Cavitation Cluster Research Articles

Mechanisms of ultrasonic de-agglomeration of oxides through in-situ high-speed observations and acoustic measurements

Ultrasonic de-agglomeration and dispersion of oxides is important for a range of applications. In particular, in liquid metal, this is one of the ways to produce metal-matrix composites reinforced with micron and nano sized particles. The associated mechanism through which the de-agglomeration occurs has, however, only been conceptualized theoretically and not yet been validated with experimental observations. In this paper, the influence of ultrasonic cavitation on SiO2 and MgO agglomerates (commonly found in lightweight alloys as reinforcements) with individual particle sizes ranging between 0.5 and 10 μm was observed for the first time in-situ using high-speed imaging. Owing to the opacity of liquid metals, a de-agglomeration imaging experiment was carried out in de-ionised water with sequences captured at frame rates up to 50 kfps. In-situ observations were further accompanied by synchronised acoustic measurements using an advanced calibrated cavitometer, to reveal the effect of pressure amplitude arising from oscillating microbubbles on oxide de-agglomeration. Results showed that ultrasound-induced microbubble clusters pulsating chaotically, were predominantly responsible for the breakage and dispersion of oxide agglomerates. Such oscillating cavitation clusters were seen to capture the floating agglomerates resulting in their immediate disintegration. De-agglomeration of oxides occurred from both the surface and within the bulk of the aggregate. Microbubble clusters oscillating with associated emission frequencies at the subharmonic, 1st harmonic and low order ultra-harmonics of the driving frequency were deemed responsible for the breakage of the agglomerates.

Three-dimensional cavitation mapping in shock wave and laser lithotripsy

Shock wave lithotripsy (SWL) and laser lithotripsy (LL) have been widely used for non-invasive treatment of kidney stones. Cavitation plays an important role in stone fragmentation, yet it may also contribute to renal injury during SWL and LL. It is therefore crucial to determine the spatiotemporal distributions of cavitation activities to maximize stone fragmentation while minimizing tissue injury. Traditional cavitation detection methods include high-speed optical imaging, active cavitation mapping (ACM), and passive cavitation mapping (PCM), among which PCM has most practical applications in biological tissues. To image the dynamics of cavitation bubble collapse, we have developed tri-modality cavitation imaging that includes 3D high-speed optical imaging, ACM, and PCM seamlessly integrated in a single imaging system. We also developed a transient angular spectrum approach for PCM reconstruction, which is ten times faster than the traditional delay-and-sum method. Using the tri-modality system, we imaged and analyzed laser-induced single cavitation bubbles in both free field and constricted space and shock wave-induced cavitation clusters. Collectively, our results have demonstrated the high reliability and spatial-temporal accuracy of the 3D PCM approach, which paves the way for future in vivo applications in large animals and human studies during SWL and LL.

Using the effects of hydrodynamic cavitation for purposeful dynamical action on the supramolecular structures

Evolution of the cavitation cluster and the level of the dynamic cavitation effects in liquid flow within a Venturi nozzle, depending on the design features and the mode of operation of this type cavitator, are considered. The experimental and theoretical investigations have been performed with the view to using the Venturi nozzle as cavitation reactor to efficient influence supramolecular structures in liquid in relation to producing stable liposome dispersions. Structural peculiarities of liposomes, closed biological nanocapsules, as well as bond energy values in these structures have been analyzed. The results of this study prove that using Venturi nozzles in large-scale production of liposome preparations allows increasing production capacity and significant reduction in the mass-related power consumption as compared with traditional acoustic and hydrodynamic methods.

Tri-modality cavitation mapping in shock wave lithotripsy

Shock wave lithotripsy (SWL) has been widely used for non-invasive treatment of kidney stones. Cavitation plays an important role in stone fragmentation, yet it may also contribute to renal injury during SWL. It is therefore crucial to determine the spatiotemporal distributions of cavitation activities to maximize stone fragmentation while minimizing tissue injury. Traditional cavitation detection methods include high-speed optical imaging, active cavitation mapping (ACM), and passive cavitation mapping (PCM). While each of the three methods provides unique information about the dynamics of the bubbles, PCM has most practical applications in biological tissues. To image the dynamics of cavitation bubble collapse, we previously developed a sliding-window PCM (SW-PCM) method to identify each bubble collapse with high temporal and spatial resolution. In this work, to further validate and optimize the SW-PCM method, we have developed tri-modality cavitation imaging that includes three-dimensional high-speed optical imaging, ACM, and PCM seamlessly integrated in a single system. Using the tri-modality system, we imaged and analyzed laser-induced single cavitation bubbles in both free field and constricted space and shock wave-induced cavitation clusters. Collectively, our results have demonstrated the high reliability and spatial-temporal accuracy of the SW-PCM approach, which paves the way for the future in vivo applications on large animals and humans in SWL.

Evidence of the formation of the shock scattering induced violent cavitation cluster during boiling histotripsy insonation: A numerical case study

Boiling histotripsy is a promising noninvasive ultrasonic technique that can be used to mechanically destroy solid tumours. In boiling histotripsy, the formations and dynamics of a boiling vapour bubble and cavitation clouds contribute towards mechanical tissue fractionation. Whilst a number of numerical and experimental studies have been performed to examine and understand the evolution of a boiling bubble at the HIFU focus in a viscoelastic medium, little is known about the subsequent generation of cavitation clouds that form in between the boiling bubble and the HIFU transducer during boiling histotripsy insonation. Previous experimental observations suggest that the shock scattering by a boiling bubble may play a significant role in producing cavitation clouds. The main objective of the present study is, therefore, to investigate the relationship between the shock scattering phenomenon and the occurrence of cavitation clouds through (a) performing a numerical simulation of nonlinear wave propagation with the presence of a bubble at the HIFU focus and (b) comparing with the previous high speed camera observations of a cavitation cluster formation. The size of a bubble (i.e., 95.7, 128.1 and 258 μm in diameter) as well as the HIFU exposure conditions (i.e., a driving frequency of 1.1 MHz, peak positive and negative pressures of 68.4 MPa and −13.9 MPa) used in the simulations were obtained from the previous boiling histotripsy experiments conducted with a tissue gel phantom. The numerical results presented in this study clearly demonstrate that the shock scattering is the main cause of the creation of a cavitation cluster in boiling histotripsy.

Characterising the cavitation activity generated by an ultrasonic horn at varying tip-vibration amplitudes

Dual-perspective high-speed imaging and acoustic detection is used to characterise cavitation activity at the tip of a commercial 20 kHz (f0) ultrasonic horn, over 2 s sonications across the range of input powers available (20 – 100%). Imaging at 1 × 105 frames per second (fps) captures cavitation-bubble cluster oscillation at the horn-tip for the duration of the sonication. Shadowgraphic imaging at 2 Mfps, from an orthogonal perspective, probes cluster collapse and shock wave generation at higher temporal resolution, facilitating direct correlation of features within the acoustic emission data generated by the bubble activity. f0/m subharmonic collapses of the primary cavitation cluster directly beneath the tip, with m increasing through integer values at increasing input powers, are studied. Shock waves generated by periodic primary cluster collapses dominate the non-linear emissions of the cavitation noise spectra. Transitional input powers for which the value of m is indistinct, are identified. Overall shock wave content within the emission signals collected during sonications at transitional input powers are reduced, relative to input powers with distinct m. The findings are relevant for the optimisation of applications such as sonochemistry, known to be mediated by bubble collapse phenomena.

Cavitation clusters in lipid systems - Ring-up, bubble population, and bifurcated streamer lifetime.

The processing of oils is vital to their ultimate use within the food industry. Control over the physical properties of such materials could be achieved through the application of high-intensity ultrasound (HIU). However, the exact mechanism, centred upon acoustic cavitation, is currently unclear. To investigate the cavitation environment in oils further, the ring-up of a HIU source in an oil media is studied in the presence and absence of a pre-existing bubble population. High-speed imaging and acoustic measurements within the system is demonstrated to be extremely useful in characterising the dynamics present under non steady-state conditions. The behaviour of the clusters generated in the first 1000ms under these conditions is shown to be significantly different depending on the bubble population. A bifurcated streamer (BiS), originating from a unique bi-cluster event, is only observable in the presence of a bubble population during the ring-up process to higher cluster orders. In addition, the lifetime of this BiS event is highly temperature dependent and is shown to be a good marker for the viscosity of the oil employed.

THE WAYS OF PRODUCING AN UNIFIED MATHEMATICAL MODEL FOR THE CAVITATING FLOW IN HYDRODYNAMIC CAVITATION REACTORS

Possible ways of constructing a general model of cavitation reactors are considered. A mathematical model is proposed that without using any limiting assumptions describes adequately the dynamics of vapor-gas bubbles and the behavior of a cavitation cluster in a wide range of regime parameters. In the framework of the model, the influence of gas content and liquid temperature on the cavitation intensity is considered. The possibility of modifying the model as applied to optimizing the operation of cavitation reactors is discussed.

Cavitation induced by shock wave focusing in eye-like experimental configurations.

During laser-induced, breakdown-based medical procedures in human eyes such as posterior capsulotomy and vitreolysis, shock waves are emitted from the location of the plasma. A part of these spherically expanding transients is reflected from the concave surface of the corneal epithelium and refocused within the eye. Using a simplified experimental model of the eye, the dominant secondary cavitation clusters were detected by high-speed camera shadowgraphy in the refocusing volume, dislocated from the breakdown position and described by an abridged ray theory. Individual microbubbles were detected in the preheated cone of the incoming laser pulse and radially extending cavitation filaments were generated around the location of the breakdown soon after collapse of the initial bubble. The generation of the secondary cavitation structures due to shock wave focusing can be considered an adverse effect, important in ophthalmology.

Cavitation Clusters in Lipid Systems: The Generation of a Bifurcated Streamer and the Dual Collapse of a Bubble Cluster

AbstractSeveral studies have reported the use of high‐intensity ultrasound (HIU) to induce the crystallization of lipids. The effect that HIU has on lipid crystallization is usually attributed to the generation of cavities but acoustic cavitation has never been fully explored in lipids. The dynamics of a particular cavitation cluster next to a piston like emitter (PLE) in an oil was investigated in this study. The lipid systems, which are important in food processing, are studied with high‐speed camera imaging, laser scattering, and acoustic pressure measurements. A sequence of stable clusters was noted. In addition, a bifurcated streamer was detected, which exists within a sequence of clusters. This is shown to originate from two clusters on the PLE tip oscillating with a 180° phase shift in time with respect to one another. Finally, the collapse phase of the cluster is shown to involve a rapid (~10 μs) two‐stage process. These results show that the dynamics of cluster formation and collapse is driven by HIU power levels and might have implications in lipid sonocrystallization.

The effects of the size of a boiling bubble on lesion production in boiling histotripsy

Boiling histotripsy employs a number of millisecond-long High Intensity Focused Ultrasound (HIFU) pulses with high acoustic peak pressures at the HIFU focus to mechanically fractionate soft tissue. Studies have shown the mechanisms underpinning this tissue fractionation process; however, the question of how HIFU exposure conditions affect lesion formation still remains unclear. In the present work, a high-speed camera and a passive cavitation detection (PCD) system were used to investigate the dynamics of bubbles induced and the corresponding mechanical damage generated in optically transparent tissue-mimicking phantoms with two different boiling histotripsy exposure conditions (1. P+ = 85.4 MPa, P- = – 15.6 MPa; 2. P+ = 71.5 MPa, P- = – 13.4 MPa at focus). Our results clearly showed that there is a positive relationship between the size of a boiling bubble and the lesion dimension. At P+ = 85.4 MPa and P- = – 15.6 MPa, a relatively larger boiling bubble was, for instance, produced at the focus in the gel phantom followed by the presence of a wider cavitation cluster progressing toward the HIFU transducer, resulting in the formation of a larger lesion compared to that with P+ = 71.5 MPa and P- = – 13.4 MPa.

Formation of a cavitation cluster in the vicinity of a quasi-empty rupture

The presentation deals with one of the experimental and numerical models of a quasi-empty rupture in the magma melt. This rupture is formed in the liquid layer of a distilled cavitating fluid under shock loading within the framework of the problem formulation with a small electromagnetic hydrodynamic shock tube. It is demonstrated that the rupture is shaped as a spherical segment, which retains its topology during the entire process of its evolution and collapsing. The dynamic behavior of the quasi-empty rupture is analyzed, and the growth of cavitating nuclei in the form of the boundary layer near the entire rupture interface is found. It is shown that rupture implosion is accompanied by the transformation of the bubble boundary layer to a cavitating cluster, which takes the form of a ring-shaped vortex floating upward to the free surface of the liquid layer. A p-κ mathematical model is formulated, and calculations are performed to investigate the implosion of a quasi-empty spherical cavity in the cavitating liquid, generation of a shock wave by this cavity, and dynamics of the bubble density growth in the cavitating cluster by five orders of magnitude.

Cavitation clusters in lipid systems – surface effects, local heating and streamer formation

Cavitation clusters and streamers are characterised in lipid materials (specifically sunflower oil) and compared to water systems. The lipid systems, which are important in food processing, are studied with high-speed camera imaging, laser scattering and pressure measurements. In these oils, clusters formed at an aged (roughened) tip of the sound source (a piston like emitter, PLE) are shown to collapse with varied periodicity in relation to the drive amplitude employed. A distinct streamer (an area of increased flow emanating from the cavitation cluster) is seen in the lipid media which is collimated directly away from the tip of the PLE source whereas in water the cavitation plume is visually less distinct. The velocity of bubbles in the lipid streamer near the cluster on the order of 10 m s-1. Local heating effects, within the streamer, are detected using a dual thermocouple measurement at extended distances. Viscosity, temperature and the outgassing within the oils are suggested to play a key role in the streamer formation in these systems.

Modeling of hydrodynamic cavitating flows considering the bubble-bubble interaction

A cavitation model considering the second-order derivative and bubble-bubble interaction is proposed. The bubble wall velocities are obtained by integrating the modified Rayleigh equation. The bubble-bubble interaction is measured by the coupling strength, which is modified for the hydrodynamic cavitating flows during the evaporation and decreases sharply at the cluster boundary during the condensation. The phase change rates are greatly reduced when the bubble-bubble interaction is considered, except at extremely low vapor volume fraction. Numerical results are given for the collapse of cavitation clusters and the unsteady cavitating flows around two NACA0015 hydrofoils. Comparisons are made with the experimental data and Zwart–Gerber–Belamri cavitation model.

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 20MHz. 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.

Smooth particle hydrodynamics method for modeling cavitation-induced fracture of a fluid under shock-wave loading

It is demonstrated that the method of smoothed particle hydrodynamics can be used to study the flow structure in a cavitating medium with a high concentration of the gas phase and to describe the process of inversion of the two-phase state of this medium: transition from a cavitating fluid to a system consisting of a gas and particles. A numerical analysis of the dynamics of the state of a hemispherical droplet under shock-wave loading shows that focusing of the shock wave reflected from the free surface of the droplet leads to the formation of a dense, but rapidly expanding cavitation cluster at the droplet center. By the time t = 500 µs, the bubbles at the cluster center not only coalesce and form a foam-type structure, but also transform to a gas-particle system, thus, forming an almost free rapidly expanding zone. The mechanism of this process defined previously as an internal “cavitation explosion” of the droplet is validated by means of mathematical modeling of the problem by the smoothed particle hydrodynamics method. The deformation of the cavitating droplet is finalized by its decomposition into individual fragments and particles.

Observations on the dynamics of bubble cluster in an ultrasonic field

The dynamics driven interaction between the bubbles in a cavitation cluster is known to be a complex phenomenon indicative of a highly active nonlinear as well as chaotic behavior in ultrasonic fields. By considering the cluster of encapsulated microbubble with a thin elastic shell in ultrasonic fields, in this paper, the dynamics of microbubbles has been studied via applying the methods of chaos physics. Bifurcation, Lyapunov exponent, and time series are plotted with respect to variables such as amplitude, initial bubble radius, frequency and viscosity. The findings of the study indicate that a bubble cluster undergoes a chaotic unstable region as the amplitude and frequency of ultrasonic pulse are increased mainly due to the period doubling phenomenon. The results of the present study are supported by findings of previous studies.

Multiple observations of cavitation cluster dynamics close to an ultrasonic horn tip

Bubble dynamics in water close to the tip of an ultrasonic horn (∼23 kHz, 3 mm diameter) have been studied using electrochemistry, luminescence, acoustics, light scattering, and high-speed imaging. It is found that, under the conditions employed, a large bubble cluster (∼1.5 mm radius) exists at the tip of the horn. This cluster collapses periodically every three to four cycles of the fundamental frequency of the horn. Following the collapse of the cluster, a short-lived cloud of small bubbles (each tens of microns in diameter) was observed in the solution. Large amplitude pressure emissions are also recorded, which correlate temporally with the cluster collapse. Bursts of surface erosion (measured in real time using an electrochemical technique) and multibubble sonoluminescence emission both also occur at a subharmonic of the fundamental frequency of the horn and are temporally correlated with the bubble cluster collapse and the associated pressure wave emission.

Acoustics cavitation in microfluidics for sonoluminescence and sonochemistry

Strong ultrasound is applied to a microfluidic channel to generate nonlinear surface waves which entrap bubbles at the gas–liquid interface to form oscillating bubbles. The ultrasound is generated by the piezoelectric transducer on the side of polydimethylsiloxane microchannel. The microchannel is attached to a glass slide through plasma bonding, while the transducer is glued by epoxy for strong coupling. The high speed photography shows that continuous cavitation clusters are formed within the microchannel as gas is injected. As they collapse rapidly, they are able to produce very intense concentration of energy that is able to emit light. This phenomenon is known as sonoluminescence. Previously, sonoluminescence is achieved via a single bubble or multiple bubbles in a bulk liquid. The authors report a realization of sonoluminescence in a microfluidic device. The same oscillating bubbles can also be used as micro-labs. They can trigger chemical reactions that require high temperature and pressure. We achieve the formation of OH radicals in a lab-on-a-chip device. In conclusion, nonequilibrium microbubbles can be induced in a microfluidic system. They oscillate and collapse, and in the process provide a source of energy concentration for the emission of light and the activation of chemical reactions.

Acoustically driven cavitation cluster collapse in planar geometry

We demonstrate the dynamics of arrays of transient cavitation bubbles exposed to a sound field in a planar geometry. Single, double, and complex configurations of cavitation bubbles are obtained by shaping a pulsed laser beam with a digital hologram and focusing it into a thin gap of liquid. The liquid is driven with an oscillating pressure field of variable phase and amplitude. We compare the dynamics of a single bubble recorded with high-speed photography with a two-dimensional Rayleigh model. For multibubble configurations we observe bubble-bubble interaction and coalescence which depends on the phase of the acoustic field. Larger clusters demonstrate drastically enhanced collapse for high-amplitude driving, enabling the study of artificial cavitation clusters under strong driving.