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.     

Generation of microbubbles from hollow cylindrical ultrasonic horn

In this study, we found that microbubbles with diameters of less than 100 μm can be easily generated by using a hollow cylindrical ultrasonic horn. Consecutive images of bubbles obtained by using high-speed and high-resolution cameras reveal that a capillary wave is formed on the gas–liquid interface under weak ultrasonic irradiation and that the wave head is detached in the form of bubbles by the fragmentation of the interface as the power of ultrasonic irradiation increases. Moreover, consecutive images of the bubble interface obtained by an ultra-high-speed camera indicate that the breakup of bubbles oscillating harmonically with the ultrasonic irradiation generates many microbubbles that are less than 100 μm in diameter. With regard to the orifice diameter of the horn end, we found that its optimum value varies with the ultrasonic power input. When the orifice diameter is small, the capillary wave generated from the horn end easily propagates all over the gas–liquid interface, thereby starting the generation of microbubbles at a lower ultrasonic power input. When the orifice diameter is large, the capillary wave is attenuated because of viscosity and surface tension. Hence, in this case, microbubble generation from the horn requires a higher ultrasonic power input. Furthermore, the maximum yield of microbubbles via primary and secondary bubble generation can be increased by increasing the gas flow rate.     

High-speed observation of acoustic cavitation erosion in multibubble systems

Cleaning and erosion of objects by ultrasound in liquids are caused by the action of acoustic cavitation bubbles. Experiments have been performed with respect to the erosive effect of multibubble structures on painted glass surfaces and on aluminium foils in an ultrasonic standing wave field at 40 kHz. High-speed imaging techniques have been employed to investigate the mechanisms at work, in particular bubble interaction and cluster formation near and at the object surfaces. It was found that different prototype bubble structures can contribute to the erosion process. Some are bound to the surface, which seems to act as a bubble source in this case, while others also exist independently from the object. Cleaning and erosion effects at the pressure antinodes can vary strongly as they depend on the emerging bubble structures. These, in turn, seem to be substantially influenced by properties and the history of the surface.     

Bubble population phenomena in acoustic cavitation

Theoretical treatments of the dynamics of a single bubble in a pressure field have been undertaken for many decades. Although there is still scope for progress, there now exists a solid theoretical basis for the dynamics of a single bubble. This has enabled useful classifications to be established, including the distinction between stable cavitation (where a bubble pulsates for many cycles) and transient cavitation (where the bubble grows extensively over time-scales of the order of the acoustic cycle, and then undergoes an energetic collapse and subsequent rebound and then, potentially, either fragmentation, decaying oscillation or a repeat performance). Departures from sphericity, such as shape and surface oscillations and jetting, have also been characterized. However, in most practical systems involving high-energy cavitation (such as those involving sonochemical, biological and erosive effects), the bubbles do not behave as the isolated entities modelled by this single-bubble theory: the cavitational effect may be dominated by the characteristics of the entire bubble population, which may influence, and be influenced by, the sound field.

The well established concepts that have resulted from the single-bubble theory must be reinterpreted in teh light of the bubble population, an appreciation of population mechanisms being necessary to apply our understanding of single-bubble theory to many practical applications of ‘power’ ultrasound. Even at a most basic level these single-bubble theories describe the response of the bubble to the local sound field at the position of the bubble, and that pressure field will be influenced by the way sound is scattered by neighbouring bubbles. The influence of the bubble population will often go further, a non-uniform sound field creating an inhomogeneous bubble distribution. Such a distribution can scatter, channel and focus ultrasonic beams, can acoustically shield regions of the sample, and elsewhere localize the cavitational activity to discrete ‘hot spots’. As a result, portions of the sample may undergo intense sonochemical activity, degassing, erosion, etc., whilst other areas remain relatively unaffected. Techniques exist to control such situations where they are desirable, and to eliminate this localization where a more uniform treatment of the sample is desired.     

Microelectrode study of single cavitational bubbles induced by 500 kHz ultrasound

Insight is gained into about the processes governing cavitational activity and acoustic streaming induced by high frequency (500 kHz) ultrasound by the use of microelectrodes with short time resolution electrochemical equipment to allow monitoring of the activity of single cavitating bubbles. Current transients are interpreted as showing the flux of solution towards the electrode surface due to microstreaming. In order to explain the current amplitude, a simplified model is produced. Important parameters such as bubble size and shape on the surface as well as the boundary layer thickness for microstreaming are taken into account. This model leads to the amplitude of the oscillations of the cavitating bubble. Introducing realistic bubble sizes, this amplitude is found to be in the order of 1 micron. The conclusions arising from this work allow a further interpretation of previous observations at millimeter scale electrodes.     

Modeling non-spherical oscillations and stability of acoustically driven shelled microbubbles

The oscillation and destruction of microbubbles under ultrasound excitation form the basis of contrast enhanced ultrasound imaging and microbubble assisted drug and gene delivery. A typical microbubble has a size of a few micrometers and consists of a gas core encapsulated by a shell. These bubbles can be driven into surface mode oscillations, which not only contribute to the measured acoustic signal but can lead to bubble destruction. Existing models of surface model oscillations have not considered the effects of a bubble shell. In this study a model was developed to study the surface mode oscillations in shelled bubbles. The effects of shell viscosity and elasticity on the surface mode oscillations were modeled using a Boussinesq-Scriven approach. Simulation was conducted using the model with various bubble sizes and driving acoustic pressures. The occurrence of surface modes and the number of ultrasound cycles needed for the occurrence were calculated. The simulation results show a significant difference between shelled bubbles and shell free bubbles. The shelled bubbles have reduced surface mode amplitudes and a narrower bubble size range within which these modes develop compared to shell free bubbles. The clinical implications were also discussed.     

Blackbody spectra for sonoluminescing hydrogen bubbles

The dynamical motion of sonoluminescing bubbles formed from a mixture of water and hydrogen gas indicates that these bubbles contain hydrogen. Their spectrum is well matched by an ideal 6000 K blackbody radiating from a surface with a radius less than 1/4 microm. According to this model, the state of matter inside the collapsed bubble is so stressed that the photon mean free path is much smaller than 1 microm. Implications for various theories of the light-emitting mechanism and the role of chemical reactions are discussed.     

Effect of an elastic wall on the dynamics of an encapsulated microbubble: A simulation study

The purpose of the present simulation study is to reveal how confining surfaces with different mechanical properties affect the acoustic response of a contrast agent microbubble. To this end, numerical simulations are carried out for three types of walls: a plastic (OptiCell) wall, an aluminium wall, and a biological tissue. For each wall, the behaviour of contrast microbubbles of three sizes is investigated. The spectral characteristics of the scattered pressure produced by the microbubbles are compared for two cases: the bubble oscillates far away from the wall and the same bubble oscillates in the immediate vicinity of the wall. The results of the simulations allow one to make the following main conclusions. The effect of the OptiCell wall on the acoustic bubble response is stronger than that of the aluminium and tissue walls. Changes in the bubble response near the wall are stronger when bubbles are excited above their resonance frequency. Considering changes in the fundamental and the 2nd harmonic with respect to the peak values of these components at different bubble radii, it is found that the changes are stronger for smaller bubbles and that the changes in the 2nd harmonic are stronger than those in the fundamental. These results allow one to gain an insight into conditions under which the effect of an elastic wall on the acoustic response of a contrast agent microbubble is easier to be detected.     

Difference frequency and its harmonic emitted by microbubbles under dual frequency excitation

Vibro-acoustography is an elasticity imaging method that uses two ultrasound beams of slightly different frequency to excite an object and detects the resulting acoustic emission (AE) at the difference frequency. This method is especially sensitive to bubbles due to their nonlinearity. This study explores the harmonic acoustic emission (HAE) at twice the difference frequency emitted from bubbles. A perturbation method based on the dynamic bubble equation is used to derive the AE and HAE from a single bubble excited by dual frequency waves. Simulation shows that HAE is generated only by microbubbles whose resonant frequencies match the incident ultrasound frequencies. In contrast, AE is more sensitive to resonance at the difference frequency, which is relevant to sub-millimeter bubbles. This finding was confirmed by experiments where HAE was produced from Optison microbubbles, but not from larger air bubbles which are off resonance at the incident ultrasound frequency. In conclusion, harmonic acoustic emission is present for microbubbles. It is very sensitive to the size of the bubble and may be used for selective detection of microbubbles.     

Effect of surfactant addition on removal of microbubbles using ultrasound

It is difficult to control the bubble in a liquid by the external operation, because the behavior of the bubble is controlled in buoyancy and flow of liquid. On the other hand, microbubbles, whose diameter is several decades μm, stably disperse in static liquid because of their small buoyancy and electrical repulsion. When an ultrasound, whose frequency was 2.4 MHz, was irradiated, the milky white microbubbles suspended solution became rapidly clear. In this study, the effects of surfactant addition on the removal of microbubbles from a liquid in an ultrasonic field were investigated. The efficiency of removal of microbubbles decreased with surfactant addition. Surfactant type influenced the size of agglomerated microbubbles, and the efficiency of removal of microbubbles changed. The surface of microbubble was modified by surfactant adsorption, and the steric inhibition influenced the removal of microbubbles.     

Resonance frequency of microbubbles: effect of viscosity

The transmitted frequency at which a gas bubble of millimeter or submillimeter size oscillates resonantly in a low-viscosity liquid is approximately equal to the undamped natural frequency (referred to as the Minnaert frequency if surface tension effects are disregarded). Based on a theoretical analysis of bubble oscillation, this paper shows that such an approximation cannot be validated for microbubbles used in contrast-enhanced ultrasound imaging. The contrast-agent microbubbles represent either encapsulated bubbles of size less than 10 microm or free (nonencapsulated) bubbles of submicron size. The resonance frequency of the microbubbles deviates significantly from the undamped natural frequency over the whole range of microbubble sizes due to the increased viscous damping coefficient. The difference between these two frequencies is shown to have a tremendous impact on the resonant backscatter by the microbubbles. In particular, the first and second harmonics of the backscattered signal from the microbubbles are characterized by their own resonance frequencies, equal to neither the microbubble resonance frequency nor the undamped natural frequency.     

A candidate mechanism for exciting sound during bubble coalescence

Coalescing bubbles are known to produce a pulse of sound at the moment of coalescence, but the mechanism driving the sound production is uncertain. A candidate mechanism for the acoustic forcing is the rapid increase in the bubble volume, as the neck of air joining the two parent bubbles expands. A simple model is presented here for the volume forcing caused by the coalescence dynamics, and its predictions are tested against the available data. The model predicts the right order of magnitude for the acoustic amplitude, and the predicted amplitudes also scale correctly with the radius of the smaller parent bubble.     

Cavitation bubble behavior and bubble–shock wave interaction near a gelatin surface as a study of in vivo bubble dynamics

The collapse of a single cavitation bubble near a gelatin surface, and the interaction of an air bubble attached to a gelatin surface with a shock wave, were investigated. These events permitted the study of the behavior of in vivo cavitation bubbles and the subsequent tissue damage mechanism during intraocular surgery, intracorporeal and extracorporeal shock wave lithotripsy. Results were obtained with high-speed framing photography. The cavitation bubbles near the gelatin surface did not produce significant liquid jets directed at the surface, and tended to migrate away from it. The period of the motion of a cavitation bubble near the gelatin surface was longer than that of twice the Rayleigh's collapse time for a wide range of relative distance, L/R_max, excepting for very small L/R_max values (L was the stand-off distance between the gelatin surface and the laser focus position, and R_max was the maximum bubble radius). The interaction of an air bubble with a shock wave yielded a liquid jet inside the bubble, penetrating into the gelatin surface. The liquid jet had the potential to damage the gelatin. The results predicted that cavitation-bubble-induced tissue damage was closely related to the oscillatory bubble motion, the subsequent mechanical tissue displacement, and the liquid jet penetration generated by the interaction of the remaining gas bubbles with subsequent shock waves. The characteristic bubble motion and liquid jet formation depended on the tissue's mechanical properties, resulting in different damage mechanisms from those observed on hard materials.     

The effects of coupling and bubble size on the dynamical-systems behaviour of a small cluster of microbubbles

This study endeavours to apply a theoretical model for predicting the dynamics of a bubble cluster of various sizes, within which each bubble may assume different initial conditions from other bubbles in the cluster. The resulting system of coupled Keller-Miksis-Parlitz equations are solved numerically, and the effects of coupling and bubble size on bubble cluster dynamics are examined for a given set of ultrasound parameters. It has been found that the effects of coupling are significant, and a bubble cluster's bifurcation characteristics and route to chaos can be altered by inter-bubble interactions. This gives rise to the possibility of suppressing the chaotic oscillations of microbubbles by varying bubble cluster size. Small equilibrium radii bubbles have little influence on the dynamics of neighbouring bubbles in a cluster via coupling. Furthermore, a bubble system consisting of smaller-sized bubbles transitions from order to chaos at lower driving pressure amplitudes.     

In-situ synchrotron X-ray imaging of ultrasound (US)-generated bubbles: Influence of US frequency on microbubble cavitation for membrane fouling remediation

Gaining an in-depth understanding of the characteristics and dynamics of ultrasound (US)--generated bubbles is crucial to effectively remediate membrane fouling. The goal of present study is to conduct in-situ visualization of US-generated microbubbles in water to examine the influence of US frequency on the dynamics of microbubbles. This study utilized synchrotron in-line phase contrast imaging (In-line PCI) available at the biomedical imaging and therapy (BMIT) beamlines at the Canadian Light Source (CLS) to enhance the contrast of liquid/air interfaces at different US frequencies of 20, 28 and 40 KHz at 60 Watts. A high-speed camera was used to capture 2,000 frames per second of the bubble cavitation generated in water under the ultrasound influence. Key parameters at the polychromatic beamlines were optimized to maximize the phase contrast of gas/liquid of the microbubbles with a minimum size of 5.5 µm. ImageJ software was used to analyze the bubble characteristics and their behavior under the US exposure including the microbubble number, size, and fraction of the total area occupied by the bubbles at each US frequency. Furthermore, the bubble characteristics over the US exposure time and at different distances from the transducer were studied. The qualitative and quantitative data analyses showed that the microbubble number or size did not change over time; however, it was observed that most bubbles were created at the middle of the frames and close to the US field. The number of bubbles created under the US exposure increased with the frequency from 20 kHz to 40 kHz (about 4.6 times). However, larger bubbles were generated at 20 kHz such that the average bubble radius at 20 kHz was about 6.8 times of that at 40 kHz. Microbubble movement/traveling through water was monitored, and it was observed that the bubble velocity increased as the frequency was increased from 20 kHz to 40 kHz. The small bubbles moved faster, and the majority of them traveled upward towards the US transducer location. The growth pattern (a correlation between the mean growth ratio and the exposure time) of bubbles at 20 kHz and 60 W was obtained by tracking the oscillation of 22 representative microbubbles over the 700 ms of imaging. The mean growth ratio model was also obtained.     

球状泡群内气泡的耦合振动

振动气泡形成辐射场影响其他气泡的运动, 故多气泡体系中气泡处于耦合振动状态. 本文在气泡群振动模型的基础上, 考虑气泡间耦合振动的影响, 得到了均匀球状泡群内振动气泡的动力学方程, 以此为基础分析了气泡的非线性声响应特征. 气泡间的耦合振动增加了系统对每个气泡的约束, 降低了气泡的自然共振频率, 增强了气泡的非线性声响应. 随着气泡数密度的增加, 振动气泡受到的抑制增强; 增加液体静压力同样可抑制泡群内气泡的振动, 且存在静压力敏感区(1–2 atm, 1 atm=1.01325×10⁵ Pa); 驱动声波对气泡振动影响很大, 随着声波频率的增加, 能够形成空化影响的气泡尺度范围变窄. 在同样的声条件、泡群尺寸以及气泡内外环境下, 初始半径小于5 μm 的气泡具有较强的声响应. 气泡耦合振动会削弱单个气泡的空化影响, 但可延长多气泡系统空化泡崩溃发生的时间间隔和增大作用范围, 整体空化效应增强.     

Effect of polymer surface activity on cavitation nuclei stability against dissolution

The persistence of acoustic cavitation in a pulsed wave ultrasound regime depends upon the ability of cavitation nuclei, i.e., bubbles, to survive the off time between pulses. Due to the dependence of bubble dissolution on surface tension, surface-active agents may affect the stability of bubbles against dissolution. In this study, measurements of bubble dissolution rates in solutions of the surface-active polymer poly(propyl acrylic acid) (PPAA) were conducted to test this premise. The surface activity of PPAA varies with solution pH and concentration of dissolved polymer molecules. The surface tension of PPAA solutions (55-72 dynes/cm) that associated with the polymer surface activity was measured using the Wilhelmy plate technique. Samples of these polymer solutions then were exposed to 1.1 MHz high intensity focused ultrasound, and the dissolution of bubbles created by inertial cavitation was monitored using an active cavitation detection scheme. Analysis of the pulse echo data demonstrated that bubble dissolution time was inversely proportional to the surface tension of the solution. Finally, comparison of the experimental results with dissolution times computed from the Epstein-Plesset equation suggests that the radii of residual bubbles from inertial cavitation increase as the surface tension decreases.     

"Compression-only" behavior: a second-order nonlinear response of ultrasound contrast agent microbubbles

Oscillating phospholipid-coated ultrasound contrast agent microbubbles display a so-called "compression-only" behavior, where it is observed that the bubbles compress efficiently while their expansion is suppressed. Here, a theoretical understanding of the source of this nonlinear behavior is provided through a weakly nonlinear analysis of the shell buckling model proposed by Marmottant et al. [J. Acoust. Soc. Am. 118, 3499-3505 (2005)]. It is shown that the radial dynamics of the bubble can be considered as a superposition of a linear response at the fundamental driving frequency and a second-order nonlinear low-frequency response that describes the negative offset of the mean bubble radius. The analytical solution deduced from the weakly nonlinear analysis shows that the compression-only behavior results from a rapid change of the shell elasticity with bubble radius. In addition, the radial dynamics of single phospholipid-coated microbubbles was recorded as a function of both the amplitude and the frequency of the driving pressure pulse. The comparison between the experimental data and the theory shows that the magnitude of compression-only behavior is mainly determined by the initial phospholipids concentration on the bubble surface, which slightly varies from bubble to bubble.     

Subharmonic behavior of phospholipid-coated ultrasound contrast agent microbubbles

Coated microbubbles, unlike tissue are able to scatter sound subharmonically. Therefore, the subharmonic behavior of coated microbubbles can be used to enhance the contrast in ultrasound contrast imaging. Theoretically, a threshold amplitude of the driving pressure can be calculated above which subharmonic oscillations of microbubbles are initiated. Interestingly, earlier experimental studies on coated microbubbles demonstrated that the threshold for these bubbles is much lower than predicted by the traditional linear viscoelastic shell models. This paper presents an experimental study on the subharmonic behavior of differently sized individual phospholipid coated microbubbles. The radial subharmonic response of the microbubbles was recorded with the Brandaris ultra high-speed camera as a function of both the amplitude and the frequency of the driving pulse. Threshold pressures for subharmonic generation as low as 5 kPa were found near a driving frequency equal to twice the resonance frequency of the bubble. An explanation for this low threshold pressure is provided by the shell buckling model proposed by Marmottant et al. [J. Acoust. Soc. Am. 118, 3499-3505 (2005)]. It is shown that the change in the elasticity of the bubble shell as a function of bubble radius as proposed in this model, enhances the subharmonic behavior of the microbubbles.     

Effect of ultrasonic frequency and surfactant addition on microcapsule destruction

In a previous study, we found that cavitation bubbles cause the ultrasonic destruction of microcapsules containing oil in a shell made of melamine resin. The cavitation bubbles can be smaller or larger than the resonance size; smaller bubbles cause Rayleigh contraction, whereas larger bubbles are not involved in the sonochemical reaction. The activity in and around the bubble (e.g., shear stress, shock wave, microjet, sonochemical reaction, and sonoluminescence) varies substantially depending on the bubble size. In this study, we investigated the mechanism of the ultrasonic destruction of microcapsules by examining the correlations between frequency and microcapsule destruction rate and between microcapsule size and cavitation bubble size. We evaluated the bubbles using multibubble sonoluminescence and the bubble size was changed by adding a surfactant to the microcapsule suspension. The microcapsule destruction was frequency dependent. The main cause of microcapsule destruction was identified as mechanical resonance, although the relationship between bubble size and microcapsule size suggested that bubbles smaller than or equal to the microcapsule size may also destroy microcapsules by applying shear stress locally.