P. Koch

Observation of acoustic cavitation bubbles at 2250 frames per second.

Acoustically induced cavitation at 20 kHz is observed by means of high speed CCD recording at a frame-rate of 2250 per second. Using digital image processing the bubbles trajectories are reconstructed. The experimental data reveal that collision and coalescence of bubbles is a predominant phenomenon that limits their individual lifetime. Measurements of bubble sizes and velocities are in agreement with previous results.

Off-label, off-limits? Parental awareness and attitudes towards off-label use in paediatrics

Off-label drug use in paediatrics is associated with an increased risk of adverse drug reactions. Any risk–benefit analysis has to be based on value judgments that should include parents views. However, nothing is known so far about the parents perspective on this critical issue. Therefore, a quantitative survey with parents of healthy and chronically ill children was carried out (nu2009=u200994). Knowledge about the practise of off-label use is generally poor in both groups. Surprisingly, this is also true for the parents of children with chronic disease. Nine percent of the parents of chronically ill children and 20% of the parents of healthy children would refuse treatment with an off-label drug. Parents who have poor knowledge about the practise of off-label use tend to refuse to volunteer their child for study participation. Therefore, the information of parents on the off-label use of drugs is important to meet ethical standards and to increase the parents acceptance of medical studies with children.

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.

Bubble dynamics in a standing sound field: The bubble habitat

Bubble dynamics is investigated numerically with special emphasis on the static pressure and the positional stability of the bubble in a standing sound field. The bubble habitat, made up of not dissolving, positionally and spherically stable bubbles, is calculated in the parameter space of the bubble radius at rest and sound pressure amplitude for different sound field frequencies, static pressures, and gas concentrations of the liquid. The bubble habitat grows with static pressure and shrinks with sound field frequency. The range of diffusionally stable bubble oscillations, found at positive slopes of the habitat-diffusion border, can be increased substantially with static pressure.

News from bubble dynamics: High static pressures, shock waves and interior dynamics

Three topics from the vast area of bubble dynamics are addressed: (i) the influence of high static pressure on the oscillation of a spherical bubble in a sound field (calculations), (ii) the response of a bubble to a shock wave (experiments), and (iii) the interior dynamics as seen by molecular dynamics calculations. The notion of a bubble habitat is used for positionally and spherically stable, not dissolving bubbles in some two-dimensional space of parameters (mainly bubble radius at rest and sound pressure amplitude) for presenting the results. An elevated static pressure substantially enlarges the bubble habitat and thus the range, where bubbles can be stably trapped. Shock wave - bubble interaction is investigated in the context of laser induced bubbles collapsing in the neighborhood of a solid, plane boundary. Jet development as influenced by both the boundary and by a shock wave propagating parallel to the boundary is studied by high speed photography. The interior of a bubble is explored by molecu...

Bubble dynamics—from single bubbles to ensembles

In a sound field a bubble is forced to oscillate and behaves like a strongly nonlinear oscillator. Moreover, this oscillator alters its parameters by diffusion of gas into and out of the bubble and opens up new dimensions when starting surface oscillations. Theoretical models that incorporate these phenomena are investigated. Part of the parameter space given by the amplitude and frequency of the sound field and by the bubble radius is scanned to get the regions of stable bubble oscillations with respect to diffusion and surface oscillations. Experimentally, single bubbles can be investigated best either by keeping them in a trap or by generating them by focused laser light. For interaction studies with two bubbles a trapped bubble and a laser bubble have been taken. The motion of the bubbles and their interaction is studied by high speed photography and compared with bubble interaction models.

Simulation of the translational motion of few cavitation bubbles in an ultrasonic field

The experimentally recorded motions of two and three bubbles in an acoustic standing wave field are simulated by a particle model approach. The parameters are similar to typical environment in acoustic cavitation at 20 kHz (bubble equilibrium sizes of a few micron and pressure amplitudes larger than 100 kPa). Observed bubble paths can be sufficiently reproduced by the model within the range of experimental error of parameters.