Interferometric Fiber Optic Probe for Measurements of Cavitation Bubble Expansion Velocity and Bubble Oscillation Time.

Cavitation bubbles are used in medicine as a mechanism to generate shock waves. The study of cavitation bubble dynamics plays a crucial role in understanding and utilizing such phenomena for practical applications and purposes. Since the lifetime of cavitation bubbles is in the range of hundreds of microseconds and the radii are in the millimeter range, the observation of bubble dynamics requires complicated and expensive equipment. High-speed cameras or other optical techniques require transparent containers or at least a transparent optical window to access the region. Fiber optic probe tips are commonly used to monitor water pressure, density, and temperature, but no study has used a fiber tip sensor in an interferometric setup to measure cavitation bubble dynamics. We present how a fiber tip sensor system, originally intended as a hydrophone, can be used to track the expansion and contraction of cavitation bubbles. The measurement is based on interference between light reflected from the fiber tip surface and light reflected from the cavitation bubble itself. We used a continuous-wave laser to generate cavitation bubbles and a high-speed camera to validate our measurements. The shock wave resulting from the collapse of a bubble can also be measured with a delay in the order of 1 µs since the probe tip can be placed less than 1 mm away from the origin of the cavitation bubble. By combining the information on the bubble expansion velocity and the time of bubble collapse, the lifetime of a bubble can be estimated. The bubble expansion velocity is measured with a spatial resolution of 488 nm, half the wavelength of the measuring laser. Our results demonstrate an alternative method for monitoring bubble dynamics without the need for expensive equipment. The method is flexible and can be adapted to different environmental conditions, opening up new perspectives in many application areas.

Flow Boiling Enhancement Using Three-Dimensional Contact-Line Pinning on Hierarchical Superbiphilic Micro/Nanostructures.

Flow boiling is a promising method for the cooling of sensitive computational and industrial components, facilitating the transportation of large quantities of heat at near-constant temperature and in a small form factor. The prevention of vapor film formation is a fundamental challenge for the enhancement of boiling systems, and an impetus therefore exists for the discovery of new techniques to segregate nucleating bubbles during their formation. Herein, we utilize the strong capillary forces generated by nanostructures to pin the liquid/vapor interface in three dimensions and thereby control the coalescence and flow interactions of developing bubbles. We demonstrate this principle on both symmetrical and asymmetrical superbiphilic microstructures, showing enhancement of peak heat transfer coefficient by 81% and 113%, respectively, when compared to the best superhydrophilic and superhydrophobic analogues. Our approach shows a potential future direction for engineered boiling micro/nanostructures, wherein bubble dynamics are directly manipulated on bespoke, three-dimensional substrates.

Hybrid quadrature moment method for accurate and stable representation of non-Gaussian processes applied to bubble dynamics.

Solving the population balance equation (PBE) for the dynamics of a dispersed phase coupled to a continuous fluid is expensive. Still, one can reduce the cost by representing the evolving particle density function in terms of its moments. In particular, quadrature-based moment methods (QBMMs) invert these moments with a quadrature rule, approximating the required statistics. QBMMs have been shown to accurately model sprays and soot with a relatively compact set of moments. However, significantly non-Gaussian processes such as bubble dynamics lead to numerical instabilities when extending their moment sets accordingly. We solve this problem by training a recurrent neural network (RNN) that adjusts the QBMM quadrature to evaluate unclosed moments with higher accuracy. The proposed method is tested on a simple model of bubbles oscillating in response to a temporally fluctuating pressure field. The approach decreases model-form error by a factor of 10 when compared with traditional QBMMs. It is both numerically stable and computationally efficient since it does not expand the baseline moment set. Additional quadrature points are also assessed, optimally placed and weighted according to an additional RNN. These points further decrease the error at low cost since the moment set is again unchanged. This article is part of the theme issue 'Data-driven prediction in dynamical systems'.

MFC: An open-source high-order multi-component, multi-phase, and multi-scale compressible flow solver.

MFC is an open-source tool for solving multi-component, multi-phase, and bubbly compressible flows. It is capable of efficiently solving a wide range of flows, including droplet atomization, shock-bubble interaction, and bubble dynamics. We present the 5- and 6-equation thermodynamically-consistent diffuse-interface models we use to handle such flows, which are coupled to high-order interface-capturing methods, HLL-type Riemann solvers, and TVD time-integration schemes that are capable of simulating unsteady flows with strong shocks. The numerical methods are implemented in a flexible, modular framework that is amenable to future development. The methods we employ are validated via comparisons to experimental results for shock-bubble, shock-droplet, and shock-water-cylinder interaction problems and verified to be free of spurious oscillations for material-interface advection and gas-liquid Riemann problems. For smooth solutions, such as the advection of an isentropic vortex, the methods are verified to be high-order accurate. Illustrative examples involving shock-bubble-vessel-wall and acoustic-bubble-net interactions are used to demonstrate the full capabilities of MFC.

A New Mechanism of Light-Induced Bubble Growth to Propel Microbubble Piston Engine.

Radiation pressure refers to the momentum transfer of photons during light "particles" impacting a surface. The force is too small to drive microengines. Different from the classical radiation pressure, the indirect radiation pressure (Fm ) is introduced, coming from the momentum change of light-induced bubble expansion. Fm is shown to obey Fm ∼ (I·rb )2 , behaving faster growth of indirect radiation pressure versus light intensity I and bubble radius rb . An effective bubble size range is identified for Fm to suppress other forces for bubble in liquid. The top laser irradiation on nanofluid is used in this experiment. A well-defined bubble pulsating flow, being a new principle of bubble piston engine, is demonstrated. During pulse on (≈ns scale), Fm exceeding other forces generates an extremely large acceleration, which is three to four orders larger than the gravity acceleration, propelling the bubble traveling downward. During pulse off, the bubble is floating upward due to the nonexistence of Fm . In such a way, the piston engine sustains the oscillating ranges of 38-347 µm for bubble diameters and 2.7-457.9 µm for traveling distances of piston. This work is useful to manipulate bubble dynamics in solar energy systems, and can find various applications in optofluidics.

Wavelet analysis techniques in cavitating flows.

Cavitating and bubbly flows involve a host of physical phenomena and processes ranging from nucleation, surface and interfacial effects, mass transfer via diffusion and phase change to macroscopic flow physics involving bubble dynamics, turbulent flow interactions and two-phase compressible effects. The complex physics that result from these phenomena and their interactions make for flows that are difficult to investigate and analyse. From an experimental perspective, evolving sensing technology and data processing provide opportunities for gaining new insight and understanding of these complex flows, and the continuous wavelet transform (CWT) is a powerful tool to aid in their elucidation. Five case studies are presented involving many of these phenomena in which the CWT was key to data analysis and interpretation. A diverse set of experiments are presented involving a range of physical and temporal scales and experimental techniques. Bubble turbulent break-up is investigated using hydroacoustics, bubble dynamics and high-speed imaging; microbubbles are sized using light scattering and ultrasonic sensing, and large-scale coherent shedding driven by various mechanisms are analysed using simultaneous high-speed imaging and physical measurement techniques. The experimental set-up, aspect of cavitation being addressed, how the wavelets were applied, their advantages over other techniques and key findings are presented for each case study.This paper is part of the theme issue 'Redundancy rules: the continuous wavelet transform comes of age'.

Numerical study of gravity effects on phase separation in a swirl chamber.

The effects of gravity on a phase separator are studied numerically using an Eulerian/Lagrangian two-phase flow approach. The separator utilizes high intensity swirl to separate bubbles from the liquid. The two-phase flow enters tangentially a cylindrical swirl chamber and rotate around the cylinder axis. On earth, as the bubbles are captured by the vortex formed inside the swirl chamber due to the centripetal force, they also experience the buoyancy force due to gravity. In a reduced or zero gravity environment buoyancy is reduced or inexistent and capture of the bubbles by the vortex is modified. The present numerical simulations enable study of the relative importance of the acceleration of gravity on the bubble capture by the swirl flow in the separator. In absence of gravity, the bubbles get stratified depending on their sizes, with the larger bubbles entering the core region earlier than the smaller ones. However, in presence of gravity, stratification is more complex as the two acceleration fields - due to gravity and to rotation - compete or combine during the bubble capture.

Theoretical study on the movements of bubbles.

The radial and translational motions of multiple interacting spherical bubbles are obtained using classical Newton mechanics. It is seen that bubbles not only move in straight line, but also in circular motion. The tracks of the bubbles show that the interactions among them include attractive, repulsive and dynamic equilibrium. There are three types of straight line corresponding to attraction, coexistence of attraction and repulsion and dynamic equilibrium, and two types of circular movement corresponding to attraction and dynamic equilibrium. The results can provide an explanation for cavitation chain and profile in cavitation field.

A new model for bubble cluster dynamics in a viscoelastic media.

Bubble cluster dynamics in viscoelastic media is instructive for ultrasound diagnosis and therapy. In this paper, we propose a statistical model for bubble cluster dynamics in viscoelastic media considering the radius distribution of bubble nuclei. By investigating and comparing the response for a bubble in three conditions: single bubble; multi-bubble with the same radius; multi-bubble with different radius, the following rules are found: The promotion or suppression of the bubble cluster on the bubble vibration is not monotonous with the increase of the number of bubbles. The promotion or suppression of the bubble cluster on the bubble vibration varies alternately with the frequency. The effect of bubble cluster on bubble vibration is mostly suppressed when the driving acoustic pressure amplitude pa is high (5000 kPa). Usually, the bubble cluster promotes the vibration of the large bubbles (R0 = 10 µm) more, or suppresses it less.

Ammonia production by microbubbles: A theoretical analysis of achievable energy intensity.

The present paper studies the energy intensity of ammonia production by a freely oscillating microbubble placed in an infinite domain of liquid. The initial content of the bubble is a mixture of hydrogen and nitrogen. The bubble is expanded isothermically to a maximum radius, then it is "released" and oscillates freely. The input energy is composed of the potential energy of the bubble at the maximum radius, the energy required to produce hydrogen, and the pumping work in case a vacuum is employed. The chemical yield is computed by solving the underlying governing equations: the Keller-Miksis equation for the radial dynamics, the first law of thermodynamics for the internal temperature and the reaction mechanism for the evolution of the concentration of the chemical species. The control parameters during the simulations are the equilibrium bubble size, initial expansion ratio, ambient pressure, the initial concentration ratio of hydrogen and the material properties of the liquid. At the optimal parameter setup, the energy intensity is 90.17GJ/t that is 2.31 times higher than the best available technology, the Haber-Bosch process. In both cases, the hydrogen is generated via water electrolysis.

Compressible effect on the mutual interaction of two cavitation bubbles in radial oscillations and translational motion.

In this study, we considered the compressible effect on the mutual interaction of two cavitation bubbles by correcting the sound field emitted by one bubble in the radial equations of the other bubble to first order in the Mach number of the flow, and the effect is represented by the incident wave acting on bubbles. The results illustrates that the incident wave can enhance the resonance response at the redistributed resonance frequency, which leads to an increase in radial acceleration and the secondary Bjerknes force, and rapid approach of bubbles. Furthermore, the influence of incident wave on the interaction of bubbles driven at lower frequencies is more significant, due to resonance enhancement caused by the proximity of natural frequencies and frequency multiplications of the external sound field. Our findings reveal that the compressible effect is not only critical to interaction in radial oscillations, but also in translational motion.

Dynamics of cavitation bubbles in viscous liquids in a tube during a transient process.

We experimentally, numerically, and theoretically investigate the dynamics of cavitation bubbles in viscous liquids in a tube during a transient process. In experiments, cavitation bubbles are generated by a modified tube-arrest setup, and the bubble evolution is captured with high-speed imaging. Numerical simulations using OpenFOAM are employed to validate our quasi-one-dimensional theoretical model, which effectively characterizes the bubble dynamics. We find that cavitation onset is minimally affected by the liquid viscosity. However, once cavitation occurs, various aspects of bubble dynamics, such as the maximum bubble length, bubble lifetime, collapse time, and collapse speed, are closely related to the liquid viscosity. We further establish that normalized bubble dynamics are solely determined by the combination of the Reynolds number and the Euler number. Moreover, we also propose a new dimensionless number, Ca2, to predict the maximum bubble length, a critical factor in determining the occurrence of liquid column separation.

An experimentally validated cavitation inception model for spring-driven autoinjectors.

Cavitation, the formation and collapse of vapor-filled bubbles, poses a problem in spring-driven autoinjectors (AIs). It occurs when the syringe accelerates abruptly during activation, causing pressure fluctuations within the liquid. These bubbles expand and then collapse, generating shock waves that can harm both the device and the drug molecules. This issue stems from the syringe's sudden acceleration when the driving rod hits the plunger. To better understand cavitation in AIs, we explore how design factors like drive spring force, air gap size, and fluid viscosity affect its likelihood and severity. We use a dynamic model for spring-driven autoinjectors to predict and analyze the factors contributing to cavitation initiation and severity. This model predicts the motion of AI components, such as the displacement and velocity of the syringe barrel, and allows us to investigate pressure wave propagation and the subsequent dynamics of cavitation under various operating conditions. We investigated different air gap heights (from 1 to 4 mm), drive spring forces (from 8 to 30 N), and drug solution viscosities (from 1 to 18 cp) to assess cavitation inception based on operational parameters. Results reveal that AI dynamics and cavitation onset and severity strongly depend upon AI operating parameters, namely drive spring force and air gap height. The maximum syringe acceleration increases with spring stiffness and decreases with air gap height; increases in air gap height prolong the time interval of syringe acceleration but diminish the maximum syringe acceleration. From actuation to injection, air gap pressure peaks twice, first due to impact with the rod/plunger and secondly due to the deacceleration event upon injection. The maximum air gap pressure increases with spring stiffness and decreases with air gap height. Results show that maximum cavitation bubble radii and collapse-driven extension rates occur with higher driver spring forces, smaller air gap heights, and less viscous solutions. A cavitation criterion is developed for cavitation in autoinjectors that concludes that cavitation in autoinjectors depends on the peak syringe acceleration.

Ultrasound for the degradation of endocrine disrupting compounds in aqueous solution: A review on mechanisms, influence of operating parameters and cost estimation.

Availability of drinking water is one of the basic humanitarian goals but remains as a grand challenge that the world is facing today. Currently, water bodies are contaminated not only with conventional pollutants but also with numerous recalcitrant pollutants, such as PPCPs, endocrine disrupting compounds, etc. These emerging pollutants require special attention because of their toxicity to living organisms, bio-resistant and can sustain even after primary and secondary treatments of wastewater. Among different treatment technologies, sonolysis is found to be an innovative and promising technique for the treatment of emerging pollutants present in aqueous solution. Sonolysis is the use of ultrasound to enhance or alter chemical reactions by the formation of free radicals and shock waves which ultimately helps in degradation of pollutants. This review summarizes several studies in the sonochemical literature, including mechanisms of sonochemical process, physical and chemical effects of ultrasound, and the influence of several process variables such as ultrasound frequency, power density, temperature and pH of the medium on degradation performance for endocrine disrupting compounds. In addition, this review highlighted techno-economic perspectives focusing on the total cost required for translating the ultrasound-based processes on a large scale. Overall, the objective of this study is to exhibit a critical review of information available in the literature to encourage and promote future research on sonolysis for the degradation of Endocrine Disrupting Compounds (EDCs).

Jet dynamics of a cavitation bubble near unequal-sized dual particles.

The jet dynamics of a cavitation bubble near unequal-sized dual particles is investigated employing OpenFOAM, and the effects of the jets on the particles are quantitatively analyzed in terms of their pressure impacts. Different from single-particle cases, the necks that evolve between dual particles are closely linked to the formation mechanism of the jets. Based on the simulation results, the jet dynamics can be divided into five scenarios: (1) the contraction of the annular depression produced by the collision of the two necks causes the bubble to split into two daughter bubbles and generates a single jet inside each daughter bubble; (2) the annular depression impacts the particle, leading to the bubble to fracture and producing a single jet inside a daughter bubble; (3) the bubble is split by a single neck constriction and produces a single jet; (4) the bubble is split by a single neck constriction and generates two jets; and (5) the bubble is split by the contraction of two necks and produces four jets together with three daughter bubbles. As the bubble-particle distance or the radius ratio of the dual particles increases, the maximum force on the small particle generated by the bubble decreases.

Investigation of mechanisms of shock wave generation by collapse of cavitation bubbles near particles.

The mechanism of generation of shock waves by the collapse of a cavitation bubble near a single particle or dual particles is numerically investigated using OpenFOAM. For the single-particle case, shock waves during bubble inception and jet impacting on the particle surface are revealed in detail. The pressure induced on the particle by the inception shock wave of the bubble decreases with increasing bubble-particle distance, and the pressure is proportional to 1/r1.26 (r being the distance from the center of the shock wave). For the dual particles, the evolution of the neck structure is closely related to the generation mechanism of the shock wave. At extremely close particle-bubble distances, two shock waves propagating in opposite directions are emitted outside and inside the bubble after two necks collide. At long particle-bubble distances, a shock wave is emitted after the neck contracts, and simultaneously the bubble splits into two daughter bubbles. The energy of the shock wave generated by the neck constriction (i.e., the pressure at its generation location) first increases and then decreases with increasing bubble-particle distance. For unequal-sized double particles, the size of the daughter bubble depends on the bubble-particle distance and the particle size. These findings provide new perspectives for understanding the damage sustained by hydro-mechanical components operating in sand-laden water flows.

Microbubble-enhanced transcranial MR-guided focused ultrasound brain hyperthermia: heating mechanism investigation using finite element method.

Recently, our group developed a synergistic brain drug delivery method to achieve simultaneous transcranial hyperthermia and localized blood-brain barrier opening via MR-guided focused ultrasound (MRgFUS). In a rodent model, we demonstrated that the ultrasound power required for transcranial MRgFUS hyperthermia was significantly reduced by injecting microbubbles (MBs). However, the specific mechanisms underlying the power reduction caused by MBs remain unclear. The present study aims to elucidate the mechanisms of MB-enhanced transcranial MRgFUS hyperthermia through numerical studies using the finite element method. The microbubble acoustic emission (MAE) and the viscous dissipation (VD) were hypothesized to be the specific mechanisms. Acoustic wave propagation was used to model the FUS propagation in the brain tissue, and a bubble dynamics equation for describing the dynamics of MBs with small shell thickness was used to model the MB oscillation under FUS exposures. A modified bioheat transfer equation was used to model the temperature in the rodent brain with different heat sources. A theoretical model was used to estimate the bubble shell's surface tension, elasticity, and viscosity losses. The simulation reveals that MAE and VD caused a 40.5% and 52.3% additional temperature rise, respectively. Compared with FUS only, MBs caused a 64.0% temperature increase, which is consistent with our previous animal experiments. Our investigation showed that MAE and VD are the main mechanisms of MB-enhanced transcranial MRgFUS hyperthermia.

Removal of surface-attached micro- and nanobubbles by ultrasonic cavitation in microfluidics.

Surface-attached micro- and nanobubbles are known for their resistance to external forces. This study experimentally and theoretically investigates their response to strong ultrasonic fields. Surface-attached micro- and nanobubbles with contact radii from 2 µm to 20 µm are generated in a microchannel and exposed to ultrasound through a vibrating glass substrate. At a driving frequency over 200 kHz up to 2 MHz tested, no significant response from the micro- and nanobubbles is observed. By contrast, at 100 kHz-200 kHz, ultrasonic cavitation bubbles appear in the microchannel and migrate toward the surface micro- and nanobubbles. Then the surface micro- and nanobubbles merge with the ultrasonic cavitation bubbles, detach from the substrate, and become free gaseous nuclei susceptible to further cavitation. Notably, the removal process leaves no observable residue. Theoretical analysis suggests that the directional migration of cavitation bubbles is driven by mutual acoustic radiation forces. This work demonstrates that ultrasonic fields can effectively remove surface micro- and nanobubbles, transforming them into free gaseous cavitation nuclei.

Investigation on the effects of an elliptical wall on the dynamic behaviors of a bubble restricted by two parallel plates.

The present paper investigates the dynamic behaviors of a bubble restricted by two parallel plates near an elliptical wall. The typical experimental phenomena of the bubble are recorded employing the high-speed photography and a theoretical Kelvin impulse model is established. The impacts of the spatial position and the curvature of the wall on the bubble collapse behaviors are quantitatively investigated through the theoretical model and verified against the experimental results. The Kelvin impulse intensity and the direction during the bubble collapse process are compared and discussed for different elliptical-shaped walls. The main conclusions include: (1) During the bubble collapse process, the phenomenon of the bubble uneven splitting is discovered. (2) At different spatial positions and wall curvatures, the bubble collapse jet angle, movement distance, and velocity are in good agreement with the theoretical Kelvin impulse predictions. (3) As the short-to-long axis ratio increases, the differences in the distributions of the Kelvin impulse intensity and the direction near the elliptical wall gradually become larger, and the range of the influence of the impulse intensity expands.

Numerical investigation of acoustic cavitation characteristics of a single gas-vapor bubble in soft tissue under dual-frequency ultrasound.

The viscoelastic tissue under dual-frequency ultrasound excitation affects the acoustic cavitation of a single gas-vapor bubble. To investigate the effect of the cavitation dynamics, the Gilmore-Akulichev-Zener (GAZ) model is coupled with the Peng-Robinson equation of state (PR EOS). Results indicate that the GAZ-PR EOS model can accurately estimate the bubble dynamics by comparing with the Gilmore PR EOS and GAZ-Van der Waals (VDW) EOS model. Furthermore, the acoustic cavitation effect in different viscoelastic tissues is investigated, including the radial stress at the bubble wall, the temperature, pressure, and the number of water molecules inside the bubble. Results show that the creep recovery and the relaxation of the stress caused by viscoelasticity can affect the acoustic cavitation of the bubble, which could inhibit the bubble's expansion and reduce the internal temperature and pressure within the bubble. Moreover, the effect of dual-frequency ultrasound on the cavitation of single gas-vapor bubbles is studied. Results suggest that dual-frequency ultrasound could increase the internal temperature of bubbles, the internal pressure of bubbles, and the radial stress at the bubble wall. More importantly, there is a specific optimal combination of frequencies for particular viscoelasticity by exploring the impact of different dual-frequency ultrasound combinations and tissue viscoelasticity on the acoustic cavitation of a single gas-vapor bubble. In conclusion, this study helps to provide theoretical guidance for dual-frequency ultrasound to improve acoustic chemical and mechanical effects, and further optimize its application in acoustic sonochemistry and ultrasound therapy.