Time-resolved monitoring of cavitation activity in megasonic cleaning systems

Origin of the broad-band noise in acoustic cavitation

The broad-band noise has been experimentally used to monitor the cavitation activity in a sonochemical reactor, an ultrasonic cleaning bath, a biological tissue, etc. However, the origin of the broad-band noise is still under debate. In the present review, two models for the mechanism of the broad-band noise are discussed. One is acoustic emissions from chaotically (non-periodically) pulsating bubbles. The other is acoustic emissions from bubbles with temporal fluctuation in the number of bubbles. It is suggested that the latter mechanism is sometimes dominant. Further studies are required on the role for bubble cluster dynamics as well as the bubble-bubble interaction in the broad-band noise especially at relatively low ultrasonic frequencies.

The in situ electrochemical detection of microbubble oscillations during motion through a channel

Bubble oscillation has many applications, from driving local fluid motion to cleaning. However, in order to exploit their action, a full understanding of this motion, particularly in confined spaces (such as crevices etc. which are important in ultrasonic decontamination) is important. To this end, here we show how a Coulter counter can be used to characterize microbubbles produced through the ultrasonication of electrolytes. These microbubbles are shown to exist in relatively high concentrations while bubble activity is driven by ultrasound. Detection of these microbubbles, and their oscillatory behaviour, is achieved via translocation through a cylindrical glass microchannel (GMC). The microbubbles oscillate within the 40 μm channel employed and this behaviour is observed to change over the translocation period. This is attributed to the acoustic environment present or changes to the physical conditions in the interior of the chamber compared to the exterior. High-speed imaging confirms the presence of microbubbles as they move or 'skate' across the surface of the structures present before translocating through the channel. The observations are useful as they show that microbubble oscillation occurs within small structures, is preceded by surface confined bubbles and could be enhanced through pressure driven flow through a structure.

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.

Interstitial Matrix Prevents Therapeutic Ultrasound From Causing Inertial Cavitation in Tumescent Subcutaneous Tissue

We search for cavitation in tumescent subcutaneous tissue of a live pig under application of pulsed, 1-MHz ultrasound at 8 W cm^-2 spatial peak and pulse-averaged intensity. We find no evidence of broadband acoustic emission indicative of inertial cavitation. These acoustic parameters are representative of those used in external-ultrasound-assisted lipoplasty and in physical therapy and our null result brings into question the role of cavitation in those applications. A comparison of broadband acoustic emission from a suspension of ultrasound contrast agent in bulk water with a suspension injected subcutaneously indicates that the interstitial matrix suppresses cavitation and provides an additional mechanism behind the apparent lack of in-vivo cavitation to supplement the absence of nuclei explanation offered in the literature. We also find a short-lived cavitation signal in normal, non-tumesced tissue that disappears after the first pulse, consistent with cavitation nuclei depletion in vivo.

An activated fluid stream--New techniques for cold water cleaning

Electrochemical, acoustic and imaging techniques are used to characterise surface cleaning with particular emphasis on the understanding of the key phenomena relevant to surface cleaning. A range of novel techniques designed to enhance and monitor the effective cleaning of a solid/liquid interface is presented. Among the techniques presented, mass transfer of material to a sensor embedded in a surface is demonstrated to be useful in the further exploration of ultrasonic cleaning of high aspect ratio micropores. In addition the effect of micropore size on the cleaning efficacy is demonstrated. The design and performance of a new cleaning system reliant on the activation of bubbles within a free flowing stream is presented. This device utilised acoustic activation of bubbles within the stream and at a variety of substrates. Finally, a controlled bubble swarm is generated in the stream using electrolysis, and its effect on both acoustic output and cleaning performance are compared to the case when no bubbles are added. This will demonstrate the active role that the electrochemically generated bubble swarm can have in extending the spatial zone over which cleaning is achieved.

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.

Electrochemical ‘bubble swarm’ enhancement of ultrasonic surface cleaning

Control and excitation of electrochemical bubble swarms enhances surface cleaning within an ultrasonically activated stream.

Low-frequency low energy ultrasound combined with microbubbles induces distinct apoptosis of A7r5 cells

The present study aimed to investigate whether low frequency low energy ultrasound combined with microbubbles induces apoptotic cell death of A7r5 rat aortic vascular smooth muscle cells, and to identify the possible mechanisms underlying this effect. Ultrasonic waves (45 kHz with 0.3 Wcm2 of intensity for 0, 10, 20 and 30 sec) were used together with different dosages of SonoVue™ microbubbles (0, 14, 28, 42 and 56 µl), respectively. The cell viability and apoptotic rate were determined by trypan blue staining immediately following treatment and flow cytometry 24 h thereafter. The treatment conditions resulting in the lowest amount of necrosis, highest apoptotic rate and lowest microbubble dosage was selected for the US+MB group, which was treated with ultrasound combined with microbubbles. The cell proliferation 24 h following treatment was determined and western blot analysis was applied to examine the expression of apoptosis‑associated proteins, B-cell lymphoma 2 (Bcl‑2) and Bcl-2-associated X (Bax). The harmonic acoustic pressure amplitude was measured to obtain the cavitation intensity. The combination of 20 sec ultrasound irradiation and 14 µl SonoVue™ was selected as the treatment conditions for the US+MB group. The results demonstrated that both ultrasound alone (the US group) and in combination with microbubbles significantly inhibited the proliferation of A7r5 cells compared with that of the control (P<0.01), and the suppression in the US+MB group was significantly greater (P<0.01). The apoptotic rate in A7r5 cells induced by this combination treatment (16.62±0.93%) was significantly higher than that in the control (3.93±0.39%; P<0.01) and US (6.88±1.87%; P<0.01) groups. Treatment with ultrasound combined with microbubbles increased the expression of Bax and decreased the ratio of Bcl‑2/Bax compared with those in the control and US groups. The cavitation induced by ultrasound combined with microbubble treatment was more intense than that by ultrasound alone. The results demonstrated that the cell death and apoptosis of A7r5 cells were associated with ultrasound duration and microbubble dosage. Low frequency ultrasound combined with microbubbles induced apoptosis in A7r5 cells through the upregulation of Bax and the downregulation of the Bcl‑2/Bax ratio, where the cavitation effect may have an important role.

Physical forces exerted by microbubbles on a surface in a traveling wave field

The effect of a wave with a varying traveling component on the bubble activity as well as the physical force generated by microbubbles on a surface has been studied. The acoustic emission from a collection of bubbles is measured in a 928 kHz sound field. Particle removal tests on a surface, which actually measures the applied physical force by the bubbles on that surface, indicate a very strong dependence on the angle of incidence. In other words, when the traveling wave component is maximized, the average physical force applied by microbubbles reaches a maximum. Almost complete particle removal for 78 nm silica particles was obtained for a traveling wave, while particle removal efficiency was reduced to only a few percent when a standing wave was applied. This increase in particle removal for a traveling wave is probably caused by a decrease in bubble trapping at nodes and antinodes in a standing wave field.

Towards an understanding and control of cavitation activity in 1 MHz ultrasound fields

Various industrial processes such as sonochemical processing and ultrasonic cleaning strongly rely on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in a vessel is strongly depending on the ultrasonic process conditions. It is therefore crucial to quantify cavitation activity as a function of the process parameters. At 1 MHz, the active cavitation bubbles are so small that it is becoming difficult to observe them in a direct way. Hence, another metrology based on secondary effects of acoustic cavitation is more suitable to study cavitation activity. In this paper we present a detailed analysis of acoustic cavitation phenomena at 1 MHz ultrasound by means of time-resolved measurements of sonoluminescence, cavitation noise, and synchronized high-speed stroboscopic Schlieren imaging. It is shown that a correlation exists between sonoluminescence, and the ultraharmonic and broadband signals extracted from the cavitation noise spectra. The signals can be utilized to characterize different regimes of cavitation activity at different acoustic power densities. When cavitation activity sets on, the aforementioned signals correlate to fluctuations in the Schlieren contrast as well as the number of nucleated bubbles extracted from the Schlieren Images. This additionally proves that signals extracted from cavitation noise spectra truly represent a measure for cavitation activity. The cyclic behavior of cavitation activity is investigated and related to the evolution of the bubble populations in the ultrasonic tank. It is shown that cavitation activity is strongly linked to the occurrence of fast-moving bubbles. The origin of this "bubble streamers" is investigated and their role in the initialization and propagation of cavitation activity throughout the sonicated liquid is discussed. Finally, it is shown that bubble activity can be stabilized and enhanced by the use of pulsed ultrasound by conserving and recycling active bubbles between subsequent pulsing cycles.

Enhancement of cavitation activity and particle removal with pulsed high frequency ultrasound and supersaturation

Megasonic cleaning as applied in leading edge semiconductor device manufacturing strongly relies on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in the cleaning liquid strongly depends on the sonication conditions. It is shown that cavitation activity as measured by means of ultraharmonic cavitation noise can be significantly enhanced when pulsed sonication is applied to a gas supersaturated liquid under traveling wave conditions. It is demonstrated that this enhancement coincides with a dramatic increase in particle removal and is therefore of great interest for megasonic cleaning applications. It is demonstrated that the optimal pulse parameters are determined by the dissolution time of the active bubbles, whereas the amount of cavitation activity depends on the ratio between pulse-off and pulse-on time as well as the applied acoustic power. The optimal pulse-off time is independent of the corresponding pulse-on time but increases significantly with increasing gas concentration. We show that on the other hand, supersaturation is needed to enable acoustic cavitation at aforementioned conditions, but has to be kept below values, for which active bubbles cannot dissolve anymore and are therefore lost during subsequent pulses. For the applicable range of gas contents between 100% and 130% saturation, the optimal pulse-off time reaches values between 150 and 340 ms, respectively. Full particle removal of 78 nm-diameter silica particles at a power density of 0.67 W/cm(2) is obtained for the optimal pulse-off times. The optimal pulse-off time values are derived from the dissolution time of bubbles with a radius of 3.3 μm and verified experimentally. The bubble radius used in the calculations corresponds to the linear resonance size in a 928 kHz sound field, which demonstrates that the recycling of active bubbles is the main enhancement mechanism. The optimal choice of the pulsing conditions however is constrained by the trade-off between the effective sonication time and the desire to have a sufficient amount of active bubbles at lower powers, which might be necessary if very delicate structures have to be cleaned.