a remarkable effect of the stripe width on the size, movement, growth, splitting, and multiplying of the bubbles. The high-speed imaging also reveals that there is a minimal width of the hydrophobic stripes that allows bubble attraction and formation. Our observations are supported by a theoretical approach based on the forces acting on the bubbles.

The potential utility of high-intensity ultrasound to treat osteoarthritis

Osteoarthritis (OA) is a widespread musculoskeletal disease that reduces quality of life and for which there is no cure. The treatment of OA is challenging since cartilage impedes the local and systemic delivery of therapeutic compounds (TCs). This review identifies high-intensity ultrasound (HIU) as a non-contact technique to modify articular cartilage and subchondral bone. HIU enables new approaches to overcome challenges associated with drug delivery to cartilage and new non-invasive approaches for the treatment of joint disease. Specifically, HIU has the potential to facilitate targeted drug delivery and release deep within cartilage, to repair soft tissue damage, and to physically alter tissue structures including cartilage and bone. The localized, non-invasive ultrasonic delivery of TCs to articular cartilage and subchondral bone appears to be a promising technique in the immediate future.

An erosion sensor based on a quartz crystal microbalance for quantitative determination of the cleaning efficiency in an ultrasonic vessel

The efficiency of ultrasonic cleaning vessels cannot be measured directly in an easy way. In the presented work, a sensor is developed which quantitatively measures the ablation of a test layer. The sensor element is a quartz crystal which is coated with a sacrificial layer. Small changes in mass of this layer can be measured by a frequency shift of the crystal oscillation. For measurements, a 10 MHz AT-cut quartz crystal was used in a cleaning vessel working at 44.9 kHz. To determine the frequency shift by the ablation of the test layer, the quartz crystal was driven by a frequency generator sweeping the frequency in the range of the resonance frequency and a characteristic frequency was determined. The test layer which was applied to the quartz crystal consisted of silica microparticles suspended in varnish. In a preliminary experiment using a commercial cleaner it could be shown that significant changes in resonance frequency by cavitation effect could be detected. The initial frequency shift of the sacrificial layer is reproducible within 10%. The test layer can be adapted to the conditions of the cleaning vessel. By changing the electrical input power of the vessel, a threshold in the cavitation erosion was found.

Study of non-spherical bubble oscillations near a surface in a weak acoustic standing wave field

The interaction of acoustically driven bubbles with a wall is important in many applications of ultrasound and cavitation, as the close boundary can severely alter the bubble dynamics. In this paper, the non-spherical surface oscillations of bubbles near a surface in a weak acoustic standing wave field are investigated experimentally and numerically. The translation, the volume, and surface mode oscillations of bubbles near a flat glass surface were observed by a high speed camera in a standing wave cell at 46.8 kHz. The model approach is based on a modified Keller-Miksis equation coupled to surface mode amplitude equations in the first order, and to the translation equations. Modifications are introduced due to the adjacent wall. It was found that a bubble's oscillation mode can change in the presence of the wall, as compared to the bubble in the bulk liquid. In particular, the wall shifts the instability pressure thresholds to smaller driving frequencies for fixed bubble equilibrium radii, or to smaller equilibrium radii for fixed excitation frequency. This can destabilize otherwise spherical bubbles, or stabilize bubbles undergoing surface oscillations in the bulk. The bubble dynamics observed in experiment demonstrated the same trend as the theoretical results.

Ultrasound assisted co-precipitation of nanostructured CuO-ZnO-Al2O3 over HZSM-5: effect of precursor and irradiation power on nanocatalyst properties and catalytic performance for direct syngas to DME

Nanostructured CuO-ZnO-Al2O3/HZSM-5 was synthesized from nitrate and acetate precursors using ultrasound assisted co-precipitation method under different irradiation powers. The CuO-ZnO-Al2O3/HZSM-5 nanocatalysts were characterized using XRD, FESEM, BET, FTIR and EDX Dot-mapping analyses. The results indicated precursor type and irradiation power have significant influences on phase structure, morphology, surface area and functional groups. It was observed that the acetate formulated CuO-ZnO-Al2O3/HZSM-5 nanocatalyst have smaller CuO crystals with better dispersion and stronger interaction between components in comparison to nitrate based nanocatalysts. Ultrasound assisted co-precipitation synthesis method resulted in nanocatalyst with more uniform morphology compared to conventional method and increasing irradiation power yields smaller particles with better dispersion and higher surface area. Additionally the crystallinity of CuO is lower at high irradiation powers leading to stronger interaction between metal oxides. The nanocatalysts performance were tested at 200-300 °C, 10-40 bar and space velocity of 18,000-36,000 cm(3)/g h with the inlet gas composition of H2/CO = 2/1 in a stainless steel autoclave reactor. The acetate based nanocatalysts irradiated with higher levels of power exhibited better reactivity in terms of CO conversion and DME yield. While there is an optimal temperature for CO conversion and DME yield in direct synthesis of DME, CO conversion and DME yield both increase with the pressure increase. Furthermore ultrasound assisted co-precipitation method yields more stable CuO-ZnO-Al2O3/HZSM-5 nanocatalyst while conventional precipitated nanocatalyst lost their activity ca. 18% and 58% in terms of CO conversion and DME yield respectively in 24 h time on stream test.

Sonochemical and high-speed optical characterization of cavitation generated by an ultrasonically oscillating dental file in root canal models

Ultrasonically Activated Irrigation makes use of an ultrasonically oscillating file in order to improve the cleaning of the root canal during a root canal treatment. Cavitation has been associated with these oscillating files, but the nature and characteristics of the cavitating bubbles were not yet fully elucidated. Using sensitive equipment, the sonoluminescence (SL) and sonochemiluminescence (SCL) around these files have been measured in this study, showing that cavitation occurs even at very low power settings. Luminol photography and high-speed visualizations provided information on the spatial and temporal distribution of the cavitation bubbles. A large bubble cloud was observed at the tip of the files, but this was found not to contribute to SCL. Rather, smaller, individual bubbles observed at antinodes of the oscillating file with a smaller amplitude were leading to SCL. Confinements of the size of bovine and human root canals increased the amount of SL and SCL. The root canal models also showed the occurrence of air entrainment, resulting in the generation of stable bubbles, and of droplets, near the air-liquid interface and leading eventually to a loss of the liquid.

Dynamic features of a laser-induced cavitation bubble near a solid boundary

This paper deals with detailed features of bubble dynamics near a solid boundary. The cavitation bubble was created by using a Q-switched Nd: YAG laser pulse and observed using a high-speed camera (up to 100,000 frames per second). A hydrophone system was employed to monitor the acoustic signals generated by the transient pressure impulses and estimate the bubble oscillation periods. Experimental observations were carried out for bubbles with various maximum expanded radius Rmax (between 1.0mm and 1.6mm) and stand-off distances, ds (defined as the distance between the solid boundary and the bubble center at inception) of 0.4≤γ≤3.0, and γ=ds/Rmax. The existence of a solid boundary created asymmetry in the flow field and forced the bubble to collapse non-spherically, which finally brought forth the jet impact phenomenon. The dimensionless first and second oscillation periods were dependent on γ. A series of expansion and collapse of the bubble with cascading loss of energy were observed after the bubble had been generated. This study revealed that most bubbles lost about two-thirds of the total energy from the first maximum expansion to the second maximum expansion.

Observations of cavitation erosion pit formation

Previous investigations showed that a single cavitation bubble collapse can cause more than one erosion pit (Philipp & Lauterborn [1]). But our preliminary study showed just the opposite - that in some cases a single cavitation pit can result from more than one cavitation event. The present study shows deeper investigation of this phenomenon. An investigation of the erosion effects of ultrasonic cavitation on a thin aluminum foil was made. In the study we observed the formation of individual pits by means of high speed cameras (>1000 fps) and quantitatively evaluated the series of images by stereoscopy and the shape from shading method. This enabled the reconstruction of the time evolution of the pit shape. Results show how the foil is deformed several times before a hole is finally punctured. It was determined that larger single pits result from several impacts of shock waves on the same area, which means that they are merely special cases of pit clusters (pit clusters where pits overlap perfectly). Finally it was shown that a thin foil, which is subjected to cavitation, behaves as a membrane. It was concluded that the physics behind erosion depends significantly on the means of generating cavitation (acoustic, hydrodynamic, laser light) and the specimen characteristics (thin foil, massive specimen), which makes comparison of results of materials resistance to cavitation from different experimental set-ups questionable. Further development of the shape from shading method in the scope of cavitation erosion testing will enable better evaluation of cavitation erosion models.

Dissolved gas and ultrasonic cavitation--a review

The physics and chemistry of nonlinearly oscillating acoustic cavitation bubbles are strongly influenced by the dissolved gas in the surrounding liquid. Changing the gas alters among others the luminescence spectrum, and the radical production of the collapsing bubbles. An overview of experiments with various gas types and concentration described in literature is given and is compared to mechanisms that lead to the observed changes in luminescence spectra and radical production. The dissolved gas type changes the bubble adiabatic ratio, thermal conductivity, and the liquid surface tension, and consequently the hot spot temperature. The gas can also participate in chemical reactions, which can enhance radical production or luminescence of a cavitation bubble. With this knowledge, the gas content in cavitation can be tailored to obtain the desired output.

The structures and evolution of Smoker in an ultrasonic field

The structures and evolution of Smoker in a 20 kHz ultrasonic field were investigated experimentally with high-speed photography. The spine-plume structure of Smoker was discovered. A few large bubbles align themselves along the central line and form the spine of Smoker. Numerous small bubbles move towards the spine and form the plume structures. The size of large bubbles differs almost by an order of magnitude from that of small bubbles. The evolution of cavitation structure from Flare to Smoker was found. When a Flare appears near a Smoker, the Flare may merge into the plume structures of the Smoker, or form a double-tipped Smoker. A double-tipped Smoker seldom splits into two Smokers, while two separate Smokers tend to merge as one. The large bubbles (or dense plume structures) in the middle part of the two separate Smokers attract each other, driving the two Smokers to bend towards each other and merge.

Model processes and cavitation indicators for a quantitative description of an ultrasonic cleaning vessel: Part I: experimental results

In this paper, four sensor types are presented for quantitative measurements in an ultrasonic cleaning vessel: (1) a hydrophone to measure spectral components of the sound field; (2) an aluminium foil technique as a model process for erosion; (3) a test tube filled with a solution of luminol to measure the emission of light; and (4) a test tube filled with potassium iodide solution to measure the oxidation of iodide. Thus a broad range of diverse cavitation effects is covered. The quantities were measured in dependence on three parameters: the electrical input power of the transducers, the temperature and the O(2) concentration of the water. To ensure constant environmental conditions, a flow system was built up which continuously exchanges the water in the vessel. The comparability of the data measured in subsequent measurement cycles is discussed and the influence of the different sensor types on the cavitation field is considered. Dependences on the three parameters are shown. A quantitative analysis of correlations between the data is carried out in the second part of the study (Koch and Jüschke, 2012 [1]).

A simple model of ultrasound propagation in a cavitating liquid. Part II: Primary Bjerknes force and bubble structures

In a companion paper, a reduced model for propagation of acoustic waves in a cloud of inertial cavitation bubbles was proposed. The wave attenuation was calculated directly from the energy dissipated by a single bubble, the latter being estimated directly from the fully nonlinear radial dynamics. The use of this model in a mono-dimensional configuration has shown that the attenuation near the vibrating emitter was much higher than predictions obtained from linear theory, and that this strong attenuation creates a large traveling wave contribution, even for closed domain where standing waves are normally expected. In this paper, we show that, owing to the appearance of traveling waves, the primary Bjerknes force near the emitter becomes very large and tends to expel the bubbles up to a stagnation point. Two-dimensional axi-symmetric computations of the acoustic field created by a large area immersed sonotrode are also performed, and the paths of the bubbles in the resulting Bjerknes force field are sketched. Cone bubble structures are recovered and compare reasonably well to reported experimental results. The underlying mechanisms yielding such structures is examined, and it is found that the conical structure is generic and results from the appearance a sound velocity gradient along the transducer area. Finally, a more complex system, similar to an ultrasonic bath, in which the sound field results from the flexural vibrations of a thin plate, is also simulated. The calculated bubble paths reveal the appearance of other commonly observed structures in such configurations, such as streamers and flare structures.

Study on the bubble transport mechanism in an acoustic standing wave field

The use of bubbles in applications such as surface chemistry, drug delivery, and ultrasonic cleaning etc. has been enormously popular in the past two decades. It has been recognized that acoustically-driven bubbles can be used to disturb the flow field near a boundary in order to accelerate physical or chemical reactions on the surface. The interactions between bubbles and a surface have been studied experimentally and analytically. However, most of the investigations focused on violently oscillating bubbles (also known as cavitation bubble), less attention has been given to understand the interactions between moderately oscillating bubbles and a boundary. Moreover, cavitation bubbles were normally generated in situ by a high intensity laser beam, little experimental work has been carried out to study the translational trajectory of a moderately oscillating bubble in an acoustic field and subsequent interactions with the surface. This paper describes the design of an ultrasonic test cell and explores the mechanism of bubble manipulation within the test cell. The test cell consists of a transducer, a liquid medium and a glass backing plate. The acoustic field within the multi-layered stack was designed in such a way that it was effectively one dimensional. This was then successfully simulated by a one dimensional network model. The model can accurately predict the impedance of the test cell as well as the mode shape (distribution of particle velocity and stress/pressure field) within the whole assembly. The mode shape of the stack was designed so that bubbles can be pushed from their injection point onto a backing glass plate. Bubble radial oscillation was simulated by a modified Keller-Miksis equation and bubble translational motion was derived from an equation obtained by applying Newton's second law to a bubble in a liquid medium. Results indicated that the bubble trajectory depends on the acoustic pressure amplitude and initial bubble size: an increase of pressure amplitude or a decrease of bubble size forces bubbles larger than their resonant size to arrive at the target plate at lower heights, while the trajectories of smaller bubbles are less influenced by these factors. The test cell is also suitable for testing the effects of drag force on the bubble motion and for studying the bubble behavior near a surface.

Motion of micrometer sized spherical particles exposed to a transient radial flow: attraction, repulsion, and rotation

It is now accepted that the physical forces in ultrasonic cleaning are due to strongly pulsating bubbles driven by the sound field. Here we have a detailed look at bubble induced cleaning flow by analyzing the transport of an individual particle near an expanding and collapsing bubble. The induced particulate transport is compared with a force balance model. We find two important properties of the flow which explain why bubbles are effectively cleaning: During bubble expansion a strong shear layer loosens the particle from the surface through particle spinning and secondly an unsteady boundary layer generates an attractive force, thus collecting the contamination in the bubble's close proximity.