We report on cavitation in confined microscopic environments which are commonly called microfluidic or lab-on-a-chip systems. The cavitation bubble is created by focusing a pulsed laser into these structures filled with a light-absorbing liquid. At the center of a 20 microm thick and 1 mm wide channel, pancake-shaped bubbles expand and collapse radially. The bubble dynamics compares with a two-dimensional Rayleigh model and a planar flow field during the bubble collapse is measured. When the bubble is created close to a wall a liquid jet is focused towards the wall, resembling the jetting phenomenon in axisymmetry. The jet flow creates two counter-rotating vortices which stir the liquid at high velocities. For more complex geometries, e.g., triangle- and square-shaped structures, the number of liquid jets recorded correlates with the number of boundaries close to the bubble.

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A high-speed camera was used to investigate the early stage of a chemical reaction within a few milliseconds. We focus on the process of color change caused by a droplet containing a pH indicator when impinging on the surface of alkaline solution. Contrary to our expectation, this reaction starts along the equatorial line, and not at the protruding edge of the droplet, where it first touches the reaction partner. Small vertical fingers emerge from the front line within 1.5 ms. The results suggest that the observed deformation of the droplet and heat diffusion play major roles during this early reaction stage. Our investigations contribute to the understanding of short-term transport processes across interfaces, including the onset of unstable behavior of reaction fronts.

Magnesium binding to cation#x2010;depleted blue bacteriorhodopsin (b‐bR) was studied spectrophotometrically as well as by following stopped‐flow kinetics. There exist three kinetically different steps in the binding process, yielding purple bacteriorhodopsin (p‐bR). Since only the firtst step is dependent on the concentration of the reactants, the reaction scheme can be proposed as the simplest model, with MgbR being the first intermediate and ΣI denoting a set of successive intermediates. According to this model k 1, k −1 and k 2 are calculated to be 2.8 × 104 M−1 · s−1, 5.0 × 10 s−1 and 1 × 10−2 s−1, respectively.

External control of oscillatory glycolysis in yeast extract has been performed by application of either homogeneous temperature oscillations or stationary, spatial temperature gradients. Entrainment of the glycolytic oscillations by the 1/2- and 1/3-harmonic, as well as the fundamental input frequency, could be observed. From the phase response curve to a single temperature pulse, a distinct sensitivity of NADH-oxidizing processes, compared with NAD-reducing processes, is visible. Determination of glycolytic intermediates shows that the feedback-regulated phosphofructokinase as well as the glyceraldehyde-3-phosphate dehydrogenase are the most temperature-sensitive steps of glycolysis. We also find strong concentration changes in ATP and AMP at varying temperatures and, accordingly, in the energy charge. Construction of a feedback loop for spatial control of temperature by means of a Peltier element allowed us to apply a temperature gradient to the yeast extract. With this setup it is possible to initiate traveling waves and to control the wave velocity.

The rupture of thin smectic bubbles is studied by means of high speed video imaging. Bubbles of centimeter diameter and film thicknesses in the nanometer range are pierced, and the instabilities of the moving rim around the opening hole are described. Scaling laws describe the relation between film thickness and features of the filamentation process of the rim. A flapping motion of the retracting smectic film is assumed as the origin of the observed filamentation instability. A comparison with similar phenomena in soap bubbles is made. The present experiments extend studies on soap films [H. Lhuissier and E. Villermaux, Phys. Rev. Lett. 103, 054501 (2009)10.1103/PhysRevLett.103.054501] to much thinner, uniform films of thermotropic liquid crystals.

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