Cavitation Bubbles In Water Research Articles

The collapse of a sonoluminescent cavitation bubble imaged with X-ray free-electron laser pulses

Single bubble sonoluminescence (SBSL) is the phenomenon of synchronous light emission due to the violent collapse of a single spherical bubble in a liquid, driven by an ultrasonic field. During the bubble collapse, matter inside the bubble reaches extreme conditions of several gigapascals and temperatures on the order of 10000 K, leading to picosecond flashes of visible light. To this day, details regarding the energy focusing mechanism rely on simulations due to the fast dynamics of the bubble collapse and spatial scales below the optical resolution limit. In this work we present phase-contrast holographic imaging with single x-ray free-electron laser (XFEL) pulses of a SBSL cavitation bubble in water. X-rays probe the electron density structure and by that provide a uniquely new view on the bubble interior and its collapse dynamics. The involved fast time-scales are accessed by sub-100 fs XFEL pulses and a custom synchronization scheme for the bubble oscillator. We find that during the whole oscillation cycle the bubble’s density profile can be well described by a simple step-like structure, with the radius R following the dynamics of the Gilmore model. The quantitatively measured internal density and width of the boundary layer exhibit a large variance. Smallest reconstructed bubble sizes reach down to R≃0.8μm , and are consistent with spherical symmetry. While we here achieved a spatial resolution of a few 100 nm, the visibility of the bubble and its internal structure is limited by the total x-ray phase shift which can be scaled with experimental parameters.

3D model for inertial cavitation bubble dynamics in binary immiscible fluids

The physics of cavitation bubbles in water has been extensively studied for over a century. However, our understanding of bubble dynamics in binary immiscible fluids, such as a water-oil system, is limited. These systems have recently gained a lot of interest due to their relevance in modern medical treatment, emulsification, and the food industry. In this study, we establish an accurate and inexpensive three-dimensional boundary integral (BI) model for inertial cavitation bubble dynamics in binary immiscible fluid systems. Our novel scheme for solving velocities on the flow boundaries, expressed in a matrix form, can be easily extended to complex situations where multiple bubbles are generated in different phases. Additionally, we employ a density potential method and a weighted moving least-squares method to maintain a high level of mesh regularity. Our results demonstrate that the proposed 3D model is comparable in accuracy to a 3D axisymmetric model by comparing it against analytical solutions and the results obtained from an axisymmetric model. Furthermore, we compare our numerical simulations against a purposely conducted experiment for the bubble-droplet interaction, and excellent agreement is achieved. For the first time, we present simulation results of two-bubble interactions with an initially flat fluid-fluid interface and inside a droplet surrounded by a second fluid. We also reveal the dependences of the two-bubble morphologies, flow patterns, and jet velocities on the density ratio between the two phases. Finally, we find a significant difference in the mechanism of fluid mixing by multiple bubbles compared to that of a single bubble case.

Early dynamics of laser-induced plasma and cavitation bubble in water

The knowledge of early laser-induced plasma and cavitation bubble dynamics are of great importance for a better understanding of underwater laser-induced breakdown spectroscopy. In this work, we investigated the temporal evolution of the plasma and bubble at an early stage before 2 μs, by using fast imaging and shadowgraph techniques. It showed that the initial plasma forms at the laser focal point and then grows up toward the incident laser. As time evolves, the plasma expands simultaneously forward and backward to its peripheral region and a sub-plasma structure can be observed at low laser energies. The plasma has a good pulse-to-pulse repeatability during the laser pulse, while soon after the laser pulse (from 20 to 50 ns), it undergoes a severe pulse-to-pulse fluctuation with an inhomogeneous emission distribution. At longer times, the plasma becomes much stable again. To clarify the interrelationship between the bubble dynamics and the plasma characteristics, shadowgraph images of the bubble were shown and compared with the plasma images. It showed that the morphology of the bubble and its evolution have a good consistency with the morphology of plasma. The bubble radius as a function of time can be well expressed by the equation R=at0.4, and a is a constant related to the laser energy. By inspecting the evolution curve of the bubble wall and the bubble pressure calculated based on the Gilmore model, a transition phase from 20 to 50 ns can be identified between the moving breakdown phase and the thermal expansion phase. In this transition phase, both the bubble expansion speed and the bubble pressure drop drastically that may account for the severe plasma fluctuations as observed in this period. These results of early bubble dynamics provide insights into the critical role of bubble-plasma interaction on the characterization of laser-induced plasma in water.

Numerical Simulation of Collapse and Rebound of a Cavitation Bubble in Water

Numerical Simulation of Collapse and Rebound of a Cavitation Bubble in Water

Cavitation-induced shock wave behaviour in different liquids

This paper follows our earlier work where a strong high frequency pressure peak has been observed as a consequence of the formation of shock waves due to the collapse of cavitation bubbles in water, excited by an ultrasonic source at 24 kHz. We study here the effects of liquid physical properties on the shock wave characteristics by replacing water as the medium successively with ethanol, glycerol and finally a 1:1 ethanol–water solution. The pressure frequency spectra obtained in our experiments (from more than 1.5 million cavitation collapsing events) show that the expected prominent shockwave pressure peak was barely detected for ethanol and glycerol, particularly at low input powers, but was consistently observed for the 1:1 ethanol–water solution as well as in water, with a slight shift in peak frequency for the solution. We also report two distinct features of shock waves in raising the frequency peak at MHz (inherent) and contributing to the raising of sub-harmonics (periodic). Empirically constructed acoustic pressure maps revealed significantly higher overall pressure amplitudes for the ethanol–water solution than for other liquids. Furthermore, a qualitative analysis revealed that mist-like patterns are developed in ethanol–water solution leading to higher pressures.

Chemical-free exfoliation of hexagonal boron nitride via cavitation-bubble plasma in water

Hexagonal boron nitride (h-BN) sheets have numerous applications, but current low-cost, environmentally friendly techniques for h-BN exfoliation yield undesirably small sheet sizes. We have developed a novel chemical-free method to prepare large h-BN nanosheets (BNNSs) by the exfoliation of h-BN using plasma with cavitation bubbles in water, named here as cavitation-bubble plasma. Unlike ordinary liquid exfoliation methods, which reduce the sheet size of BNNSs compared with that of the starting h-BN, the cavitation-bubble plasma process exfoliated h-BN without reducing the sheet size. Furthermore, the plasma dramatically improved the exfoliation yield.

Comprehensive analysis of spherical bubble oscillations and shock wave emission in laser-induced cavitation

The dynamics of spherical laser-induced cavitation bubbles in water is investigated by plasma photography, time-resolved shadowgraphs and sensitive single-shot probe beam scattering that portrays the transition from initial nonlinear to late linear oscillations. The frequency of late oscillations yields the bubble's gas content. Numerical simulations with an extended Gilmore model using plasma size as input and oscillation times as fit parameter provide insights into experimentally not accessible bubble parameters and shock wave emission. Model extensions include a term covering the initial shock-driven acceleration of the bubble wall, an automated method determining shock front position and pressure decay and a complete energy balance for the partitioning of absorbed laser energy into vaporization, bubble and shock wave energy and dissipation through viscosity and condensation. These tools are used for analysing a scattering signal covering 102 oscillation cycles from a bubble with 36 μm maximum radius produced by a plasma with 1550 K average temperature. Predicted bubble wall velocities during expansion agree well with experimental data. Upon first collapse, most energy was stored in the compressed liquid around the bubble and radiated away acoustically. The collapsed bubble contained more vapour than gas and had a pressure of 13.5 GPa. The decay of the rebound shock wave pressure with radius r was initially $\mathrm{\ \propto }{r^{ - 1.8}}$ , and energy dissipation at the shock front heated the liquid near the bubble wall to temperatures above the superheat limit. The shock-induced temperature rise reduces damping during late bubble oscillations. Damping in first collapse increases significantly for small bubbles with less than 10 μm radius.

Astigmatism and spherical aberrations as main causes for degradation of ultrafast laser-induced cavitation in water

The generation of cavitation bubbles in water by focused laser pulses has been thoroughly studied, particularly due to the wide range of applications in biomedical fields, in particular for ophthalmic laser surgery. The presence of optical aberrations in any optical system deteriorates the precision of light focusing and may reduce the laser interaction efficiency in transparent media. In this paper, we analyze the influence of several controlled optical geometrical aberrations in the formation and dynamics of cavitation bubbles in water caused by laser-induced optical breakdown using femtosecond laser pulses. The maximum cavitation bubble diameter reduces with increasing aberrations, showing a drop of the energy coupling efficiency, ultimately leading to the absence of cavitation bubble. The study reveals that the secondary astigmatism and spherical aberrations degrade more rapidly the beam concentration than the other aberrations studied here (namely astigmatism first and second, coma, spherical and trefoil). The cavitation energy threshold becomes equally increased. This observation relates very well to numerical simulations evaluating the laser fluence at and around the focusing plane in the presence of aberrations. These results open the door to more controlled ultrafast laser assisted surgeries and processing in transparent media.

Acoustic cavitation model based on a novel reduced order gas pressure law

The thermal behavior of a spherical gas bubble in a liquid excited by an acoustic pressure signal is investigated by constructing an iterative solution of the energy balance equations between the gas bubble and the surrounding liquid in the uniform pressure approximation. This iterative solution leads to hierarchy equations for the radial partial derivatives of the temperature at the bubble wall, which control the temporal rate of change of the gas pressure and gas temperature within the bubble. In particular, a closure relation for the hierarchy equations is introduced based on the ansatz that approximates the rapid change of state during the collapse of the bubble from almost isothermal to almost adiabatic behavior by time averaging the complex dynamics of change of state over a relatively short characteristic time. This, in turn, leads to the desired reduced order gas pressure law exhibiting power law dependence on the bubble wall temperature and on the bubble radius, with the polytropic index depending on the isentropic exponent of the gas and on a parameter that is a function of the Péclet number and a characteristic time scale. Results of the linear theory for gas bubbles are recovered by identifying this parameter as a function of the Péclet number based on the Minnaert frequency. The novel gas pressure law is then validated against the near-isothermal solution and against the results of the numerical simulations of the original energy balance equations for large amplitude oscillations using spectral methods. Consequently, an acoustic cavitation model that accounts for phase change but that neglects mass diffusion is constructed by employing the reduced order gas pressure law together with the Plesset–Zwick solution for the bubble wall temperature and the Keller–Miksis equation for spherical bubble dynamics. Results obtained using variable interface properties for acoustically driven cavitation bubbles in water show that the time variations of the bubble radius and the bubble wall temperature lie between those obtained by the isothermal and adiabatic laws depending on the value of the Péclet number and the characteristic time scale.

Outgoing shock waves at collapse of a cavitation bubble in water

Formation and propagation of a shock pulse arising at collapse of a cavitation bubble in water are numerically studied. The initial bubble radius R0 is varied in the range 1–4 mm, the liquid pressure p∞ in the range 1–20 bar, and the liquid temperature T∞ in the range 20–80 °C. The dynamics of the vapor in the bubble and the surrounding liquid is governed by the equations of gas dynamics with allowing for the heat conductivity, heat and mass exchange on the bubble surface. Wide-range equations of state are applied. The numerical technique is based on the Godunov method on moving grids thickened to the bubble surface. A discontinuous approximation of the numerical shock pulse in the vicinity of its front part is used. The case of R0=1 mm, p∞=1 bar, T∞=20 °C is considered as a reference one. It is revealed that the influence of R0 in terms of the dimensionless distance from the collapse center r/R0 is not large and is mainly realized during the shock pulse formation. The variation in p∞ mostly leads to quantitative changes of the pulse characteristics. In particular, the maximum rate of decrease in the pulse pressure peak grows with r (the distance to the collapse center) from that of a regression proportional to r−3/2 to that of a regression according to r−4/3. With rising T∞, the pulse gets weaker and becomes pronounced at larger distance from the collapse center. With increasing T∞ to about 45 °C at R0=1.92 mm and p∞=1 bar, the pulse turns out to be shockless. With the increase of T∞ to approximately 40 °C, the maximum rate of decay in the pulse pressure amplitude is reduced to that of a regression proportional to r−1. It is shown that the obtained results are in good agreement with known experimental data.

Pump-probe X-ray holographic imaging of laser-induced cavitation bubbles with femtosecond FEL pulses

Cavitation bubbles can be seeded from a plasma following optical breakdown, by focusing an intense laser in water. The fast dynamics are associated with extreme states of gas and liquid, especially in the nascent state. This offers a unique setting to probe water and water vapor far-from equilibrium. However, current optical techniques cannot quantify these early states due to contrast and resolution limitations. X-ray holography with single X-ray free-electron laser pulses has now enabled a quasi-instantaneous high resolution structural probe with contrast proportional to the electron density of the object. In this work, we demonstrate cone-beam holographic flash imaging of laser-induced cavitation bubbles in water with nanofocused X-ray free-electron laser pulses. We quantify the spatial and temporal pressure distribution of the shockwave surrounding the expanding cavitation bubble at time delays shortly after seeding and compare the results to numerical simulations.

Nanosecond timing and synchronization scheme for holographic pump-probe studies at the MID instrument at European XFEL.

Single-pulse holographic imaging at XFEL sources with 1012 photons delivered in pulses shorter than 100 fs reveal new quantitative insights into fast phenomena. Here, a timing and synchronization scheme for stroboscopic imaging and quantitative analysis of fast phenomena on time scales (sub-ns) andlength-scales (≲100 nm) inaccessible by visible light is reported. A fully electronic delay-and-trigger system has been implemented at the MID station at the European XFEL, and applied to the study of emerging laser-driven cavitation bubbles in water. Synchronization and timing precision have been characterized to be better than 1 ns.

Impact of bubble coalescence in the determination of bubble sizes using a pulsed US technique: Part 2 – Effect of the nature of saturating gas

Knowledge on cavitation bubble size distribution, ambient radius of bubbles is of interest for many applications that include therapeutic and diagnostic medicine. It however becomes a hard quest when increasing the ultrasonic frequency, when direct observation of bubble dynamics is no longer possible. An indirect method based on the estimation of the bubble dissolution time under pulsed ultrasound (362 kHz) is used here under optimized conditions to derive ambient radii of cavitation bubbles in water saturated with He, Ar, Xe, O2, N2 and air: 3.0 µm for Ar, 1.2 µm for He, 3.1 µm for Xe, 2.8 µm for O2, around 1 µm for N2 and air. If the pulse on-time is increased, bubble coalescence occurs, the extent of which is rather limited for Ar but extremely high for He or N2.

Capturing the final stage of the collapse of cavitation bubbles generated during nanosecond laser ablation of submerged targets

We observe and study the growth and collapse of cavitation bubbles generated during the pulsed laser ablation of submerged targets in three liquid environments, water, ethanol, and n-butanol, utilizing a stroboscopic shadowgraphy system. The transient images of the final stage of bubble collapse when the bubble volume is at a minimum are captured. Ethanol and n-butanol are subject to cavitation-induced reactions, which produce gaseous products that hinder the implosive collapse of the bubbles. In contrast, laser-produced cavitation bubbles in water achieve the smallest radii at the final stage of collapse, resulting in a sharper increase in localized temperature and pressure that can drive a potential second etching. Our results emphasize the role of the chemical effects of liquids on the implosive collapse of cavitation bubbles, providing a deeper understanding of cavitation-related phenomena and liquid-assisted laser processing.

Evaluation of Thermal and Acoustic Energy during Collapse of Cavitation Bubbles

The results of a numerical study of the thermal and acoustic energies released during the collapse of a single spherical cavitation bubble in water at a pressure of 10 bar and a temperature of 20°С are given. In the model used, we take into account the thermal conductivity of the vapor in the bubble and the surrounding liquid, heat transfer, evaporation/condensation on the surface of the bubble, and the fluid compressibility. The conversion of mechanical energy into heat due to the fluid viscosity is not accounted for. When the bubble collapses, the energy of acoustic radiation due to radial pulsations of the bubble is shown to be approximately nine times greater than the energy spent on heating the liquid. The value of this energy is proportional to the cube of the initial bubble radius.

Signal Improvement for Underwater Measurement of Metal Samples Using Collinear Long-Short Double-Pulse Laser Induced Breakdown Spectroscopy

A collinear long-short double-pulse laser induced breakdown spectroscopy (LS-DP-LIBS) is employed to detect the underwater metal samples. The emission spectra, time-resolved signal, plasma images and sound characteristics of plasma shockwave are experimentally investigated in this work. The results show that the underwater signal of Al, Cu and Fe spectral lines are significantly improved by collinear LS-DP-LIBS with inter-pulse delay of 35 us. The mechanism of the signal improvement is considered to be the pre irradiation effect of long pulse laser beam. In the collinear LS-DP-LIBS method, the long pulse firstly generates a cavitation bubble in water and provides the gaseous environment. Then the short pulse generates the plasma from sample surface. The present experiments show that the collinear LS-DP-LIBS method has acquired significantly signal improvement in underwater measurement of metal samples. This new method has great potential in deep-sea exploration using LIBS.

Influence of ultrasound on the concentration of hydrogen ions in water

The paper deals with the physical and chemical effects arising in water under the action of the energy of elastic vibrations of ultrasonic frequency in the cavitation mode. Cavitation state is characterized by the formation of the gas and air cavities - bubbles in the aquatic environment. The theory of occurrence of high temperatures at the moment of collapse of a cavitation bubble in water is given. It is noted that there is a dissimilation of the substance of free active radicals, dissociated and ionized molecules. The kinetics of chemical reactions in the ultrasonic field under the cavitation regime is described. Methods for obtaining ultrasonic cavitation in a liquid medium with a description of the generation of ultrasonic vibrations and a list of transducers and types of emitters are given. A relatively wide spectrum of radiated frequencies and instability of frequency and amplitude of oscillations are considered. The choice of ultrasonic generator is justified due to the necessary frequencies and intensities for the excitation of cavitation in water (f = 18-22 kHz, I = 2.0 W/cm2), technological considerations about the possibility of mounting the transducer in an existing production line, as well as safety and efficiency issues. The relevance, purpose and objectives of the work are formulated. The equipment and methods of experimental research are presented. Laboratory experimental studies on the effect of ultrasonic vibrations in the cavitation mode on the change of water pH were carried out. It is shown that bubbles formed in the ultrasonic field in the studied water collapse and create micro-explosions, shock waves and micro-flows. The results of studies on the change of pH in water are shown and the analysis of experimental data is performed. The experimental studies made it possible to substantiate the conclusions of this work and to formulate a conclusion about the influence of the ultrasonic field in the cavitation mode on the pH of the aqueous medium and its further application in water treatment processes.

Liquid Heating During the Collapse of a Single Cavitation Bubble

This paper presents the results of a numerical study of the liquid heating process during collapse of a single spherical cavitation bubble in water at a liquid pressure of 10 bar, temperature of 20°C, and an initial bubble radius of 500 μm. The simulation of this phenomenon took into account the thermal conductivity of the vapor in the bubble and the surrounding liquid, heat transfer and evaporation/condensation on the surface of the bubble, and the effects of viscosity and compressibility of the liquid. It is shown that, as a result of bubble collapse, the liquid heats up in a region of about 60 μm radius. The temperature in the center of this region is about 50°C higher than in the surrounding liquid. The thermal energy expended on heating the liquid in this region is approximately equal to 25 μJ.

Opto-acoustic effect of Au nanoparticles in water under irradiation of pulse laser

The cavitation bubbles in water generated by laser irradiated Au nanoparticles (NPs) exhibit potential applications in the photothermal therapy and enhancing photoacoustic imaging. The exploration of the related processes is crucial to improve the opto-acoustic efficiency that is important to these applications. Herein a model including the Mie theory, gas state equation, Rayleigh-Plesset equation, and sound pressure equation is established. Through this model, the optimal diameter of an Au NP, the threshold value of the laser induced cavitation, the time varying volume and pressure of the bubble is obtained. The influences of laser flux and NP diameter to the volume and pressure of the bubble are simulated based on this model. The acoustic pressure of many Au NPs is obtained through the superposition of the opto-acoustic wave of a single Au NP, in which the Au NPs are assumed to be distributed randomly in the laser irradiation area. The opto-acoustic wave of many Au NPs is also investigated experimentally, which coincides with the theoretical analyses.

High-speed film-thickness measurements between a collapsing cavitation bubble and a solid surface with total internal reflection shadowmetry

The time evolution of the liquid-film thickness of a single cavitation bubble in water collapsing onto a solid surface is measured. To this end, total internal reflection (TIR) shadowmetry is developed, a technique based on TIR and the imaging of shadows of an optical structure on a polished glass surface. The measurements are performed at frame rates up to 480 kHz. Simultaneous high-speed imaging of the bubble shape at up to 89 kHz allows relating the evolution of the film thickness to the bubble dynamics. With a typical maximum bubble radius of 410 µm, we varied the nondimensional stand-off distance γ from 0.47 to 1.07. We find that during the first collapse phase, the bubble does not come in direct contact with the solid surface. Instead, when the bubble collapses, the jet impacts on a liquid film that always resides between the bubble and solid. At jet impact, it is 5–40 µm thick, depending on γ. Also, during rebound, at any given point in time, most or all of the then overall toroidal bubble is not in contact with the solid surface.