Cavitation bubble dynamics

The dynamics of cavitation bubbles on water is investigated for bubbles produced optically and acoustically. Single bubble dynamics is studied with laser produced bubbles and high speed photography with framing rates up to 20.8 million frames per second. Examples for jet formation and shock wave emission are given. Acoustic cavitation is produced in water in the interior of piezoelectric cylinders of different sizes (up to 12 cm inner diameter). The filementary structure composed of bubbles is investigated and their light emission (sonoluminescence) studied for various driving strengths.     

Experimental studies of bubble dynamics inside a corner

The dynamics of a bubble near a corner formed by two flat rigid boundaries (walls), is studied experimentally using a spark-generated bubble. The expansion, collapse, rebound, re-collapse and migration of the bubble, along with jetting and protrusion, are captured using a high-speed camera. Our experimental observations reveal the behaviour of the bubble in terms of the corner angle and the dimensionless standoff distances to the near and far walls in terms of the maximum bubble radius. The bubble remains approximately spherical during expansion except for its surface becoming flattened when in close proximity to a wall. When a bubble is initiated at the bisector of the two walls, the bubble becomes oblate along the bisector during the late stages of collapse. A jet forms towards the end of collapse, pointing to the corner. The closer the bubble to the two walls, the more oblate along the bisector the bubble becomes, and the wider the jet. A bubble initiated near one of the two walls is mainly influenced by the nearer wall. The jet formed is pointing to the near wall but inclined towards the corner. After the jet penetrates through the bubble surface, the bubble becomes a bubble ring, and a bubble protrusion forms following the jet. The bubble ring collapses and subsequently disappears, while the protrusion firstly expands, and then collapses and migrates to the corner.     

Influence of gas motion inside a charged bubble in a liquid on the parameters of bubble oscillations

The influence of a finite rate of leveling of the gas pressure inside a charged bubble in an ideal incompressible liquid on the bubble volume and surface oscillations is studied in a linear approximation with respect to the surface oscillation amplitude. It is shown that the bubble shape is governed by superposition of spherical harmonics with amplitudes strongly depending on their frequencies, as well as on the physical properties of the gas inside the bubble and the ambient liquid.     

Cavitation bubble dynamics studied by high speed photography and holography: part two

For a thorough study of bubble dynamics in cavitation bubble fields one has to resort to holography to circumvent the problems caused by bubbles moving out of focus during their three-dimensional motion. The development and application of high speed holography and holocinematography in cavitation physics is reviewed in this paper. The state of the art is that several thousand holograms with frame size a few square millimetres can be taken at rates up to 300 000 per second. Digital image processing, a valuable tool which is used to cope with the huge information content of series of holograms, is discussed. Results on bubble dynamics obtained via holography are presented, in particular on shock wave emission and bubble oscillation, splitting and interaction in a sound field. Global field dynamics and streamer stability are also discussed.     

Cavitation bubble dynamics studied by high speed photography and holography: part one

Bubble dynamics has been recognized as being responsible for many effects in ultrasonic fields, for example cleaning and destruction of surfaces, and emulsification. To improve the performance of ultrasonic devices which make use of these effects some knowledge of the dynamics of cavitation bubbles is needed. One of the main means of studying bubble dynamics is high speed photography in conjunction with some suitable bubble production mechanism to get rid of the statistical appearance of bubbles encountered in ultrasonic cavitation. Results of jet and vortex ring formation obtained mainly with spark-and laser-produced bubbles are reported and demonstrated in a series of frames.     

The collapsing bubble in a liquid by molecular dynamics simulations

Molecular dynamics simulations have been made of a collapsing bubble or cavity in a simple liquid. Simulations of a Lennard-Jones liquid reveal that the collapsing process takes place in a series of stages. First, the ‘hottest’ molecules from the high kinetic energy tail in the Maxwell—Boltzmann distribution diffuse into the empty cavity. This is followed by a gradual filling in of the cavity until the density in the centre is a little lower than that of the bulk liquid. The system eventually reaches a final new equilibrium liquid state through a subsequent slower equilibration phase. The bubble fills in an oscillatory manner, by partly filling in, and then partially emptying, and so on, with ever decreasing amplitude towards the final uniform liquid state. These density oscillations are more obvious in systems with a larger bubble. Similar oscillations are observed in the kinetic energy of the molecules at selected radii from the centre of the initial bubble. The maximum temperature occurs typically at the end of the initial fillingin stage during which the density of the core undergoes a vapour-to-liquid phase transition, the released latent heat probably contributing to the temperatures achieved in this region. The average maximum temperature found in the smallest system examined is about nine times the critical temperature, which is about 6000 K for water, thus suggesting a simple mechanism for producing molecules with the sorts of kinetic energies and lifetimes required for sonoluminescence.     

两种气泡混合的声空化

将非线性声波方程和改进的Rayleigh-Plesset方程联立可以描述空化环境中的声场及相应的气泡动力学特征. 用时域有限差分方法模拟了圆柱形容器内两种气泡相互混合时的空化情况. 在烧杯内的稳态背景声场形成过程中, 瓶壁耗散吸收扮演了重要的角色. 在稳态背景声场的基础上, 分析了混合气泡与声场的相互作用、气泡之间的相互作用、混合情况下的频谱特性. 结果表明: 两种气泡平衡半径都不太大时, 气泡与声场的相互作用不强, 声场及气泡的行为也比较规律; 相反, 当其中一种气泡平衡半径相对比较大时, 声场与气泡具有较强的非线性相互作用, 声场及气泡的行为表现出复杂的特性.     

行波驱动下空泡在可压缩流场中的运动特性研究

基于波动方程给出了计及可压缩性的边界积分方程. 以此为基础,求解行波驱动下非球状空泡的运动规律及其运动稳定性,并分析比较了行波频率、幅值以及初相位对空泡运动特性的影响. 研究结果表明：较高的行波频率与较低的幅值是空泡稳定运动的充分条件. 在一定幅值和频率的行波驱动下,空泡将在收缩阶段末期形成与行波传播方向相同的高速射流;计及流场可压缩性后,空泡脉动一次的时间减短,幅度减弱,射流顶点速度以及空泡内部压力的峰值随之减小;随着行波频率的增大或是幅值的降低,空泡脉动幅度与射流强度逐渐减弱;行波初相位的变化使空泡的初始运动状态随之改变,并影响非球状变形时的射流强度. 关键词： 可压缩 空泡 行波 运动特性     

激光空泡在刚性壁面附近空蚀特性

采用脉冲激光聚焦于透明液体中产生单个激光空泡,利用高速相机拍摄空泡在透明液体中整个运动过程,并对刚性壁面造成的空蚀现象进行了观测.实验研究发现,空泡溃灭将产生高速冲击波和高速射流,这是造成刚性壁面损伤的两种主要原因.空泡与刚性壁面的无量纲距离在0.4～1.4之同时,刚性壁面首先受到高速冲击波的破坏,由于空泡的趋壁效应,空泡在第二次收缩过程中将在壁面附近对实验靶材产生高速微射流的空蚀破坏.且这两种作用机制在无量纲距离为1.0时,高速微射流对壁面的空蚀效果更加明显.     

激光空泡在刚性壁面附近空蚀特性

采用脉冲激光聚焦于透明液体中产生单个激光空泡,利用高速相机拍摄空泡在透明液体中整个运动过程,并对刚性壁面造成的空蚀现象进行了观测。实验研究发现,空泡溃灭将产生高速冲击波和高速射流,这是造成刚性壁面损伤的两种主要原因。空泡与刚性壁面的无量纲距离在0.4~1.4之间时,刚性壁面首先受到高速冲击波的破坏,由于空泡的趋壁效应,空泡在第二次收缩过程中将在壁面附近对实验靶材产生高速微射流的空蚀破坏。且这两种作用机制在无量纲距离为1.0时,高速微射流对壁面的空蚀效果更加明显。     

Cavitation erosion by shockwave self-focusing of a single bubble

The ability of cavitation bubbles to effectively focus energy is made responsible for cavitation erosion, traumatic brain injury, and even for catalyse chemical reactions. Yet, the mechanism through which material is eroded remains vague, and the extremely fast and localized dynamics that lead to material damage has not been resolved. Here, we reveal the decisive mechanism that leads to energy focusing during the non-spherical collapse of cavitation bubbles and eventually results to the erosion of hardened metals. We show that a single cavitation bubble at ambient pressure close to a metal surface causes erosion only if a non-axisymmetric energy self-focusing is at play. The bubble during its collapse emits shockwaves that under certain conditions converge to a single point where the remaining gas phase is driven to a shockwave-intensified collapse. We resolve the conditions under which this self-focusing enhances the collapse and damages the solid. High-speed imaging of bubble and shock wave dynamics at sub-picosecond exposure times is correlated to the shockwaves recorded with large bandwidth hydrophones. The material damage from several metallic materials is detected in situ and quantified ex-situ via scanning electron microscopy and confocal profilometry. With this knowledge, approaches to mitigate cavitation erosion or to even enhance the energy focusing are within reach.     

异质原子对液体气泡成核影响的分子动力学模拟

采用分子动力学方法模拟了池沸腾中液体层加入异质原子对气泡成核的影响.分析了异质原子能量参数对液体起始气泡成核时间和温度的影响及其机理.结果表明,当异质原子能量参数小于液氩能量参数时,液体起始气泡成核时间缩短,起始温度降低.当异质原子剂能量参数大于液氩能量参数时,液体起始气泡成核时间增加,起始温度升高.异质原子在壁面上的吸附及在液体中的扩散行为影响固液界面性质,较大能量参数的异质原子扩散系数较小,更多能量参数较大的原子吸附在固体表面上使得壁面势能壁垒增加,导致沸腾时间延迟,液体需要吸收更多的热量克服势能壁垒,进而提高沸腾起始温度.能量参数较小的异质原子扩散系数较大,异质原子更容易分散到液体中,使得壁面附近液体层势能减小,液体层更容易气泡成核行为.     

Cavitation bubble collapse studied at 20 million frames per second

The collapse of laser-induced bubbles in water is investigated by high speed photography at framing rates as high as 20 million frames per second. The case of a spherical bubble in an unbounded liquid is compared with the Gilmore model. Bubbles collapsing in front of a solid wall show a rich dynamics depending on their normalized distance. Unprecedented details are given of the generic sequence of events leading to multiple shock waves and bubble shape metamorphosis upon collapse.     

Electrical properties of water drops inside the dropwise cluster

The Letter shows that inside a dropwise cluster formed over the heated water surface, water drops are electrically charged. The charge of a separate drop reaches 10³ units of an elementary charge. The drops are positioned from each other at the distance of double Debye radius length. It is fixed up that drops levitate over the water surface in consequence of the Stokes force acting from the side of gas-vapor flow rising from water surface. The Stokes force thousand times exceeds the Coulomb drop repulsion force from the water surface.     

高强度聚焦超声场中空化泡群的结构及其形成过程

空化是高强度聚焦超声(HIFU)引起组织损伤的作用机制之一。本文使用高速摄影技术研究了HIFU场中空化泡群结构的空间分布特征、超声功率对空化泡群结构的影响、空化泡群结构的建立过程尤其是起始过程。空化泡群的结构以空化泡在声场中的区域化分布为特征.在一定功率范围内空化泡群的结构保持稳定。从建立过程来看,空化泡群首先出现在焦区,然后在焦后区(远离换能器的区域)形成,焦前区(靠近换能器的区域)空化泡群最后形成。在对空化泡群起始的研究中发现最初拍摄到的空化泡群形状与焦区形状接近,并且观察到空化泡在焦区发生聚集然后破裂的现象。     

Jump probability and bubble dynamics

The existence of a jump probability is introduced from the interpretation of transport measurements performed on magnetic bubbles. As a consequence the shape of the experimental curve giving the mobility and the extrapolated coercivity is shown to be dependent on the duration of the applied field gradient.     

Hamiltonian description of bubble dynamics

The dynamics of a nonspherical bubble in a liquid is described within the Hamiltonian formalism. Primary attention is focused on the introduction of the canonical variables into the computational algorithm. The expansion of the Dirichlet-Neumann operator in powers of the displacement of a bubble wall from an equilibrium position is obtained in the explicit form. The first three terms (more specifically, the second-, third-, and fourth-order terms) in the expansion of the Hamiltonian in powers of the canonical variables are determined. These terms describe the spectrum and interaction of three essentially different modes, i.e., monopole oscillations (pulsations), dipole oscillations (translational motions), and surface oscillations. The cubic nonlinearity is analyzed for the problem associated with the generation of Faraday ripples on the wall of a bubble in an acoustic field. The possibility of decay processes occurring in the course of interaction of surface oscillations for the first fifteen (experimentally observed) modes is investigated.