Robert Mettin

Collapse and rebound of a laser-induced cavitation bubble

A strong laser pulse that is focused into a liquid produces a vapor cavity, which first expands and then collapses with subsequent rebounds. In this paper a mathematical model of the spherically symmetric motion of a laser-induced bubble is proposed. It describes gas and liquid dynamics including compressibility, heat, and mass transfer effects and nonequilibrium processes of evaporation and condensation on the bubble wall. It accounts also for the occurrence of supercritical conditions at collapse. Numerical investigations of the collapse and first rebound have been carried out for different bubble sizes. The results show a fairly good agreement with experimental measurements of the bubble radius evolution and the intensity of the outgoing shock wave emitted at collapse. Calculations with a small amount of noncondensable gas inside the bubble show its strong influence on the dynamics.

Spatio–temporal dynamics of acoustic cavitation bubble clouds

Bubble clouds forming in an extended volume of liquid in acoustic cavitation show a slowly varying filamentary structure, whose origin is still not completely understood. Experimental observations are reported that provide some characteristics of the phenomenon, such as bubble distributions and sound–field measurements. A discussion of relevant physical interactions in bubbly liquids is comprised of wave dynamics, Bjerknes and drag forces, nucleation and coalescence. For describing the structure formation process, continuum and particle approaches are employed. In the framework of the continuum model it is shown that homogeneous bubble distributions are unstable, and regions with high bubble concentration emerge in the course of a self–concentration process. In the particle model, all bubbles are treated as interacting objects that move in the liquid. This approach is complementary to the continuum model. It allows the inclusion of some particular features, for instance Bjerknes forces based on nonlinear bubble oscillations. Both models are discussed and results are compared with experimentally observed patterns.

Acoustic cavitation structures and simulations by a particle model

Cavitation bubbles in acoustic resonators are observed to arrange in branch-like patterns. We give a brief review of the anatomy of such structures and outline an approach for simulation by individual, moving bubbles. This particle model can reproduce an experimentally observed transition between different structure types in a rectangular resonator cell.

Observations of pressure-wave-excited contrast agent bubbles in the vicinity of cells

Levovist contrast agent bubble dynamics after spark-induced pressure wave exposure is investigated in the vicinity of normal rat kidney fibroblast cells. The bubble dynamics is calculated using the Keller–Miksis model and results are compared with the experimental radius time curves. High-speed microscopic images show rapid bubble expansion and collapse. Even at moderate peak negative pressure amplitudes of less than 2 MPa the contrast agent bubbles have been observed to expand to more than 30 times their original radius and to rupture cells upon collapse.

Shock wave induced interaction of microbubbles and boundaries

In the present study we experimentally investigate bubble dynamics after laser induced shock wave exposure in the vicinity of salt crystals suspended in water. High-speed microscopic images show aspherical collapse and rebound of single and multiple bubbles with initial radii between 5 and 150 μm. Radius time curves of bubbles close to one boundary are compared to the bubble dynamics of a spherical model. The bubble dynamics strongly depends on the position of neighboring bubbles and on the number of boundaries given by the surrounding salt grains. After excitation bubbles are drawn to the closest particles in their vicinity. Subsequent application of shock waves leads to jet formation against the rigid boundaries. The bubbles often tend to form in or migrate into cracks on the crystal surfaces and sometimes lead to the breakage of particles due to rapid bubble dynamics. Similar behavior may occur in other cases where material damage is induced by shock waves in liquids such as lithotripsy or shock wave c...

Stereoscopic high-speed recording of bubble filaments

Filamentary formations of acoustic cavitation bubbles in an ultrasonic resonator are recorded by high-speed stereoscopic means. The bubble locations and motions are reconstructed in three dimensions, and a velocity distribution of bubbles is obtained. Experimental bubble trajectories are compared to a one-to-one simulation by a particle modeling approach which shows reasonable agreement. Such investigations are important for a better understanding of the mechanisms taking place in applications of intense ultrasound in liquids, and for verification and improvement of particle modeling of cavitation bubbles.

Two-frequency driven single-bubble sonoluminescence

Sonoluminescing single bubbles driven simultaneously by two harmonic frequencies were recently reported to increase the maximum light output up to a factor of 3 with respect to single mode excitation. In this paper, experimental and numerical results on single-bubble sonoluminescence (SBSL) in an air/water system using the fundamental mode of 25 kHz and the second harmonic at 50 kHz are presented. The region of light emission is mapped in the three-dimensional parameter space spanned by the two driving pressure amplitudes and their relative phase. Good agreement was seen between measured light output, maximum bubble radius, and stability boundaries and the numerical model which is based on spherical bubble oscillations regarding diffusive and shape stability. The maximum brightness was enhanced by a factor up to 2.5 with respect to single mode SBSL. However, long-term measurements reveal great variation of the emission at fundamental mode driven SBSL and of the boost factor reached with two frequencies. The overall brightness maxima of both excitation methods within a period of several hours turn out to show little difference.

Optimized periodic control of chaotic systems

Abstract We demonstrate the open-loop control of chaotic systems by means of optimized periodic signals. The use of such signals enables us to eliminate chaotic motion with significantly reduced control power compared to simple harmonic perturbations. It is found that the stabilized periodic dynamics can be changed by small, specific alterations of the control signal. Thus, low power switching between different periodic states can be achieved without feedback. The performance of the proposed control method in the presence of noise is discussed.

Study on the bubble transport mechanism in an acoustic standing wave field.

The use of bubbles in applications such as surface chemistry, drug delivery, and ultrasonic cleaning etc. has been enormously popular in the past two decades. It has been recognized that acoustically-driven bubbles can be used to disturb the flow field near a boundary in order to accelerate physical or chemical reactions on the surface. The interactions between bubbles and a surface have been studied experimentally and analytically. However, most of the investigations focused on violently oscillating bubbles (also known as cavitation bubble), less attention has been given to understand the interactions between moderately oscillating bubbles and a boundary. Moreover, cavitation bubbles were normally generated in situ by a high intensity laser beam, little experimental work has been carried out to study the translational trajectory of a moderately oscillating bubble in an acoustic field and subsequent interactions with the surface. This paper describes the design of an ultrasonic test cell and explores the mechanism of bubble manipulation within the test cell. The test cell consists of a transducer, a liquid medium and a glass backing plate. The acoustic field within the multi-layered stack was designed in such a way that it was effectively one dimensional. This was then successfully simulated by a one dimensional network model. The model can accurately predict the impedance of the test cell as well as the mode shape (distribution of particle velocity and stress/pressure field) within the whole assembly. The mode shape of the stack was designed so that bubbles can be pushed from their injection point onto a backing glass plate. Bubble radial oscillation was simulated by a modified Keller-Miksis equation and bubble translational motion was derived from an equation obtained by applying Newtons second law to a bubble in a liquid medium. Results indicated that the bubble trajectory depends on the acoustic pressure amplitude and initial bubble size: an increase of pressure amplitude or a decrease of bubble size forces bubbles larger than their resonant size to arrive at the target plate at lower heights, while the trajectories of smaller bubbles are less influenced by these factors. The test cell is also suitable for testing the effects of drag force on the bubble motion and for studying the bubble behavior near a surface.

Effects of argon sparging rate, ultrasonic power, and frequency on multibubble sonoluminescence spectra and bubble dynamics in NaCl aqueous solutions.

The sonoluminescence spectra from acoustic cavitation in aqueous NaCl solutions are systematically studied in a large range of ultrasonic frequencies under variation of electrical power and argon sparging. At the same time, bubble dynamics are analysed by high-speed imaging. Sodium line and continuum emission are evaluated for acoustic driving at 34.5, 90, 150, 365, and 945kHz in the same reactor vessel. The results show that the ratio of sodium line to continuum emission can be shifted by the experimental parameters: an increase in the argon flow increases the ratio, while an increase in power leads to a decrease. At 945kHz, the sodium line is drastically reduced, while the continuum stays at elevated level. Bubble observations reveal a remarkable effect of argon in terms of bubble distribution and stability: larger bubbles of non-spherical shapes form and eject small daughter bubbles which in turn populate the whole liquid. As a consequence, the bubble interactions (splitting, merging) appear enhanced which supports a link between non-spherical bubble dynamics and sodium line emission.