Claus-Dieter Ohl

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.

Bubble dynamics, shock waves and sonoluminescence

Sound and light emission by bubbles is studied experimentally. Single bubbles kept in a bubble trap and single laser–generated bubbles are investigated using ultrafast and high–speed photography in combination with hydrophones. The optical observation at 20 million frames per second of the shock waves emitted has proven instrumental in revealing the dynamic process upon bubble collapse. When jet formation is initiated by a non–spherically symmetric environment, several distinct shock waves are emitted within a few hundred nanoseconds, originating from different sites of the bubble. The counterjet phenomenon is interpreted in this context as a secondary cavitation event. Furthermore, the light emission of laser–generated cavities (termed cavitation bubble luminescence) is studied with respect to the symmetry of collapse. The prospects of optical cavitation and multibubble trapping in the study of few–bubble systems and bubble interactions are briefly discussed. Finally, the behaviour of bubble clouds, their oscillations, acoustic noise and light emission are described. Depending on the strength of the driving sound field, period doubling and chaotic oscillations of the collective bubble dynamics are observed.

Surface cleaning from laser-induced cavitation bubbles

When bubbles expand and collapse close to boundaries, a shear flow is generated which is able to remove particles from the surface, thus locally cleaning it. Here the authors demonstrate experimentally with microparticle tracking velocimetry that the strongest forcing of particles occurs during a very brief time interval of the bubble oscillation period. During this interval a jet flow impacts and spreads radially along the surface, thus transporting the particles with it.

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.

Laser-induced cavitation based micropump

Lab-on-a-chip devices are in strong demand as versatile and robust pumping techniques. Here, we present a cavitation based technique, which is able to pump a volume of 4000 microm3 within 75 micros against an estimated pressure head of 3 bar. The single cavitation event is created by focusing a laser pulse in a conventional PDMS microfluidic chip close to the channel opening. High-speed photography at 1 million frames s(-1) resolves the flow in the supply channel, pump channel, and close to the cavity. The elasticity of the material affects the overall fluid flow. Continuous pumping at repetition rates of up to 5 Hz through 6 mm long square channels of 20 microm width is shown. A parameter study reveals the key-parameters for operation: the distance between the laser focus and the channel, the maximum bubble size, and the chamber geometry.

Thermoacoustic resonance effect and circuit modelling of biological tissue

In this letter, thermoacoustic resonance effect is predicted from theoretical analysis with series resistor-inductor-capacitor resonance circuit model and then observed experimentally using muscle tissue illuminated by multi-pulse microwave source. Through model fitting, the circuit parameters are extracted to characterize quantitatively the resonant response of the tissue. Coherent demodulation is applied to obtain the enhanced signal-to-noise ratio and spatial information by treating tissue as a communication channel. This physical phenomenon shows significantly higher sensitivity than conventional single microwave pulse induced thermoacoustic effect, enabling the potential design of low-power thermoacoustic imaging device for portable and on-site diagnosis.

The 'acoustic scallop': a bubble-powered actuator

The device described here consists of a millimeter-size tube immersed in a liquid, closed at one end, and partially filled with gas. A sound field in the liquid causes the gas volume to pulsate alternately expelling and drawing liquid through the open end of the tube. According to general fluid mechanical principles, the liquid exits the tube as a jet, while it enters it from the entire solid angle available. Averaged over a cycle, this flow pattern results in a net source of momentum which, by reaction, exerts a force on the tube. Possible applications include the self-propulsion of the tube, a pump and a rotary actuator. Thereby a single device can be made to respond to different frequencies, for example, switching the direction of the force by changing the sound frequency. The acoustic power required is well below biologically hazardous levels, which would permit, among others, powering the device remotely through living tissue.

Controlled cavitation–cell interaction: trans-membrane transport and viability studies

Cavitation bubble dynamics close to a rigid surface gives rise to a rapid and transient fluid flow. A single bubble is created with a laser pulse at different stand-off distances from the rigid surface, where the stand-off distance gamma is defined by gamma = h/R(max), with h being the initial distance and R(max) being the maximum bubble radius. When the surface is covered with adherent cells, molecular delivery and cell detachment after single cavitation activity are observed at different locations. We find a maximum of cell detachment at a normalized stand-off distance of gamma approximately 0.65. In contrast, the maximum of the molecular uptake is found when gamma approaches 0. The single cavitation event has only little effect on the viability of cells in the non-detached area. We find apoptosis of cells only very close to the area of detachment and, additionally, the metabolism of the non-detached cells shows no pronounced difference compared to control cells according to an MTS assay. Thus, although the cavitation event is responsible for the detachment of cells, only few of the remaining cells undergo a permanent change.

Effect of nuclei concentration on cavitation cluster dynamics

Cavitation cluster dynamics after the passage of a single pressure wave is studied for different concentrations of artificial cavitation nuclei (30 to 3x10(5) nuclei/ml). With increasing concentration of cavitation nuclei the lifetime of the cavitation cluster is prolonged. Additionally, it is found that the spatial extent of the cluster decreases with higher nuclei concentration. The experimental data for concentrations less than 400 nuclei/ml are compared to simulations with a Rayleigh-Plesset-type equation, taking into account bubble-bubble interaction. For higher concentrations (more than 1000 nuclei/ml) the observed radial cluster dynamics is compared with calculations from an axisymmetric cavity-collapse model.

Spray and microjets produced by focusing a laser pulse into a hemispherical drop

We use high-speed video imaging to study laser disruption of the free surface of a hemispheric drop. The drop sits on a glass surface and the Nd:YAG (yttrium aluminum garnet) laser pulse propagates through the drop and is focused near the free surface from below. We focus on the evolution of the cylindrical liquid sheet and spray which emerges out of the drop and resembles typical impact crowns. The tip of the sheet emerges at velocities over 1 km/s. The tip of the crown breaks up into fine spray some of which is sucked back into the growing cavity at about 100 m/s. We measure the size of the typical spray droplets to be about 3 μm. We also show the formation of fine microjets, which are produced when the laser is focused inside the drop and the shock front hits small bubbles sitting under the free surface. For water these microjets are 5–50 μm in diameter and exit at 100–250 m/s. For higher viscosity drops, these jets can emerge at over 500 m/s.