Sonoluminescence

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.     

水中针-板结构小能量脉冲火花放电特性北大核心CSCD

开展了J量级系统储能下电脉冲参数对水中火花放电特性影响研究。驱动源采用参数可调的固态重频纳秒脉冲电源,放电负载为水中针-板结构(间距1 mm),在低重频条件(约5 Hz)下进行实验。通过调节放电参数、拍摄高速阴影图像、光谱诊断以及声信号测量,研究水中脉冲放电的物理特性,得到不同放电参数下放电演化规律及其对声学、光谱特性影响。实验发现:在J量级储能下,放电通道连通两极后,回路电流在几百ns内快速上升至10 A左右,随后缓慢下降,持续50~60μs。发现预设脉宽对放电影响较大,短脉宽条件下放电会被电源固态开关强制截断出现反向放电,而长脉宽条件下放电通道在后期变得不稳定甚至熄弧中断,出现气泡中二次放电现象。辐射光谱揭示了更多等离子体信息,推断通道电子密度在1018 cm-3量级,随着脉宽增加,特征谱线强度增加,表明活性粒子数密度增加,但粒子种类不变。短脉冲(<150μs)作用下产生的脉冲声波的特征宽度在110~150μs,而当脉宽继续增大,声波脉宽并不继续增加而是保持不变,保持在150μs左右。研究结果对水中小能量火花放电的机理研究有一定参考价值,为水声学、液相等离子体等领域的应用提供思路。     

Evolution of disturbances of the sphericity of a bubble under strong compression

The growth in the amplitude of the small nonsphericity of cavitation bubbles in acetone and water under strong compression is considered. A hydrodynamic model is used in which the compressibility of the liquid, the nonstationary thermal conduction of the vapor and the liquid, and nonequilibrium evaporation–condensation processes, as well as imperfection of the vapor, are considered. It has been shown that the increase in the amplitude of the small nonsphericity of cavitation bubbles in the form of separate spherical harmonics during compression in water is substantially (more than 10 times) higher than during compression in acetone. This indicates that acetone is much more advantageous over water to implement a process of the nearer-to-spherical extreme compression of the medium in the cavitation bubbles.     

Stationary soliton waves in a liquid with heat-conducting gas bubbles

Formation of an undamped soliton wave in the process of propagation of sound perturbations in a liquid with uniformly distributed gas bubbles is first revealed. The stationary soliton wave exists only in the presence of two pairs of competing factors, one of which is a balance between the nonlinearity of the medium and the linear wave dispersion, and another is a balance between the heat inflow into and outflow from the gas phase. The thermodynamic convertibility of dissipation of gas bubbles in the process of soliton wave propagation is revealed. The Korteweg-de Vries equation is derived for adiabatic bubbles. It differs radically from the Korteweg-de Vries equation obtained by Van Wijngaarden. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 84–88, June, 2006.     

Ultrasonic scattering cross sections of shell-encapsulated gas bubbles immersed in a viscoelastic liquid: first and second harmonics

The oscillations of gas bubbles, without shell, immersed in viscoelastic liquids and driven by an acoustic wave have been the subject of several investigations. They demonstrate that the viscosity coefficient and the spring constant of the liquid have significant influence on the scattering cross section of the gas bubble. For shell-encapsulated gas bubbles, the investigations have been concentrated to bubbles immersed in a pure viscous liquid. This present work computes the ultrasonic scattering cross section, first and second harmonics, of shell-encapsulated gas bubbles immersed in a viscoelastic liquid. The theoretical model of the bubble oscillation is based on the generalized Rayleigh-Plesset equation of motion of a spherical cavity immersed in a viscoelastic liquid represented by a three-parameter linear Oldroyd model. The scattering cross section is computed for Albunex type of bubble (shell thickness=15 nm, shell shear viscosity=1.77 Pas, shell modulus of rigidity=88.8 MPa) irradiated by a 3.5 MHz ultrasonic pressure wave with an amplitude of 30 kPa. The results demonstrate that encapsulated bubbles respond independently of the surrounding liquid being pure viscous or viscoelastic as long as the surrounding liquid shear viscosity is as low as 10(-3) Pas. Nevertheless, for higher shear viscosities, the bubble responds differently if the surrounding liquid is pure viscous or viscoelastic. In general, the scattering cross sections of first and second harmonics are larger for the viscoelastic liquid.     

Processes of dissolution and hydrate formation behind a shock wave in a liquid with carbon dioxide bubbles

The processes of dissolution and hydrate formation behind the front of a shock wave of moderate amplitude in water with carbon dioxide bubbles are studied experimentally at various initial static pressures. The influence of a surface-active substance (SAS) in the medium on the processes of dissolution and hydrate formation behind the shock wave is investigated. It is demonstrated that behind a shock wave of moderate amplitude in a liquid with carbon dioxide bubbles an intensive process of dissolution and hydrate formation takes place, resulting in complete disappearance of the gas phase in a matter of a few milliseconds. The presence of an SAS in the medium does not significantly influence the processes of dissolution and hydrate formation within the investigated periods of time.     

Modeling of pulsed spark discharge in water and its application to well cleaning

This paper presents a simulation model for the generation of strong pressure wave by means of pulsed spark discharge in water and its application to well cleaning. In the simulation model, one-dimensional time-dependent magnetohydrodynamic equations are coupled to a capacitive discharge circuit equation. A cylindrical conducting spark channel formed by electrical breakdown of water gap between a pair of electrodes is treated as a load of which resistance and inductance are allowed to change with time. For describing the spark channel properties accurately, precise calculations on thermodynamic properties and electrical conductivity are included in the simulation model. The simulation results show an excellent agreement with the experimentally measured shock pressure as well as the current and voltage waveforms. The simulation reveals that Joule heating of the spark channel during the very early phase of electrical discharge plays a key role in the formation of shock wave in water. The voltage on a capacitor at breakdown, the circuit inductance, and the resistance of the spark channel are found to be the most important parameters for the shock wave formation. With this technique, a pilot test for the cleaning of a clogged well has been performed in a water well which was constructed as a test-bed for riverbank filtration near the Anseong-cheon (river) in Korea. Well treatments have been carried out with an electrical energy of 510 J stored on a pulsed power system, at which the maximum shock pressure is measured to be around 7 MPa at the position of the well screen, i.e. 0.1 m away from the spark gap. A slug test shows 2.9 times improvement in the hydraulic conductivity of the well, which, combined with a visual inspection inside the well using an underwater camera, clearly demonstrates that the strong pressure wave generated by underwater spark discharge can effectively remove almost all incrustations formed in the well screen and thus improve well performance. Operating parameters for controlling the strength of shock pressure are discussed using the simulation model for extensive applications of the present technique to various types of water wells.     

Dependence of the characteristics of bubbles on types of sonochemical reactors

Computer simulations of bubble oscillations in liquid water irradiated by an ultrasonic wave have revealed that the characteristic of bubbles depends on types of sonochemical reactors: a horn-type reactor and a standing-wave type reactor. When the acoustic amplitude is large at 20 kHz, the bubble content is mostly water vapor even at the end of the bubble collapse and the temperature inside a bubble at the collapse is relatively low. On the other hand, when the acoustic amplitude is relatively low, the bubble content is mostly noncondensable gas at the end of the bubble collapse and the bubble temperature is relatively high. In a horn-type sonochemical reactor, the former type of bubbles are dominant because many bubbles exist near the horn-tip where the acoustic amplitude is large, while in a standing-wave type reactor the latter type of bubbles are dominant because the Bjerknes force gathers bubbles at a region where acoustic amplitude is relatively low.     

Shock-wave-induced jetting of micron-size bubbles

Free gas bubbles in water with radii between 7 and 55 microm subjected to a shock wave exhibit a liquid jetting phenomenon with the jet pointing in the direction of the propagating shock wave. With increasing bubble radius, the length of the jet tip increases and a lower estimate of the averaged jet velocity increases linearly from 20 to 150 m/s. At a later stage, the jet breaks up and releases micron-size bubbles. In the course of shock wave permeabilization and transfection of biological cells, this observation suggests a microinjection mechanism when the cells are near bubbles exposed to a shock wave.     

Ultrasonic-assisted spark plasma discharge for gold nanoparticles synthesis

We examine the ultrasonic-assisted spark discharge in gold nanoparticles production. A 1 kHz pulsed power supply is used to generate spark discharge plasma. The appropriate range of peak power (pulse peak current) for nanoparticles production was determined by the Dynamic Light Scattering (DLS) method. Colloidal gold nanoparticles were synthesized in 100 mL deionized water and pure ethanol as liquid dielectrics. Scanning Electron Microscopy (SEM) micrographs show that the nanoparticles diameter in the deionized water is larger than in pure ethanol. Transmission Electron Microscopy (TEM) micrographs of the gold nanoparticles also confirm the SEM results with more accuracy. The present study revealed that the ultrasonic wave increases the shape uniformity of the nanoparticles and decreases their size.     

Circular motion of submillimeter-sized acoustic bubbles attached to a boundary by high-speed image analysis

The circular motion of submillimeter-sized bubbles attached to a boundary in an 18.5 kHz ultrasonic field are investigated experimentally by high-speed photography and image analysis. It is found that the vibration of gas bubbles with diameters of 0.2–0.4 mm is between spherical radial vibration and regular surface fluctuation. Different from the circular motion of suspended bubbles in water, the circular motion of gas bubbles attached to a boundary presents some new characteristics. These bubbles attached to a boundary (wandering bubbles) will rotate around a fixed bubble array (holding bubbles). Both the wondering bubbles and holding bubbles are “degas” bubbles. The primary Bjerknes force acting on wandering bubbles in the acoustic wave field and the secondary Bjerknes force between the wandering bubbles and the holding bubbles strongly affects the circular motion. The circling and residence behavior of gas bubbles is described and analyzed in detail, which is helpful to understand and improve industrial applications such as ultrasonic cleaning, sonochemical treatment, aeration and cavitation reduction.     

Director-configurational transitions around microbubbles of hydrostatically regulated size in liquid crystals

A high-pressure technique is introduced which allows a continuous variation of the inclusion size in liquid crystal colloids. We use a nematic liquid crystal host into which micrometer-sized gas bubbles are injected. By applying hydrostatic pressures, the diameter of these gas bubbles can be continuously decreased via compression and absorption of gas into the host liquid crystal, so that the director configurations around a single bubble can be investigated as a function of the bubble size. The theoretically predicted transition from a hyperbolic hedgehog to a Saturn-ring configuration, on reduction of the particle size below a certain threshold, is confirmed to occur at the radius of a few micrometers.     

含气泡液体中的非线性声传播

考虑到分布在液体中的气泡是声波在含气泡液体中传播时引起非线性的一个很重要的因素,本文研究了声波在含气泡液体中的非线性传播.将气体含量的影响引入到声波在液体中传播的方程中,从而得到声波在气液混合物中传播的数学模型.通过对该模型进行数值模拟发现,气体含量、驱动声场声压幅值及驱动声场作用时间均会影响到气液混合物中的声场分布及声压幅值大小.液体中的气泡会"阻滞"液体中声场的传播并将能量"聚集"在声源附近.对于连续大功率的驱动声场来说,液体中的气泡会"阻滞"气液混合物中声场及其能量的传播.     

Interaction between phases in the liquid–gas system

This work analyzes the equilibrium between a liquid and a gas over this liquid separated by an interface. Various gas forms exist inside the liquid: dissolved gas molecules attached to solvent molecules, free gas molecules, and gaseous bubbles. Thermodynamic equilibrium is maintained between two phases; the first phase is the liquid containing dissolved and free molecules, and the second phase is the gas over the liquid and bubbles inside it. Kinetics of gas transition between the internal and external gas proceeds through bubbles and includes the processes of bubbles floating up and bubble growth as a result of association due to the Smoluchowski mechanism. Evolution of a gas in the liquid is considered using the example of oxygen in water, and numerical parameters of this system are given. In the regime under consideration for an oxygen–water system, transport of oxygen into the surrounding air proceeds through micron-size bubbles with lifetimes of hours. This regime is realized if the total number of oxygen molecules in water is small compared with the numbers of solvated and free molecules in the liquid.     

Propagation of pressure waves in a three-phase medium of cluster structure

The propagation of a step-shaped shock wave in a liquid is investigated experimentally. The liquid contains spherical three-phase clusters (liquid, solid balls, gas bubbles). A comparison of the experimental data on the velocity and wave structure with calculations with the use of the Boussinesq equation for a three-phase cluster medium is made. It is shown that the sound speed in a three-phase medium of cluster structure is higher than in a homogeneous three-phase medium.     

含少量气泡流体饱和孔隙介质中的弹性波

考虑孔隙流体中含有少量气泡,且气泡在声波作用下线性振动,研究声波在这种孔隙介质中的传播特性.本文先由流体质量守恒方程和孔隙度微分与流体压力微分的关系推导出了含有气泡形式的渗流连续性方程;在处理渗流连续性方程中的气体体积分数时间导数时,应用Commander气泡线性振动理论导出气体体积分数时间导数与流体压强时间导数的关系,进而得到了修正的Biot形式的渗流连续性方程;最后结合Biot动力学方程求得了含气泡形式的位移场方程,便可得到两类纵波及一类横波的声学特性.通过对快、慢纵波的频散、衰减及两类波引起的流体位移与固体位移关系的考察,发现少量气泡的存在对快纵波和慢纵波的传播特性影响较大.     

Features of the dynamics of postdetonation waves

We present the results of numerical investigations of the parameters of postdetonation waves forming at a passage from the zone occupied with a bubbly liquid formed by the detonation wave to a zone filled with a liquid without bubbles. The dependence of the pressure amplitude of detonation waves and postdetonation waves on the gas volumetric content of bubbles has been studied. A possibility of the detonation transfer through the layer of a bubble-free liquid separating the regions of the bubbly liquid has been shown, the map of possible situations at the detonation transfer through the layer of this liquid has been presented.     

气体火花开关初始放电等离子体演化过程北大核心CSCD

气体火花开关的初始放电过程对研究其工作状态有着非常重要的影响,通过基于网格粒子法-直接蒙特卡罗法(PIC-DSMC)耦合算法模拟了气体火花开关从放电开始到等离子体通道初步形成的完整过程,得到了电子和离子的数密度时空分布变化,分析了间隙中电场分布随时间变化规律,完整清晰地揭示了气体火花开关从放电初始到等离子体通道初步形成的物理过程,并初步开展了气体火花开关击穿过程的光学诊断实验,为进一步深入研究气体火花开关的物理机理打下了基础。     

Towards a triggerable switchless TEA CO2 laser

The possibility of triggering a switch-less TE gas laser has been experimentally explored. To this end, the parallel spark preioniser array that functions as a switch of the switch-less laser has been optically triggered by transporting UV photons from the triggering discharge by means of an optical fiber. The maximum triggerable range was studied as a function of the number of spark channels in the triggering discharge and the gas composition in the triggered parallel spark preioniser.     

Calculation of shock wave propagation in water containing reactive gas bubbles

The entry of a shock wave from air into water containing reactive gas (stoichiometric acetylene–oxygen mixture) bubbles uniformly distributed over the volume of the liquid has been numerically investigated using equations describing two-phase compressible viscous reactive flow. It has been demonstrated that a steady-state supersonic self-sustaining reaction front with rapid and complete fuel burnout in the leading shock wave can propagate in this bubbly medium. This reaction front can be treated as a detonation-like front or “bubble detonation.” The calculated and measured velocities of the bubble detonation wave have been compared at initial gas volume fraction of 2 to 6%. The observed and calculated data are in satisfactory qualitative and quantitative agreement. The structure of the bubble detonation wave has been numerically studied. In this wave, the gas volume fraction behind the leading front is approximately 3–4 times higher than in the pressure wave that propagates in water with air bubbles when the other initial conditions are the same. The bubble detonation wave can form after the penetration of the shock wave to a small depth (~300 mm) into the column of the bubbly medium. The model suggested here can be used to find optimum conditions for maximizing the efficiency of momentum transfer from the pressure wave to the bubbly medium in promising hydrojet pulse detonation engines.