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Acoustic cavitation: Bubble dynamics in high-power ultrasonic fields
... Tiny cavities (microbubbles) are produced during each "stretching" phase and are then collapsed violently in the subsequent cycles, creating enormous shear forces near the bubble. Such a jet causes damage to solid surfaces, leading to a decrease in particle size and dispersion throughout the medium [14]. At present, HIU is successfully applied to improve the emulsification, foaming, viscosity and gelation characteristics of proteins. ...
... Notably, the H 0 of ISPP samples treated for 20 min significantly increased with an increase in the ultrasonic voltage from 200 to 400 W but decreased with voltage increment (600 W). The increase in H 0 was attributed to the promotion of the breakdown of the macromolecular aggregates of ISPP upon cavitation and acoustic streaming, thereby exposing the partially buried interior hydrophobic groups [14,23]. Meanwhile, it was another reason of the increase of H 0 that the energy liberated during bubble collapse provided the energy of hydrophobic interaction [14]. ...
... The increase in H 0 was attributed to the promotion of the breakdown of the macromolecular aggregates of ISPP upon cavitation and acoustic streaming, thereby exposing the partially buried interior hydrophobic groups [14,23]. Meanwhile, it was another reason of the increase of H 0 that the energy liberated during bubble collapse provided the energy of hydrophobic interaction [14]. Similar results have also obtained in pea protein and whey isolates [20,24], showing that the quaternary structures of proteins were disrupted under HIU treatment. ...
The denaturation and lower solubility of commercial potato proteins generally limited their industrial application. Effects of high-intensity ultrasound (HIUS) (200, 400, and 600 W) and treatment time (10, 20, and 30 min) on the physicochemical and functional properties of insoluble potato protein isolates (ISPP) were investigated. The results revealed that HIUS treatment induced the unfolding and breakdown of macromolecular aggregates of ISPP, resulting in the exposure of hydrophobic and R-SH groups, and reduction of the particle size. These active groups contributed to the formation of a dense and uniform gel network of ISPP gel and insoluble potato proteins/egg white protein (ISPP/EWP) hybrid gel. Furthermore, the increase of solubility and surface hydrophobicity and the decrease of particle size improved the emulsifying property of ISPP. However, excessive HIUS treatment reduced the emulsification and gelling properties of the ISPP. It could be speculated that the formation of a stable secondary structure of ISPP initiated by cavitation and shearing effect might play a dominant role on gel strengthens and firmness. Meanwhile, the decrease in relative content of β-turn had a positive effect on the formation of small particle to improve emulsifying property of ISPP.
... The mechanisms by which pressure waves interact with a viscoelastic surface such as biofilm have been studied even less (Koo et al. 2017). A gap in knowledge has been identified between the basic cavitation phenomenon and the practical applications (Lauterborn and Mettin 2015). Therefore, it is important to understand which parameters will optimise cleaning with cavitation for clinical biofilm removal. ...
... Acoustic cavitation occurs when the local pressure of a liquid falls below the saturated vapour pressure, which can occur when ultrasound is applied to the fluid (Young 1999). This negative pressure required to form a cavity in a liquid is called the cavitation threshold (Lauterborn and Mettin 2015). When this occurs, during the rarefaction phase of the propagating ultrasound wave, bubbles grow from small pockets of gas (nuclei) present in the liquid (Brennen 2013). ...
... In non-inertial cavitation, bubbles oscillate repeatedly at low energy (Lauterborn and Mettin 2015). This pulsation usually occurs when the bubbles are in a lowamplitude sound field (Leighton 1994). ...
Bacterial biofilms are a cause of contamination in a wide range of medical and biological areas. Ultrasound is a mechanical energy that can remove these biofilms using cavitation and acoustic streaming, which generate shear forces to disrupt biofilm from a surface. The aim of this narrative review is to investigate the literature on the mechanical removal of biofilm using acoustic cavitation to identify the different operating parameters affecting its removal using this method. The properties of the liquid and the properties of the ultrasound have a large impact on the type of cavitation generated. These include gas content, temperature, surface tension, frequency of ultrasound and acoustic pressure. For many of these parameters, more research is required to understand their mechanisms in the area of ultrasonic biofilm removal, and further research will help to optimise this method for effective removal of biofilms from different surfaces.
... The entire process of bubble formation, growth, and collapse is referred to as UL cavitation. During this process, the temperature and pressure inside the bubble increase significantly, reaching above 5,000 K and 1,000 atm, respectively, leading to the emission of bright light known as sonoluminescence [31]. ...
This study investigated the synergistic application of ultrasonic (UL) and microbubble (MB) technologies for the disinfection of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Optimal conditions for both techniques were determined through a comprehensive experimental design, resulting in an effective disinfection rate of 100% as assessed by the tenfold dilution spread plate count method. The study evaluated three key parameters of the UL/MB system, i.e., UL duration time, MB duration time, and gas flow rate within the ranges of 30-60 sec, 30-60 sec, and 15-30 ml/min, respectively. A 2k full factorial design with three replications, five center points, and blocking was employed for robust statistical analysis. Based on the empirical data and rigorous statistical examination, the optimal conditions for achieving sterilization of E. coli and S. aureus were determined as 30 sec UL duration, 30 sec MB duration, and 30 ml/min gas flow rate; and 60 sec UL duration, 60 sec MB duration, and 15 ml/min gas flow rate, respectively. Utilizing these optimal conditions, the disinfection efficacy was assessed, revealing an inhibition rate of 54% for E. coli and an inhibition rate of 19% for S. aureus, with consistent improvement observed across the three replication trials. These findings underscore the potential of UL/MB technology as an effective disinfection strategy against common bacterial pathogens.
... Propagation of the sound waves into an aquatic system, results in cavitation [22,23]. Due to successive compression (positive pressure) and rarefaction (negative pressure), micro-to nano-sized bubbles form [24,25] and collapse violently when they reach their maximum resonant size, releasing concentrated energy into the surrounding environment. This phenomenon is accompanied by both sonophysical effects, such as microjets, microstreaming, and shock waves, as well as sonochemical effects, including pyrolysis, and radical reaction [26][27][28][29][30]. ...
... The study of bubble dynamics is of great significance in various industrial and biomedical domains. This research area contributes significantly to improvement in pump efficiency and longevity, and also finds its application in various methods such as ultrasonic cleaning, sonochemistry, therapeutic ultrasound, and biofilm removal from biomaterial surfaces [1][2][3][4][5][6][7]. Therefore, a thorough understanding of bubble dynamics is crucial to the development of more effective techniques in these fields [8]. ...
Frequently, smoothing techniques have to be applied to tackle the numerical instability and mesh distortion in modelling bubble dynamics using the boundary element method (BEM). Laplacian smoothing (LS) and its variants have been commonly used in the literature due to their ease of implementation and computational efficiency. Nevertheless, these methods are prone to inducing shrinking and oversmoothing effects. To address this issue, an efficient smoothing technique, called the Extended LS (ELS) method, is devised based on the LS method to accurately capture the salient features of the liquid jet of a collapsing bubble near complex boundaries. The efficacy of the ELS method is demonstrated through its application to various test cases for which theoretical, numerical, or experimental data are available in the literature. The ELS method is then used to simulate intricate bubble dynamics, including the oscillation of a 3D gas bubble near two rigid, fixed spherical particles. The influence of the dimensionless distance d* between the bubble and the particles on the dynamics of the bubble is thoroughly examined. As d* decreases, the bubble moves closer to the particles, causing its lower side surface to change from concave to convex.
... The impact of the jet from a collapsing bubble creates high stresses on the material surface (with or without an elastic layer). This phenomenon is being utilized for many applications, such as ultrasonic cleaning 31,32 , lithotripsy for kidney stones 9,33,34 and sonoporation of cells 35,36 . It is known that the jetting of the bubble is correlated to the cleaned area generated in acoustic surface cleaning 37,38 . ...
A cavitation bubble imposes shear stresses onto a nearby structure during its expansion and
collapse. Experimentally we probe the tangential stresses on an elastic surface by measur-
ing the displacement of embedded particles and the deformation of an elastic structure.
Corresponding numerical simulations are done using a fluid-structure interaction Volume-
of-Fluid solver in OpenFOAM, where a linear elastic solid is coupled to two viscous, im-
miscible and compressible fluids. We find good agreement in terms of bubble dynamics
and displacement motions. During the initial bubble expansion and its first collapse, the
experiment agrees with the simulation that the strain of the elastic sheet at a distance of
1.25 Rmax from the stagnation point center is larger than at 0.51 Rmax. The maximum lateral
strain occurs at a non-dimensionalized bubble stand-off distance of γ ≈ 1.1. The highest
calculated wall shear stress is 250 kPa (for position y = 0). However, the largest overall
shear stress of 1.9 MPa is found within the elastic sheet at y = 24 μm that corresponds to a
maximum displacement of Dx = 44.5 μm. Thus fracture may start from within the elastic
material rather than from the surface. To further examine the fluid-structure interaction, we
construct a simple axisymmetrical elastic ring and analyze its deformation. In this case,
we find strong deformations during the bubble collapse but also during the bubble’s initial
expansion.
... Acoustic cavitation is a phenomenon associated with the propagation of intense sound waves in liquids generating bubbles [1]. This phenomenon involves three discrete stages: nucleation, bubble growth, and, under proper conditions, implosive collapse [2,3]. ...
An intermediate-scale reactor with 10L capacity and two transducers operating at 700 and 950 kHz frequencies was developed to study the scalability of the sonolytic destruction of Per and Polyfluoroalkyl substance (PFAS). The impact of frequency, height of liquid or power density, and transducer position on reactor performance was evaluated with the potassium iodide (KI) oxidation and calorimetric power. The dual frequency mode of operation has a synergistic effect based on the triiodide concentration, and calorimetric power. The triiodide concentration, and calorimetric power were higher in this mode compared to the combination of both frequencies operating individually. The sonochemical efficiency for an intermediate-scale reactor (10L) was similar that obtained from a bench-scale reactor (2L), showing the scalability of the sonolytic technology. The placement of the transducer at the bottom or side wall of the reactor had no significant impact on the sonochemical reactivity. The superposition of the ultrasonic field from the dual transducer mode (side and bottom) did not produce a synergistic effect compared to the single transducer mode (bottom or side). This can be attributed to a disturbance due to the interaction of ultrasonic fields of two frequencies from each transducer. With the encouraging results scaling up is in progress for site implementation.
... The application of high-frequency ultrasound, also known as sonolysis, has demonstrated to be one of the most effective treatment technologies for the mineralization of PFAS in water (Campbell & Hoffmann, 2015;Gole et al., 2018;Lu et al., 2020;Rodriguez-Freire et al., 2015Vecitis et al., 2008a). When high-frequency ultrasound is applied to a liquid medium, micro-to nano-sized bubbles (MNBs) are generated through the process known as acoustic cavitation (Lauterborn & Mettin, 2014;Ultrawaves, n.d.). During acoustic cavitation, MNBs formed grow in size, and, upon reaching a resonant size, adiabatically collapse (see Fig A1 in supplementary information). ...
This work uses a computational approach to provide a mechanistic explanation for the experimentally observed destruction of per- and polyfluoroalkyl substances (PFAS) in water due to ultrasound. The PFAS compounds have caused a strong public and regulatory response due to their ubiquitous presence in the environment and toxicity to humans. In this research, ReaxFF -based Molecular Dynamics simulation under several temperatures ranging from 373 K to 5,000 K and different environments such as water vapor, O2, N2, and air were performed to understand the mechanism of PFAS destruction. The simulation results showed greater than 98% PFAS degradation was observed within 8 ns under a temperature of 5,000 K in a water vapor phase, replicating the observed micro/nano bubbles implosion and PFAS destruction during the application of ultrasound. Additionally, the manuscript discusses the reaction pathways and how PFAS degradation evolves providing a mechanistic basis for the destruction of PFAS in water due to ultrasound. The simulation showed that small chain molecules C1 and C2 fluoro-radical products are the most dominant species over the simulated period and are the impediment to an efficient degradation of PFAS. Furthermore, this research confirms the empirical findings observations that the mineralization of PFAS molecules occurs without the generation of byproducts. These findings highlight the potential of virtual experiments in complementing laboratory experiments and theoretical projections to enhance the understanding of PFAS mineralization during the application of ultrasound.
Acoustic cavitation bubbles drive chemical processes through their dynamic lifecycle in liquids. These bubbles are abundant within sonoreactors, where their behavior becomes complex within clusters. This study quantifies their chemical effects within well-defined clusters using a new laser-based method. We focus a laser beam into water, inducing a breakdown that generates a single cavitation bubble. This bubble undergoes multiple collapses, releasing several shockwaves. These shockwaves propagate into the surrounding medium, leading to the formation of secondary bubbles near a reflector, separated from the input laser beam. We evaluate the chemical activity of these bubble clusters of various sizes by KI dosimetry, and to gain insights into their dynamics, we employ high-speed imaging. Hydrophone measurements show that conversion from focused shockwave energy to chemical reactions increases to a maximum of 16.5%. Additional increases in shockwave energy result in denser bubble clusters and a slightly decreased conversion rate, falling to 14.9%, highlighting the key role of bubble dynamics in the transformation of mechanical to chemical energy and as a result in the efficiency of the sonoreactors. The size and frequency of bubble collapses influence the cluster’s chemical reactivity. We introduce a correlation for predicting the conversion rate of cluster energy to chemical energy, based on the cluster’s energy density. The maximum conversion rate occurs at a cluster energy density of 2500 J/L, linked to a cluster with an average bubble diameter of 91 μ\upmum, a bubble density of 3500 bubbles/ml, and a bubble-to-bubble distance ratio of 8.
The primary significance of this work is that the commercial yeast proteins particles were successfully used to characterize the high internal phase Pickering emulsions (HIPPEs). The different sonication time (0,3,7,11,15 min) was used to modulate the structure and interface characteristics of yeast proteins (YPs) that as Pickering particles. Immediately afterward, the influence of YPs particles prepared at different sonication time on the rheological behavior and coalescence mechanism of HIPPEs was investigated. The results indicate that the YPs sonicated for 7 min exhibited a more relaxed molecular structures and conformation, the smallest particle size, the highest H0 and optimal amphiphilicity (the three-phase contact (θ) was 88.91°). The transition from extended to compact conformations of YPs occurred when the sonication time exceeded 7 min, resulting in an augmentation of size of YPs particles, a reduction in surface hydrophobicity (H0), and an elevation in hydrophilicity. The HIPPEs stabilized by YPs particles sonicated for 7 min exhibited the highest adsorption interface protein percentage and a more homogeneous three-dimensional (3D) protein network, resulting in the smallest droplet size and the highest storage (G′). The HIPPEs sample that stabilized by YPs particles sonicated for 15 min showed the lowest adsorption protein percentage. This caused a reduction in the thickness of its interface protein layer and an enlargement in the droplet diameter (D [3,2]). It was prone to droplet coalescence according to the equation used to evaluate the coalescence probability of droplets (Eq (2)). And the non-adsorbed YPs particles form larger aggregation structures in the continuous phase and act as “structural agents” in 3D protein network. Therefore, mechanistically, the interface protein layer formed by YPs particles sonicated 7 min contributed more to HIPPEs stability. Whereas the “structural agents” contributed more to HIPPEs stability when the sonication time exceeded 7 min. The present results shed important new light on the application of commercial YPs in the functional food fields, acting as an available and effective alternative protein.
Acoustic cavitation bubbles drive chemical processes through their dynamic lifecycle in liquids. These bubbles are abundant within sonoreactors, where their behavior becomes complex within clusters. This study quantifies their chemical effects within well-defined clusters using a new laser-based method. We focus a laser beam into water, inducing a breakdown that generates a single cavitation bubble. This bubble undergoes multiple collapses, releasing several shockwaves. These shockwaves propagate into the surrounding medium, leading to the formation of secondary bubbles near a reflector, separated from the input laser beam. We evaluate the chemical activity of these bubble clusters of various sizes by KI dosimetry, and to gain insights into their dynamics, we employ high-speed imaging. Hydrophone measurements show that conversion from focused shockwave energy to chemical reactions increases to a maximum of 16.55 %. Additional increases in shockwave energy result in denser bubble clusters and a slightly decreased conversion rate, falling to 14.94 %, highlighting the key role of bubble dynamics in the transformation of mechanical to chemical energy and as a result in the efficiency of the sonoreactors. The size and frequency of bubble collapses influence the cluster’s chemical reactivity. We introduce a correlation for predicting the conversion rate of cluster energy to chemical energy, based on the cluster's energy density. The maximum conversion rate occurs at a cluster energy density of 2500 J/L, linked to a cluster with an average bubble diameter of 91.42 μm, a bubble density of 3422 bubbles/ml, and a bubble-to-bubble distance ratio of 8.
This articles reviews till-date available literature on metal matrix composites reinforced with graphene, CNT and other carbonaceous materials. The article has a special focus on the mechanical, tribological and challenges associated with the fabrication of nanocomposites. Simultaneously, it reviews the synthesis, strengthening mechanism and applications of graphene along with research gap associated with graphene metal matrix nanocomposites (GMMNC). Carbonaceous nanofillers, e.g. Graphene, are known to have extraordinary mechanical, thermal and electrical properties along with multifaceted characteristics. These materials have the potential to become an ideal material in numerous application which requires reinforcement. Graphene nanoplatelets (GNP) suffers various challenges starting from its synthesis to the uniform distribution within the matrix material. Our concern is to give details on the challenges associated with graphene and metal matrix composites along with the solution so that new research can be done at its ease. Section 1 of the article gives a detailed analysis of various carbonaceous reinforcement materials. Preparation, processing and dispersion technique for graphene and composite material is given in section 2. Section 3 of the article deals with different matrix material used in MMNC along with the properties and challenges associated with it in tabulated form. Strengthening mechanism used for the enhancement of mechanical properties of composites is described in section 4, whereas, Section 5 deals with the applications and Research gap.
The present paper reports a numerical investigation of the feasibility of a hybrid concept associating the 1-Butyl-3-methylimidazolium Acetate [C4mim][CH3COO] to sonication, in terms of cavitation formation and generated extreme conditions allowing cellulose decomposition in the second reactivity site. The results of the proposed model revealed an acoustic power threshold of 1.8 atm, in order to expect a transient cavitation in the ionic liquid, leading to harsh conditions of 1559.8 K and 49 bar within the bulk volume of the acoustic cavitation bubble. The spatial and temporal variation of the temperature was simulated within the bulk volume of the bubble as well as in the thermal boundary layer jointly with the chemical kinetics. The first stage of the polymerization reduction was clearly attained and demonstrated through the decomposition rate of cellulose and the molar rate of emergence of anhydrocellulose, reaching the respective orders of magnitude of 1.71 × 10⁴ mol/m³⋅s and 7.91 × 10⁴ mol/m³⋅s.
Although most of researchers agree on the elementary reactions behind the sonolytic formation of molecular hydrogen (H2) from water, namely the radical attack of H2O and H2O2 and the free radicals recombination, several recent papers ignore the intervention of the dissolved gas molecules in the kinetic pathways of free radicals, and hence may wrongly assess the effect of dissolved gases on the sonochemical production of hydrogen. One may fairly ask to which extent is it acceptable to ignore the role of the dissolved gas and its eventual decomposition inside the acoustic cavitation bubble? The present opinion paper discusses numerically the ways in which the nature of dissolved gas, i.e., N2, O2, Ar and air, may influence the kinetics of sonochemical hydrogen formation. The model evaluates the extent of direct physical effects, i.e., dynamics of bubble oscillation and collapse events if any, against indirect chemical effects, i.e., the chemical reactions of free radicals formation and consequently hydrogen emergence, it demonstrates the improvement in the sonochemical hydrogen production under argon and sheds light on several misinterpretations reported in earlier works, due to wrong assumptions mainly related to initial conditions. The paper also highlights the role of dissolved gases in the nature of created cavitation and hence the eventual bubble population phenomena that may prevent the achievement of the sonochemical activity. This is particularly demonstrated experimentally using a 20 kHz Sinaptec transducer and a Photron SA 5 high speed camera, in the case of CO2-saturated water where degassing bubbles are formed instead of transient cavitation.
The previous chapter presented physical characterizations of cavitation bubbles on the microscopic scale, looking, e.g., on the bubble shape, on its stability and evolution, and on the way bubble dynamics can explain energy focusing that leads to sonochemistry and sonoluminescence. These latter two phenomena are macroscopic manifestations of acoustic cavitation and can also serve to characterize bubbles and their activity.
Sonochemical splitting of thermodynamically very stable water molecule provides the evidence for drastic conditions inside the cavitation bubble. Kinetics of OH• radicals or H2O2 molecules formation during sonolysis of water can be used for quantification of acoustic power delivered to the system. This chapter focuses on the influence of several fundamental parameters, such as ultrasonic frequency, saturating gas, and some soluble nitrogen compounds on chemical reactivity of multibubble cavitation in homogeneous aqueous media in connection with the recent data on multibubble sonoluminescence.
This book presents the latest research on fundamental aspects of acoustic bubbles, and in particular on various complementary ways to characterize them. It starts with the dynamics of a single bubble under ultrasound, and then addresses few-bubble systems and the formation and development of bubble structures, before briefly reviewing work on isolated bubbles in standing acoustic waves (bubble traps) and multibubble systems where translation and interaction of bubbles play a major role. Further, it explores the interaction of bubbles with objects, and highlights non-spherical bubble dynamics and the respective collapse geometries. It also discusses the important link between bubble dynamics and energy focusing in the bubble, leading to sonochemistry and sonoluminescence.
The second chapter focuses on the emission of light by cavitation bubbles at collapse (sonoluminescence) and on the information that can be gained by sonoluminescence (SL) spectroscopy, e.g. the conditions reached inside the bubbles or the nature of the excited species formed. This chapter also includes a section on the use of SL intensity measurement under pulsed ultrasound as an indirect way to estimate bubble size and size distribution.
Lastly, since one very important feature of cavitation systems is their sonochemical activity, the final chapter presents chemical characterizations, the care that should be taken in using them, and the possible visualization of chemical activity. It also explores the links between bubble dynamics, SL spectroscopy and sonochemical activity.
This book provides a fundamental basis for other books in the Molecular Science: Ultrasound and Sonochemistry series that are more focused on applied aspects of sonochemistry. A basic knowledge of the characterization of cavitation bubbles is indispensable for the optimization of sonochemical processes, and as such the book is useful for specialists (researchers, engineers, PhD students etc.) working in the wide area of ultrasonic processing.
Acoustic emission from acoustic cavitation is called acoustic cavitation noise. In the present study, numerical simula-tions of acoustic cavitation noise have been performed under the experimental condition of Ashokkumar et al.[J.Am.Chem.Soc. 129, 2250-2258 (2007)] at 515 kHz taking into account the temporal fluctuation in the number of bubbles. It is shown that the temporal fluctuation in the number of bubbles results in the braod-band noise. As the temporal fluctuation in the number of bubbles is manly caused by fragmentation of bubbles, transient cavitation bub-bles which have short lifetimes such as one or a few acoustic cycles results in the broad-band noise. On the other hand, stable cavitation bubbles do not cause the broad-band noise even if they emit shock waves.
Experimental analysis of cavitation bubble dynamics typically uses optical imaging and photographic recording. However, the images are often severely affected by distortions and shadows due to refraction and total reflection of the illuminating light at the liquid–gas interface of the bubble. Optical ray tracing may become a powerful tool for the analysis process by assisting in the comparison of experiments to numerical two-phase flow simulations. The novelty of the present approach consists in digitizing almost the complete experimental arrangement with all its optically relevant elements and objects—including a numerical model of the yet unknown bubble—and numerically photographing the scene via ray tracing. The method is applied to the jetting dynamics of single bubbles collapsing at a solid wall. Here, ray tracing can help in the interpretation of raw experimental data concerning the complex bubble interface deformations and internal structures during the collapse. The precise shape of the highly dynamical bubbles can be inferred, thus ray tracing provides a correction method for velocity values of the liquid jets. Strong evidence is found for the existence of an ultra-short-time, fast jet, exceeding velocities known to date in the field.
Graphic abstract
Surface nanobubbles are gaseous domains found at immersed substrates, whose remarkable persistence is still not fully understood. Recently, it has been observed that the formation of nanobubbles is often associated with a local high gas oversaturation at the liquid–solid interface. Tan, An and Ohl have postulated the existence of an effective potential attracting the dissolved gas to the substrate and producing a local oversaturation within 1 nm from it that can stabilize nanobubbles by preventing outgassing in the region where gas flow would be maximum. It is this effective solid–gas potential – which is not the intrinsic, mechanical interaction between solid and gas atoms – its dependence on chemical and physical characteristics of the substrate, gas and liquid, that controls the stability and the other characteristics of surface nanobubbles. Here, we perform free energy atomistic calculations to determine, for the first time, the effective solid–gas interaction that allows us to identify the molecular origin of the stability and other properties of surface nanobubbles. By combining the Tan–An–Ohl model and the present results, we provide a comprehensive theoretical framework allowing, among others, the interpretation of recent unexplained experimental results, such as the stability of surface nanobubbles in degassed liquids, the very high gas concentration in the liquid surrounding nanobubbles, and nanobubble instability in organic solvents with high gas solubility.
The main aim of this paper is to demonstrate the benefit of the application of high-performance computing techniques in the field of non-linear science through two kinds of dynamical systems as test models.
It is shown that high-resolution, multi-dimensional parameter scans (in the order of millions of parameter combinations) via an initial value problem solver are an efficient tool to discover new features of dynamical systems that are hard to find by other means. The employed initial value problem solver is an in-house code written in C++ and CUDA C software environments, which can exploit the high processing power of professional graphics cards (GPUs). The first test model is the Keller–Miksis equation, a non-linear oscillator describing the dynamics of a driven single spherical gas bubble placed in an infinite domain of liquid. This equation is important in the field of cavitation and sonochemistry. Here, the high-resolution parameter scans gave us the opportunity to lay down the basis of a non-feedback technique to control multi-stability in which direct selection of the desired attractor is possible. The second test model is related to a pressure relief valve that can exhibit a special kind of impact dynamics called grazing impact. A fine scan of the initial conditions revealed a second focal point of the grazing lines in the initial-condition space that was hidden in previous studies.
The active cavitation threshold of a dual-frequency driven single spherical gas bubble is studied numerically. This threshold is defined as the minimum energy required to generate a given relative expansion (Rmax-RE)/RE, where RE is the equilibrium size of the bubble and Rmax is the maximum bubble radius during its oscillation. The model employed is the Keller–Miksis equation that is a second order ordinary differential equation. The parameter space investigated is composed by the pressure amplitudes, excitation frequencies, phase shift between the two harmonic components and by the equilibrium bubble radius (bubble size). Due to the large 6-dimensional parameter space, the number of the parameter combinations investigated is approximately two billion. Therefore, the high performance of graphics processing units is exploited; our in-house code is written in C++ and CUDA C software environments. The results show that for (Rmax-RE)/RE=2, the best choice of the frequency pairs depends on the bubble size. For small bubbles, below 3μm, the best option is to use equal frequencies and low ones in the giant response region. For medium sized bubbles, between 3μm and 6μm, the optimal choice is the mixture of low frequency (giant response) and main resonance frequency. For large bubbles, above 6μm, the main resonance dominates the active cavitation threshold. Increasing the prescribed relative expansion value to (Rmax-RE)/RE=3, the optimal choice is always equal frequencies with the lowest values (20kHz here). Thus, in this case, the giant response always dominates the active cavitation threshold. The phase shift between the harmonic components of the driving has no effect on the threshold.
A bubble expanding and collapsing in water near a flat solid boundary is studied numerically. According to current knowledge, it develops a high-speed liquid jet towards the boundary of typically ∼100 m/s. However, the character of jet formation and the jet properties alter strongly when the bubble expands and collapses very close to a solid boundary. In this case, a much thinner and much faster jet of typically ∼1000 m/s is generated. The respective mechanism is demonstrated by solving the Navier-Stokes equations for a model of a laser-induced bubble. The results add substantially to the understanding of the erosion process caused by imploding cavitation bubbles near solid boundaries.
A detailed experimental study on acoustically-induced bubble jets produced far from any solid wall is presented. This particular type of bubble jets is observed when a laser-induced gas cavity is seeded in a liquid by an optical dielectric breakdown while applying a strong ultrasonic field. This generation method allows a high degree of control over jet dynamics, achievable by changing critical experimental parameters as the laser pulse energy (i.e. the size of the bubbles), the driving signal frequency, the seeding phase (relative to the sound field), the acoustic pressure amplitude and in particular the pressure gradient at the bubble inception location. The temporal evolution of the characteristic dimensions of the bubble jets is analyzed by means of numerical simulations of spherical bubble dynamics and its dependence on the local acoustic field. The jet strength is presented in terms of the jet elongation ratio, defined as the axial length over the maximum width. The most relevant parameters related to jet strength are the maximum radius reached by the bubble prior to the jet formation and the pressure gradient during the bubble collapse phase. The existence of optimum driving parameters and bubble seeding position to obtain the strongest jets achievable in a given acoustic pressure distribution is discussed. This study is a first step to shed some light on the jet formation process due to known time-variable pressure gradients. The work can be extended to further understanding the role of bubble jets on sonochemical reactions and biological or medical applications.
A novel method to control multistability of nonlinear oscillators by applying dual-frequency driving is presented. The test model is the Keller–Miksis equation describing the oscillation of a bubble in a liquid. It is solved by an in-house initial-value problem solver capable to exploit the high computational resources of professional graphics cards. During the simulations, the control parameters are the two amplitudes of the acoustic driving at fixed, commensurate frequency pairs. The high-resolution bi-parametric scans in the control parameter plane show that a period-2 attractor can be continuously transformed into a period-3 one (and vice versa) by proper selection of the frequency combination and by proper tuning of the driving amplitudes. This phenomenon has opened a new way to drive the system to a desired, pre-selected attractor directly via a non-feedback control technique without the need of the annihilation of other attractors. Moreover, the residence in transient chaotic regimes can also be avoided. The results are supplemented with simulations obtained by the boundary-value problem solver AUTO, which is capable to compute periodic orbits directly regardless of their stability, and trace them as a function of a control parameter with the pseudo-arclength continuation technique.
Laser generated cavitation bubbles are numerically modeled with the finite volume method using the open source software package OpenFOAM. The alterations applied to the native code are described and validated by solving the spherical bubble collapse problem and comparing the solution with the Gilmore model and experiments. Problems of spherical stability and their connection with the mesh are discussed. Shock wave emission upon laser bubble generation and bubble collapse is modeled by inserting the Tait equation for the liquid (water) into the code. The results are compared with calculations by Hickling and Plesset and experiments. The calculations are extended to the problem of cavitation bubble collapse in front of a solid boundary.
Surface nanobubbles are nanoscopic gaseous domains on immersed substrates which can survive for days. They were first speculated to exist about 20 years ago, based on stepwise features in force curves between two hydrophobic surfaces, eventually leading to the first atomic force microscopy (AFM) image in 2000. While in the early years it was suspected that they may be an artifact caused by AFM, meanwhile their existence has been confirmed with various other methods, including through direct optical observation. Their existence seems to be paradoxical, as a simple classical estimate suggests that they should dissolve in microseconds, due to the large Laplace pressure inside these nanoscopic spherical-cap-shaped objects. Moreover, their contact angle (on the gas side) is much smaller than one would expect from macroscopic counterparts. This review will not only give an overview on surface nanobubbles, but also on surface nanodroplets, which are nanoscopic droplets (e.g., of oil) on (hydrophobic) substrates immersed in water, as they show similar properties and can easily be confused with surface nanobubbles and as they are produced in a similar way, namely, by a solvent exchange process, leading to local oversaturation of the water with gas or oil, respectively, and thus to nucleation. The review starts with how surface nanobubbles and nanodroplets can be made, how they can be observed (both individually and collectively), and what their properties are. Molecular dynamic simulations and theories to account for the long lifetime of the surface nanobubbles are then reported on. The crucial element contributing to the long lifetime of surface nanobubbles and nanodroplets is pinning of the three-phase contact line at chemical or geometric surface heterogeneities. The dynamical evolution of the surface nanobubbles then follows from the diffusion equation, Laplace's equation, and Henry's law. In particular, one obtains stable surface nanobubbles when the gas influx from the gas-oversaturated water and the outflux due to Laplace pressure balance. This is only possible for small enough surface bubbles. It is therefore the gas or oil oversaturation ζ that determines the contact angle of the surface nanobubble or nanodroplet and not the Young equation. The review also covers the potential technological relevance of surface nanobubbles and nanodroplets, namely, in flotation, in (photo)catalysis and electrolysis, in nanomaterial engineering, for transport in and out of nanofluidic devices, and for plasmonic bubbles, vapor nanobubbles, and energy conversion. Also given is a discussion on surface nanobubbles and nanodroplets in a nutshell, including theoretical predictions resulting from it and future directions. Studying the nucleation, growth, and dissolution dynamics of surface nanobubbles and nanodroplets will shed new light on the problems of contact line pinning and contact angle hysteresis on the submicron scale. © 2015 American Physical Society.
Many gas bubbles emerge when a liquid is strongly sonicated [1]. At least some of them represent the active sites in sonochemistry and multibubble sonoluminescence. Each bubble shows an individual fast dynamics by volume and shape oscillations that are responsible for the specific chemical or luminescent process. However, a bubble field shows a collective dynamics on a slower time scale as well: nucleation, drift, diffusion, coalescence and destruction picture a bubble’s life, and many of such histories determine the bubble distribution in space and time. Experiments reveal that this distribution in typical applications is far from homogeneous. On the contrary, the bubbles tend to organize themselves in filaments (“streamers”) that can form complex tree-like structures (in analogy to electrical discharge patterns we call them “Acoustic Lichtenberg Figures” or ALFs; see Fig. 1). The inhomogeneous bubble population in strong sound fields is a major complication for the investigation, modeling and enhancement of sonochemical processes, and more insight in the underlying mechanisms are of high demand. Here we report a first step towards a simulation of bubble structure formation by a many particle approach. The basis are recent calculations of Bjerknes forces in strong acoustic fields that govern the bubble motion [2,3].
A mathematical model which describes the behavior of a gas bubble
cluster in an external acoustic field is presented. According to this
model the bubbles of different ambient radii are assumed to be submerged
into a spherical liquid volume. The boundary of this volume is
considered as a cluster's boundary. This model includes a set of coupled
Rayleigh-type equations for bubble radii and cluster radius. The
dynamics of a single bubble cluster excited by external periodic
pressure is considered. The problem has been solved numerically under
the assumption of spherically-symmetric oscillations of bubbles and the
cluster's boundary. A synchronization phenomenon has been found; namely,
the numerical results have shown that bubbles of different initial radii
in a cluster collapse in phase. Moreover, it has been noted that at some
parameter values such a synchronization may lead to the intensification
of bubble collapse. .
Liquid water at atmospheric pressure can be supercooled to -41°C
(ref. ) and superheated to +302°C (ref. ). Experiments involving
fluid inclusions of water in quartz suggest that water is capable of
sustaining pressures as low as -140 MPa before it breaks by cavitation.
Other techniques, for which cavitation occurs consistently at around
-30MPa (ref. ), produce results that cast doubt on this claim. Here we
reproduce the fluid-inclusion experiment, performing repeated
measurements on a single sample--a method used in meteorology,
bioprotection and protein crystallization, but not yet in liquid water
under large mechanical tension. The resulting cavitation statistics are
characteristic of a thermally activated process, and both the free
energy and the volume of the critical bubble are well described by
classical nucleation theory when the surface tension is reduced by less
than 10%, consistent with homogeneous cavitation. The line of density
maxima of water at negative pressure is found to reach
922.8kgm-3 at around 300K, which further constrains its
contested phase diagram.
The dynamics of the medium within a collapsing and rebounding cavitation bubble is investigated by means of molecular dynamics (MD) simulations adopting a hard sphere model for the species inside the bubble. The dynamics of the surrounding liquid (water) is modelled using a Rayleigh–Plesset (RP)-type equation coupled to the bubble interior by the gas pressure at the wall obtained from the MD calculations. Water vapour and vapour chemistry are included in the RP-MD model as well as mass and energy transfer through the bubble wall. The calculations reveal the evolution of temperature, density and pressure within a bubble at conditions typical of single-bubble sonoluminescence and predict how the particle numbers and densities of different vapour dissociation and reaction products in the bubble develop in space and time. Among the parameters varied are the sound pressure amplitude of a sonoluminescence bubble in water, the noble gas mixture in the bubble and the accommodation coefficients for mass and energy exchange through the bubble wall. Simulation particle numbers up to 10 million are used; most calculations, however, are performed with one million particles to save computer run time. Validation of the MD code was done by comparing MD results with solutions obtained by continuum mechanics calculations for the Euler equations.
In this study we report several experimental and numerical results on the influence of static pressure (P_{0}) over the main parameters in single bubble sonoluminescence (SBSL), using a sulfuric acid aqueous solution (SA) with low concentrations of argon gas dissolved. Bifrequency driving was used in the experiments to enhance spatial stability of the bubbles. The experimental results were compared with simulations provided by a numerical code that models the radial dynamics of the bubbles. The results showed that an increase on the static pressure of the system shifts the Bjerknes instability threshold, allowing the bubble to access higher acoustic pressures (P_{Ac}^{}). Furthermore, a decrease in the measured ambient radius R_{0} and the calculated relative gas concentration c_{∞}/c_{0} were observed. A notorious increment in the bubble collapse violence and energy focusing for P_{0} above 1 bar was achieved. These were mainly indicated by the growth of the bubble expansion ratio (R_{max}/R_{0}), the bubble mechanical energy density, and the maximum bubble wall velocity dR/dt. In agreement with the previous statement, the maximum temperature during the bubble collapse predicted by the model is augmented as well. The use of different harmonics in the ultrasound pressure field regarding energy focusing is also discussed. Finally, we analyzed the stability regions of the R_{0}-P_{Ac}^{} parameter space via numerical predictions for P_{0} above the measured, identifying the shape instabilities as the main limiting agent to obtain further energy concentration in SA systems at high static pressures.
The dynamic equilibrium model suggests that surface nanobubbles can be stable due to an influx of gas in the vicinity of the bubble contact line, driven by substrate hydrophobicity, that balances the outflux of gas from the bubble apex. Here, we develop an alternate formulation of this mechanism that predicts rich behavior in agreement with recent experimental measurements. Namely, we find that stable nanobubbles exist in narrow temperature and dissolved gas concentration ranges, that there is a maximum and minimum possible bubble size, and that nanobubble radii decrease with temperature.
The torsion of the local flow around closed orbits and its relation to the superstructure in the bifurcation set of strictly dissipative nonlinear oscillators is investigated. The torsion number describing the twisting behaviour of the flow turns out to be a suitable invariant for the classification of local bifurcations and resonances in those systems. Furthermore, the notions of winding number and resonance are generalized to arbitrary one-dimensional dissipative oscillators. © 1986, Verlag der Zeitschrift für Naturforschung. All rights reserved.
A small spherical bubble of high internal pressure is inserted into water at constant ambient pressure as a model of a laser-induced bubble. Its subsequent dynamics near a flat solid boundary is studied in dependence on the distance of the bubble to the boundary by numerically solving the Navier-Stokes equations with the help of the open source software environment OpenFOAM. Implemented is the finite-volume method for discretization of the equations of motion and the volume-of-fluid method for capturing the interface between the bubble interior and exterior. The bubble contains a small amount of noncondensable gas that is treated as an ideal gas. The liquid is water obeying the Tait equation. Surface tension is included where necessary. The evolution of the bubble shape and a selection of pressure and velocity fields are given for normalized distances D*=D/Rmax between 0 and 3 (D is the initial distance of the bubble center to the boundary and Rmax is the maximum radius the bubble would attain without any boundary). The value Rmax= 500 μm is chosen for the study. Normal axial-jet formation (∼100 m/s) by axial flow focusing is found for 0.24≤D*≤3 and the change to a different type of axial-jet formation (∼1000 m/s) by annular-liquid-flow collision for bubbles very close to the solid boundary (0≤D*≤0.2). The transition region (0.2<D*<0.24) is characterized by additional inbound and outbound annular jets. Remarkably, the inclusion of the viscosity of the water is decisive to get the fast jets.
A feedforward control technique is presented to steer a harmonically driven, non-linear system between attractors in the frequency–amplitude parameter plane of the excitation. The basis of the technique is the temporary addition of a second harmonic component to the driving. To illustrate this approach, it is applied to the Keller–Miksis equation describing the radial dynamics of a single spherical gas bubble placed in an infinite domain of liquid. This model is a second-order, non-linear ordinary differential equation, a non-linear oscillator. With a proper selection of the frequency ratio of the temporary dual-frequency driving and with the appropriate tuning of the excitation amplitudes, the trajectory of the system can be smoothly transformed between specific attractors; for instance, between period-3 and period-5 orbits. The transformation possibilities are discussed and summarized for attractors originating from the subharmonic resonances and the equilibrium state (absence of external driving) of the system.
The existence of bulk nanobubbles has long been regarded with scepticism, due to the limitations of experimental techniques and the widespread assumption that spherical bubbles cannot achieve stable equilibrium. We develop a model for the stability of bulk nanobubbles based on the experimental observation that the zeta potential of spherical bubbles abruptly diverges from the planar value below 10 μm. Our calculations recover three persistently reported—but disputed—properties of bulk nanobubbles: that they stabilize at a typical radius of ∼100 nm, that this radius is bounded below 1 μm, and that it increases with ionic concentration.
The theoretical understanding of surface nanobubbles—nanoscale gaseous domains on immersed substrates—revolves around two contrasting perspectives. One perspective, which considers gas transport in the nanobubbles’ vicinity, explains numerous stability-related properties but systematically underestimates the dynamical response timescale by orders of magnitude. The other perspective, which considers gas transport as the bulk liquid equilibrates with the external environment, recovers the experimentally observed dynamical timescale but incorrectly predicts that nanobubbles progressively shrink until dissolution. We propose a model that couples both perspectives, which is capable of explaining the stability, dynamics, and unexpected tolerance of surface nanobubbles to undersaturated environments.
The acoustic waves being generated during the motion of a bubble in water near a solid boundary are calculated numerically. The open source package OpenFOAM is used for solving the Navier-Stokes equation and extended to include nonlinear acoustic wave effects via the Tait equation for water. A bubble model with a small amount of gas is chosen, the gas obeying an adiabatic law. A bubble starting from a small size with high internal pressure near a flat, solid boundary is studied. The sequence of events from bubble growth via axial microjet formation, jet impact, annular nanojet formation, torus-bubble collapse, and bubble rebound to second collapse is described. The different pressure and tension waves with their propagation properties are demonstrated.
When a cavitation bubble oscillates and collapses in the vicinity of a solid boundary (a substrate), it induces intense microconvection in the surrounding liquid and—of high practical importance—directly at the substrate. As the involved flows are fast, highly unsteady, and possess an impressive shear, experiments are difficult and data are scarce. Here, insight into the generation and dynamics of the liquid flows from individual cavitation bubbles collapsing in the vicinity of a solid boundary is provided. Single laser-induced cavitation bubbles (maximum radius around 375 μm) are seeded at precisely defined standoff distances to a substrate by a focused laser pulse. The bubble shape dynamics are imaged by synchronized high-speed cameras from two perpendicular viewing angles. Recording of the shape dynamics is combined with the simultaneous time-resolved measurement of the full flow field on a micrometer-resolution. Measurements employ a high-speed hybrid particle imaging velocimetry and particle tracking velocimetry technique, with a temporal sampling of up to 135 kHz, using fluorescent microparticles as tracers. The time evolution of the unsteady flow field induced by one and the same bubble over a time period much longer than the bubble lifetime is determined. The shear flow at the substrate is analyzed and a liquid transport toward and away from the substrate surface is demonstrated. Depending on the bubble standoff distance, very different flow patterns are observed. The dominant liquid displacement is caused by the long-lived vortex ring being produced during bubble collapse. Most peculiar, the bubble standoff distance determines the sense of direction of the circulation associated with the vortex ring and, consequently, whether the vortex is ejected from the substrate or radially stretches over it. The results are relevant for the understanding of cavitation effects, such as surface cleaning, erosion, and mixing or in biomedical context and may serve as basis for numerical simulations.
Multibubble sonoluminescence (MBSL) is the emission of light from imploding cavitation bubbles in dense ensembles or clouds. We demonstrate a technique of high-speed recording that allows imaging of bubble oscillations and motion together with emitted light flashes in a nonstationary multibubble environment. Hereby a definite experimental identification of light emitting individual bubbles, as well as details of their collapse dynamics can be obtained. For the extremely bright MBSL of acoustic cavitation in xenon saturated phosphoric acid, we are able to explore effects of bubble translation, deformation, and interaction on MBSL activity. The recordings with up to 0.5 million frames per second show that few and only the largest bubbles in the fields are flashing brightly, and that emission often occurs repetitively. Bubble collisions can lead to coalescence and the start or intensification of the emission, but also to its termination via instabilities and splitting. Bubbles that develop a liquid jet during collapse can flash intensely, but stronger jetting gradually reduces the emissions. Estimates of MBSL collapse temperature peaks are possible by numerical fits of transient bubble dynamics, in one case yielding 38 000 K.
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, 35 kHz and 130 kHz, and different driving powers are tested for their cleaning effectiveness. No cleaning is found for 35 kHz, whereas good cleaning results are obtained for 130 kHz, 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.
Forced oscillations of spherical bubbles in a compressible viscous liquid (water) are calculated numerically. The response of bubbles to sound fields for a special parameter set is given along with examples of the pressure distribution around a single bubble during oscillation.
Sonoluminescence is the production of electromagnetic radiation, much of it in the form of visible light, that is emitted from a gas-filled cavity that has grown and collapsed under the influence of a varying pressure field. This resource paper provides a guide to the literature of sonoluminescence, from its early history to the present.
Cavities (cavitation bubbles) are formed by focusing giant pulses of a ruby laser into liquids. Their dynamics is investigated by high-speed cinematography with picture repetition rates up to 900,000 pictures per second. The radial motion of single spherical bubbles (decaying oscillations, collapse time) is compared with existing theoretical bubble models. Bubble collapse and jet formation due to shock waves, near presence of a solid boundary, the interaction of bubbles, and nonspherical bubbles are investigated.
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.
Copyright © 2015 Elsevier B.V. All rights reserved.
A new approach for the theoretical description of structure formation in acoustic cavitation is developed. The model consists of two coupled partial differential equations describing the spatiotemporal evolution of the sound field amplitude and the bubble concentration. Linear stability analysis and numerical simulations of the pattern formation are presented. The relation between this approach and streamer formation is discussed.
The conditions influencing the onset of acoustically-induced cavitation in tap water have been investigated, with special attention to the effect of air-filled cavitation nuclei. Cavitation was induced by exciting an acoustic radial mode in the water in a spherical resonator at a frequency near 25 kc/sec. Air-filled nuclei were detected by observing the reverberant decay of sound in the resonator at frequencies from 150 to 550 kc/sec, the presence of air nuclei causing an increase in the decay rate.
Measurements have been made of the sound pressure required for cavitation inception, and of the content of air nuclei, for the following treatments of the water: (1) allowing the water to stand undisturbed after drawing from the tap, (2) partially deaerating the water, and (3) subjecting the water to increased static pressure. Some measurements were also made of the threshold for rectified diffusion into air bubbles.
The experimental results have been compared with theoretical predictions based on three alternate forms of air nuclei: (1) free air bubbles, (2) air trapped in cracks on suspended solid particles, and (3) air bubbles surrounded by skins of organic impurities.
Forced oscillations of a spherical gas bubble in an incompressible, viscous liquid (water) are calculated numerically. The information gathered is mainly displayed in the form of frequency response curves of the steady-state solutions showing the harmonics, subharmonics, and ultraharmonics. Bubbles oscillating ultraharmonically at frequencies below the main resonance may emit half the driving frequency. This fact gives rise to a new explanation for the occurrence of the first subharmonic in the spectrum of the cavitation noise in ultrasonic cavitation.
Subject Classification: [43]30.70, [43]30.75.
The pressures and sound spectra for cavitation set up in the focus of a transducer array of 15 kc/s are measured with a magneto-strictive probe.In weak cavitation, bubbles are formed and set in oscillation at their natural frequencies. They stabilise themselves with a definite radius (depending on air content), which can lie below that for resonance with the source, and burst under strong excitation. In hard cavitation strong pressure pulses are set up, which mainly influence the noise spectrum.Other matters considered are relationship between pulse and excitation; pulse form and duration; statistical fluctuations from the mean position.
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.
Sonoluminescence (SL) is the name given to the light emitted when a
liquid is cavitated in a particular (rather violent) manner. The
appropriate cavitation conditions can be realized by using high
intensity ultrasound, a spark discharge, a laser pulse, or by flowing
the liquid through a Venturi tube. SL occurs in a wide variety of
liquids, its intensity and spectrum depending on the nature of the
solvent and the solute (including dissolved gas). The intensity, but
apparently not the spectrum, also depends on the frequency of the sound
and on the temperature and hydrostatic pressure of the liquid. In a
standing wave sound field the SL originates from bubbles attracted to
the pressure antinodes and has its maximum intensity when the bubble
volume is a minimum. The phase of the sound cycle at which this occurs
depends on the amplitude and frequency of the sound field. Spectral
measurements show that SL originates mainly from the recombination of
free radicals created within the high temperature and high pressure
environment of a bubble undergoing an adiabatic compression, as may
happen either during transient cavitation or during highly non-linear,
but stable, cavitation. In discussing these, and other, attributes of SL
this review emphasizes developments over the past 20 years. Because of
the importance of the dynamical theory of bubbles to a full
understanding of SL, it includes an account of bubble dynamics. In
addition, it describes the various experimental techniques employed in
the creation and analysis of SL. Although the review lays particular
stress on the SL produced via acoustic cavitation, it also examines the
characteristics of the SL produced using other methods of cavitation.
This chapter examines the types, stages, and effects of acoustic cavitation. Cavitation is defined as the formation of one or more pockets of gas in a liquid. The cavity's gas content refers to the liquid's vapor, some other gas, or combinations thereof. Cavitation tests are one way of characterizing the liquid medium. In very carefully cleaned liquids, the cavitation threshold may approach or even equal the tensile strength of the liquid. Stable cavities are usually gas bubbles excited by relatively low acoustic pressures. There are two basic stages of acoustic cavitation, which include the inception, or initial formation of the cavity and the subsequent cavity dynamics involving growth and collapse. The collapse becomes symmetric, surface waves grow into shape instabilities, single bubbles shatter into pulsating microbubbles that interact hydrodynamically, and bubbles translate as well as pulsate in pressure gradients. The dynamics of acoustically initiated cavities depend on many factors, such as acoustic pressure amplitude, acoustic frequency, gas content of the cavity, temperature.
It has been an experimental challenge to test the rupture of liquids with homogeneous nucleation of vapor bubbles. Many prior studies suffered from the ubiquitous presence of impurities in liquids or at container surfaces that spontaneously nucleate and grow under tension. Here, we propose a microfluidic approach to eliminate such impurities and obtain homogeneous bubble nucleation. We stretch the liquid dynamically via the interaction between a laser-induced shock and an air-liquid interface in a microchannel. Reproducible observations of the nucleation of vapor bubbles are obtained, supporting our claim of homogeneous nucleation. From comparisons of the distribution of vapor cavities with Euler flow simulations, the nucleation threshold for water at room temperature is predicted to be -60 MPa.
For small Mach numbers the Rayleigh--Plesset equations (modified to include acoustic radiation damping) provide the hydrodynamic description of a bubble's breathing motion. Measurements are presented for the bubble radius as a function of time. They indicate that in the presence of sonoluminescence the ratio of maximum to minimum bubble radius is about 100. Scaling laws for the maximum bubble radius and the temperature and duration of the collapse are derived in this limit. Inclusion of mass diffusion enables one to calculate the ambient radius. For audible sound fields these equations yield picosecond hot spots, such as are observed experimentally. However, the analysis indicates that a detailed description of sonoluminescence requires the use of parameters for which the resulting motion reaches large Mach numbers. Therefore the next step toward explaining sonoluminescence will require the extension of bubble dynamics to include nonlinear effects such as shock waves.
Acoustic cavitation, a complex, spatio-temporal dynamical system, is investigated with respect to its chaotic properties. The sound output, the {open_quote}{open_quote}noise{close_quote}{close_quote}, is subjected to time series analysis. The spatial dynamics of the bubble filaments is captured by high speed holographic cinematography and subsequent digital picture processing from the holograms. Theoretical models are put forward for describing the pattern formation. {copyright} {ital 1996 American Institute of Physics.}
A vocabulary useful in describing cavitation in an acoustic field will be defined, and concepts will be introduced to facilitate the interpretation of complex, many‐bubble phenomena in terms of single‐bubble dynamics. A distinction between stable cavities and transient cavities will be made and the roles of these two limiting types of bubbles in “gaseous” and “vaporous” cavitation will be examined. Under the influence of an acoustic field, cavitation bubbles grow either slowly or explosively from small gas nuclei in a liquid. The conditions determining whether a gas nucleus will become an explosively growing, transient cavity or a slowly growing, stable cavity will be summarized, and the dynamics of the two types of cavities will be contrasted. In particular, a conceptual model of a transient cavity will be proposed, and calculations based on this model will be used to characterize the life history of such a cavity from its nucleation to its catastrophic end. The rationalization of specific cavitation phenomena (such as the erosion of solids and the acceleration of chemical reactions) in terms of the pulsations of a stable cavity or the violent collapse of a transient cavity will be considered. Finally, an attempt will be made to indicate where our understanding of the basic physical processes of cavitation needs to be deepened.
The origin of cavitation bubbles, cavitation nuclei, has been a subject of debate since the early years of cavitation research. This paper presents an analysis of a representative selection of experimental investigations of cavitation inception and the tensile strength of water. At atmospheric pressure, the possibility of stabilization of free gas bubbles by a skin has been documented, but only within a range of bubble sizes that makes them responsible for tensile strengths up to about 1.5 bar, and values reaching almost 300 bar have been measured. However, cavitation nuclei can also be harbored on the surface of particles and bounding walls. Such nuclei can be related to the full range of tensile strengths measured, when differences of experimental conditions are taken into consideration. The absence or presence of contamination on surfaces, as well as the structure of the surfaces, are central to explaining why the tensile strength of water varies so dramatically between the experiments reported. A model for calculation of the critical pressure of skin-covered free gas bubbles as well as that of interfacial gaseous nuclei covered by a skin is presented. This model is able to bridge the apparently conflicting results of the many scientists, who have been working in the field over the years.
The hypothesis discussed that the cavitation nuclei consist in gas bubbles. Due to surface tension, small bubbles would dissolve in a very short time. If the bubbles are larger than 5×10−3 cm, or if the liquid is supersaturated, they may last longer or even be stable, but then no cavitation threshold exists.
The hypothesis expressed that the nuclei are very small bubbles, stabilized by an organic skin, which mechanically prevents loss of gas by diffusion. The cavitation occurs when the skin breaks and the threshold is determined by the breaking strength of the film and the size of the bubble.