Selective aggregation by ultrasonic standing waves through gas nuclei on the particle surface

Gas nuclei in water are usually too small to be directly observed. They will grow into bubbles under the negative pressure, which is called cavitation (heterogeneous cavitation). In this study, the gas nuclei in the hydrophilic and hydrophobic silica particle suspension were investigated using the transient cavitation threshold measured by a high-intensity focused ultrasound (HIFU). The transient cavitation bubbles were also observed by a high-speed camera. The results showed that the nuclei only exist on the surface of hydrophobic particles. Furthermore, the aggregation experiments revealed that the aggregates were only formed in the hydrophobic silica suspension by ultrasonic standing waves (USW) at 200 kHz. This distinct difference was mainly due to the formation of gas nuclei on hydrophobic silica particles, which grew and coalesced into stable bubbles under the 200 kHz USW. The aggregation process in suspension was observed by a CCD camera. Moreover, the cavitation thresholds and acoustic radiation forces were analyzed to explain the mechanism of the acoustic aggregation. This study showed a very promising acoustic method for the selective aggregation of hydrophobic particles, which might be efficiently used in the mineral separation industry.     

An acoustic backscattering technique for the detection of transient cavitation produced by microsecond pulses of ultrasound

An acoustic backscattering technique for detecting transient cavitation produced by 10-microseconds-long pulses of 757-kHz ultrasound is described. The system employs 10-microseconds-long, 30-MHz center frequency tone bursts that scatter from cavitation microbubbles. Experiments were performed with suspensions of hydrophobic polystyrene spheres in ultraclean water. Transient cavitation threshold pressures measured with the active cavitation detector (ACD) were always less than or equal to those measured using a passive acoustic detection scheme. The measured cavitation thresholds decreased with increasing dissolved gas content and increasing suspended particle concentration. Results also show that ultrasonic irradiation of the polystyrene sphere suspensions by the ACD lowered the threshold pressure measured with the passive detector. A possible mechanism through which suspensions of hydrophobic particles might nucleate bubbles is presented.     

Numerical modelling of acoustic cavitation threshold in water with non-condensable bubble nuclei

Numerical modelling of acoustic cavitation threshold in water is presented taking into account non-condensable bubble nuclei, which are composed of water vapor and non-condensable air. The cavitation bubble growth and collapse dynamics are modeled by solving the Rayleigh-Plesset or Keller-Miksis equation, which is combined with the energy equations for both the bubble and liquid domains, and directly evaluating the phase-change rate from the liquid and bubble side temperature gradients. The present work focuses on elucidating acoustic cavitation in water with a wide range of cavitation thresholds (0.02–30 MPa) reported in the literature. Computations for different nucleus sizes and acoustic frequencies are performed to investigate their effects on bubble growth and cavitation threshold. The numerical predictions are observed to be comparable to the experimental data in the previous works and show that the cavitation threshold in water has a wide range depending on the bubble nucleus size.     

Oscillation and Migration of Bubbles within Ultrasonic Field

The oscillation and migration of bubbles within an intensive ultrasonic field are important issues concerning acoustic cavitation in liquids.We establish a selection map of bubble oscillation mode related to initial bubble radius and driving sound pressure under 20 kHz ultrasound and analyze the individual-bubble migration induced by the combined effects of pressure gradient and acoustic streaming.Our results indicate that the pressure threshold of stable and transient cavitation decreases with the increasing initial bubble radius.At the pressure antinode,the Bjerknes force dominates the bubble migration, resulting in the large bubbles gathering toward antinode center,whereas small bubbles escape from antinode.By contrast,at the pressure node,the bubble migration is primarily controlled by acoustic streaming,which effectively weakens the bubble adhesion on the container walls,thereby enhancing the cavitation effect in the whole liquid.     

The effect of static pressure on the inertial cavitation threshold

The amplitude of the acoustic pressure required to nucleate a gas or vapor bubble in a fluid, and to have that bubble undergo an inertial collapse, is termed the inertial cavitation threshold. The magnitude of the inertial cavitation threshold is typically limited by mechanisms other than homogeneous nucleation such that the theoretical maximum is never achieved. However, the onset of inertial cavitation can be suppressed by increasing the static pressure of the fluid. The inertial cavitation threshold was measured in ultrapure water at static pressures up to 30?MPa (300 bars) by exciting a radially symmetric standing wave field in a spherical resonator driven at a resonant frequency of 25.5 kHz. The threshold was found to increase linearly with the static pressure; an exponentially decaying temperature dependence was also found. The nature and properties of the nucleating mechanisms were investigated by comparing the measured thresholds to an independent analysis of the particulate content and available models for nucleation.     

Closed loop cavitation control – A step towards sonomechatronics

In the field of sonochemistry, many processes are made possible by the generation of cavitation. This article is about closed loop control of ultrasound assisted processes with the aim of controlling the intensity of cavitation-based sonochemical processes. This is the basis for a new research field which the authors call “sonomechatronics”. In order to apply closed loop control, a so called self-sensing technique is applied, which uses the ultrasound transducer’s electrical signals to gain information about cavitation activity. Experiments are conducted to find out if this self-sensing technique is capable of determining the state and intensity of acoustic cavitation. A distinct frequency component in the transducer’s current signal is found to be a good indicator for the onset and termination of transient cavitation. Measurements show that, depending on the boundary conditions, the onset and termination of transient cavitation occur at different thresholds, with the onset occurring at a higher value in most cases. This known hysteresis effect offers the additional possibility of achieving an energetic optimization by controlling cavitation generation.Using the cavitation indicator for the implementation of a double set point closed loop control, the mean driving current was reduced by approximately 15% compared to the value needed to exceed the transient cavitation threshold. The results presented show a great potential for the field of sonomechatronics. Nevertheless, further investigations are necessary in order to design application-specific sonomechatronic processes.     

Reproducible cavitation activity in water-particle suspensions

The study of cavitation inception in liquids rarely yields reproducible data, unless special control is taken on the cleanliness of the experimental environment. In this paper, an experimental technique is demonstrated which allows repeatable measurements of cavitation activity in liquid-particle suspensions. In addition, the method is noninvasive: cavitation bubbles are generated using a shock-wave generator, and they are photographed using a digital camera. The cavitation activity is obtained after suitable image processing steps. From these measurements, the importance of the particle's surface structure and its chemical composition is revealed, with polystyrene and polyamide particles generating the highest yields. Further findings are that cavitation nuclei become depleted with an increasing number of experiments, and the existence of nuclei with varying negative pressure thresholds. Finally, a decrease of the cavitation yield is achieved by prepressurization of the suspension-indicating that the cavitation nuclei are gaseous.     

Acoustic cavitation inception

This article presents a selective review of the subject of acoustic cavitation inception. Topics covered include cavitation threshold measurements, the role of dirt or pre-existing bubble nuclei in cavitation inception, radiation-induced acoustic cavitation, and a discussion of some unsolved problems such as cavitation thresholds in liquid helium and the effects of electric fields on thresholds.     

Correlation between microbubble-induced acoustic cavitation and hemolysis in vitro

Microbubbles promise to enhance the efficiency of ultrasound-mediated drug delivery and gene therapy by taking advantage of artificial cavitation nuclei.The purpose of this study is to examine the ultrasound-induced hemolysis in the application of drug delivery in the presence of microbubbles.To achieve this goal,human red blood cells mixed with microbubbles were exposed to 1-MHz pulsed ultrasound.The hemolysis level was measured by a flow cytometry,and the cavitation dose was detected by a passive cavitation detecting system.The results demonstrate that larger cavitation dose would be generated with the increase of acoustic pressure,which might give rise to the enhancement of hemolysis.Besides the experimental observations,the acoustic pressure dependence of the radial oscillation of microbubble was theoretically estimated.The comparison between the experimental and calculation results indicates that the hemolysis should be highly correlated to the acoustic cavitation.     

Possible explanation for the unexpected absence of gross biological damage to membranes of cells insonated in suspension and in surface culture in chambers exposed to standing and progressive wave fields

Quoted values of cavitation thresholds reported in the literature vary over several orders of magnitude. This paper describes an investigation of the threshold in a chamber with acoustically transparent windows, situated around the last axial maximum of a 0.75 MHz standing wave field. The chamber, the transducer and the reflector are submerged in a tank containing distilled water which also fills the chamber. The formation of bubbles outside the chamber, both above and below it, occurred at lower intensities than those needed to generate bubbles in the chamber. This unexpected finding led to a theoretical study of streaming patterns in progressive and standing wave fields. These showed that there is a region around the last axial maximum in which there is no gradient of energy density. Therefore if this region is enclosed by an acoustically transparent chamber, no significant bulk streaming occurs within it. We speculate on how this lack of streaming raises the cavitation threshold. The cavitation phenomenon is also examined by Doppler ultrasound of frequency 8 MHz. This confirms the occurrence of increased intensity thresholds within the chamber. It also shows that the time between filling the chamber and the start of sonication strongly influences the magnitude of the cavitation threshold. We expect that the effects described here may have consequences for sonication of cells in suspension culture when the samples are held in chambers situated around the last axial maximum of an ultrasound beam from a plane transducer.     

亚临界水中超声激励空化泡动力学分析

考察亚临界水中压力和温度对超声空化泡动力学的影响。应用非线性Rayleigh-Plesset方程模拟空化泡运动过程,并利用Matlab软件编程求数值解,用碘量法测定超声在亚临界水中的声空化产额。结果表明:当亚临界水的压力相似文献   

Harmonic responses and cavitation activity of encapsulated microbubbles coupled with magnetic nanoparticles

Encapsulated microbubbles coupled with magnetic nanoparticles, one kind of hybrid agents that can integrate both ultrasound and magnetic resonance imaging/therapy functions, have attracted increasing interests in both research and clinic communities. However, there is a lack of comprehensive understanding of their dynamic behaviors generated in diagnostic and therapeutic applications. In the present work, a hybrid agent was synthesized by integrating superparamagnetic iron oxide nanoparticles (SPIOs) into albumin-shelled microbubbles (named as SPIO-albumin microbubbles). Then, both the stable and inertial cavitation thresholds of this hybrid agent were measured at varied SPIO concentrations and ultrasound parameters (e.g., frequency, pressure amplitude, and pulse length). The results show that, at a fixed acoustic driving frequency, both the stable and inertial cavitation thresholds of SPIO-albumin microbubble should decrease with the increasing SPIO concentration and acoustic driving pulse length. The inertial cavitation threshold of SPIO-albumin microbubbles also decreases with the raised driving frequency, while the minimum sub- and ultra-harmonic thresholds appear at twice and two thirds resonance frequency, respectively. It is also noticed that both the stable and inertial cavitation thresholds of SonoVue microbubbles are similar to those measured for hybrid microbubbles with a SPIO concentration of 114.7 μg/ml. The current work could provide better understanding on the impact of the integrated SPIOs on the dynamic responses (especially the cavitation activities) of hybrid microbubbles, and suggest the shell composition of hybrid agents should be appropriately designed to improve their clinical diagnostic and therapeutic performances of hybrid microbubble agents.     

The effect of temperature and viscoelasticity on cavitation dynamics during ultrasonic ablation

Inertial cavitation has been shown to enhance heating rates during high intensity focused ultrasound treatments. Cavitation dynamics will be affected by heating and by the changes in mechanical properties of tissue resultant from thermal denaturation; however, the nature of the change is not known and forms the focus of the current study. A Keller-Miksis equation is used to find the variation in inertial cavitation threshold with temperature in water and, when coupled with a Kelvin-Voigt viscoelastic model, in biological tissue. Simulated thermal ablation treatments in liver and muscle are used to explore the changes in cavitation dynamics, and the resultant frequency spectra of secondary acoustic emissions, due to tissue denaturation. Results indicate that viscosity is the key parameter controlling cavitation dynamics in biological tissues. The increase in viscosity during denaturation is predicted to increase inertial cavitation thresholds, leading to a substantial decrease in the higher harmonic content of the emitted pressure signal across a wide range of bubble radii. Experimental validation of these observations could offer improved methods to monitor therapeutic ultrasound treatments.     

Effect of temperature on rectified diffusion during ultrasound-induced heating

Experimental observations of delayed-onset cavitation during ultrasound insonation have been suggested as being caused by a change in the size distribution of the bubble population due to rectified diffusion. To investigate this hypothesis, a single bubble model is used here to explore the effect of heating and the subsequent elevated temperatures on the rectified diffusion process. Numerical solution of the model, which includes the temperature dependences of seven relevant physical parameters, allows quantification of the change in the pressure threshold for rectified diffusion, as well as the importance of the bulk liquid saturation concentration in determining bubble evolution. Although elevated temperatures and liquid supersaturation reduce the rectified diffusion threshold, it remains coincident with the inertial cavitation thresholds at submicron bubble sizes at all temperatures. This observation suggests that changes in the nucleation environment, rather than bubble growth due to rectified diffusion, is a more likely cause of delayed-onset cavitation events.     

Characterization of ultrasound-induced fracture of polymer-shelled ultrasonic contrast agents by correlation analysis

Beyond a characteristic value of the negative peak pressure, ultrasound fracture the shell of ultrasonic contrast agents (UCAs). Existing criteria for ascertaining this threshold value exploit the dependence of the amplitude of the UCA acoustic response on the incident pressure. However, under the common experimental conditions used in this work, these criteria appear to be unreliable when they are applied to UCAs that are stabilized by a thick polymeric shell. An alternative criterion for determining the onset of shell fracture is introduced here, which uses variations of the shape of the acoustic time-domain response of an UCA suspension. Experimental evidence is presented that links the changes of the cross-correlation coefficient between consecutive time-domain signals to the fracture of the shells, and consequent release of air microbubbles. In principle, this criterion may be used to characterize similar properties of other types of particles that cannot undergo inertial cavitation.     

Dependence of cavitation,chemical effect,and mechanical effect thresholds on ultrasonic frequency

Cavitation, chemical effect, and mechanical effect thresholds were investigated in wide frequency ranges from 22 to 4880 kHz. Each threshold was measured in terms of sound pressure at fundamental frequency. Broadband noise emitted from acoustic cavitation bubbles was detected by a hydrophone to determine the cavitation threshold. Potassium iodide oxidation caused by acoustic cavitation was used to quantify the chemical effect threshold. The ultrasonic erosion of aluminum foil was conducted to estimate the mechanical effect threshold. The cavitation, chemical effect, and mechanical effect thresholds increased with increasing frequency. The chemical effect threshold was close to the cavitation threshold for all frequencies. At low frequency below 98 kHz, the mechanical effect threshold was nearly equal to the cavitation threshold. However, the mechanical effect threshold was greatly higher than the cavitation threshold at high frequency. In addition, the thresholds of the second harmonic and the first ultraharmonic signals were measured to detect bubble occurrence. The threshold of the second harmonic approximated to the cavitation threshold below 1000 kHz. On the other hand, the threshold of the first ultraharmonic was higher than the cavitation threshold below 98 kHz and near to the cavitation threshold at high frequency.     

Thresholds for cavitation produced in water by pulsed ultrasound

The threshold for transient cavitation produced in water by pulsed ultrasound was measured as a function of pulse duration and pulse repetition frequency at both 0.98 and 2.30 MHz. The cavitation events were detected with a passive acoustic technique which relies upon the scattering of the irradiation field by the bubble clouds associated with the events. The results indicate that the threshold is independent of pulse duration and acoustic frequency for pulses longer than approximately 10 acoustic cycles. The threshold increases for shorter pulses. The cavitation events are likely to be associated with bubble clouds rather than single bubbles.     

Numerical investigation of the inertial cavitation threshold by dual-frequency excitation in the fluid and tissue

Inertial cavitation thresholds, which are defined as bubble growth by 2-fold from the equilibrium radius, by two types of ultrasonic excitation (at the classical single-frequency mode and dual-frequency mode) were calculated. The effect of the dual-frequency excitation on the inertial cavitation threshold in the different surrounding media (fluid and tissue) was studied, and the paramount parameters (driving frequency, amplitude ratio, phase difference, and frequency ratio) were also optimized to maximize the inertial cavitation. The numerical prediction confirms the previous experimental results that the dual-frequency excitation is capable of reducing the inertial cavitation threshold in comparison to the single-frequency one at the same output power. The dual-frequency excitation at the high frequency (i.e., 3.1 + 3.5 MHz vs. 1.1 + 1.3 MHz) is preferred in this study. The simulation results suggest that the same amplitudes of individual components, zero phase difference, and large frequency difference are beneficial for enhancing the bubble cavitation. Overall, this work may provide a theoretical model for further investigation of dual-frequency excitation and guidance of its applications for a better outcome.     

Acoustic cavitation

This article reviews the physics of cavitation generated by acoustic fields, covering the basic theoretical and experimental data needed for a proper understanding of the many effects of cavitation and their practical applications. The dynamics of bubble motion are developed, stressing the relation between stable and transient types of cavitation and their thresholds. Non-radial types of bubble motion are now known to initiate several important effects, including erosive action, and these are dealt with in detail. Direct verification of theories of bubble dynamics is obtained using high-speed cinematography. The most recent techniques and results are described, as are the highly-sophisticated experimental methods now being applied to the bubble-fields and aggregates, including the acoustic emission from stable and transient cavitation fields.     

Precise spatial control of cavitation erosion in a vessel phantom by using an ultrasonic standing wave

In atherosclerotic inducement in animal models, the conventionally used balloon injury is invasive, produces excessive vessel injuries at unpredictable locations and is inconvenient in arterioles. Fortunately, cavitation erosion, which plays an important role in therapeutic ultrasound in blood vessels, has the potential to induce atherosclerosis noninvasively at predictable sites. In this study, precise spatial control of cavitation erosion for superficial lesions in a vessel phantom was realised by using an ultrasonic standing wave (USW) with the participation of cavitation nuclei and medium-intensity ultrasound pulses. The superficial vessel erosions were restricted between adjacent pressure nodes, which were 0.87 mm apart in the USW field of 1 MHz. The erosion positions could be shifted along the vessel by nodal modulation under a submillimetre-scale accuracy without moving the ultrasound transducers. Moreover, the cavitation erosion of the proximal or distal wall could be determined by the types of cavitation nuclei and their corresponding cavitation pulses, i.e., phase-change microbubbles with cavitation pulses of 5 MHz and SonoVue microbubbles with cavitation pulses of 1 MHz. Effects of acoustic parameters of the cavitation pulses on the cavitation erosions were investigated. The flow conditions in the experiments were considered and discussed. Compared to only using travelling waves, the proposed method in this paper improves the controllability of the cavitation erosion and reduces the erosion depth, providing a more suitable approach for vessel endothelial injury while avoiding haemorrhage.