The Quest for the Most Spherical Bubble

We describe a recently realized experiment producing the most spherical cavitation bubbles today. The bubbles grow inside a liquid from a point-plasma generated by a nanosecond laser pulse. Unlike in previous studies, the laser is focussed by a parabolic mirror, resulting in a plasma of unprecedented symmetry. The ensuing bubbles are sufficiently spherical that the hydrostatic pressure gradient caused by gravity becomes the dominant source of asymmetry in the collapse and rebound of the cavitation bubbles. To avoid this natural source of asymmetry, the whole experiment is therefore performed in microgravity conditions (ESA, 53rd and 56th parabolic flight campaign). Cavitation bubbles were observed in microgravity (~0g), where their collapse and rebound remain spherical, and in normal gravity (1g) to hyper-gravity (1.8g), where a gravity-driven jet appears. Here, we describe the experimental setup and technical results, and overview the science data. A selection of high-quality shadowgraphy movies and time-resolved pressure data is published online.Comment: 18 pages, 14 figures, 1 tabl

Energy partition at the collapse of spherical cavitation bubbles

Spherically collapsing cavitation bubbles produce a shock wave followed by a rebound bubble. Here we present a systematic investigation of the energy partition between the rebound and the shock. Highly spherical cavitation bubbles are produced in microgravity, which suppress the buoyant pressure gradient that otherwise deteriorates the sphericity of the bubbles. We measure the radius of the rebound bubble and estimate the shock energy as a function of the initial bubble radius (2-5.6 mm) and the liquid pressure (10-80 kPa). Those measurements uncover a systematic pressure dependence of the energy partition between rebound and shock. We demonstrate that these observations agree with a physical model relying on a first-order approximation of the liquid compressibility and an adiabatic treatment of the non-condensable gas inside the bubble. Using this model we find that the energy partition between rebound and shock is dictated by a single non-dimensional parameter $\xi = \Delta p\gamma^6/[{p_{g0}}^{1/\gamma} (\rho c^2)^{1-1/\gamma}]$, where $\Delta p=p_\infty-p_v$ is the driving pressure, $p_{\infty}$ is the static pressure in the liquid, $p_v$ is the vapor pressure, $p_{g0}$ is the pressure of the non-condensable gas at the maximal bubble radius, $\gamma$ is the adiabatic index of the non-condensable gas, $\rho$ is the liquid density, and $c$ is the speed of sound in the liquid.Comment: 7 pages, 7 figure

Confined Shocks inside Isolated Liquid Volumes -- A New Path of Erosion?

The unique confinement of shock waves inside isolated liquid volumes amplifies the density of shock-liquid interactions. We investigate this universal principle through an interdisciplinary study of shock-induced cavitation inside liquid volumes, isolated in 2 and 3 dimensions. By combining high-speed visualizations of ideal water drops realized in microgravity with smoothed particle simulations we evidence strong shock-induced cavitation at the focus of the confined shocks. We extend this analysis to ground-observations of jets and drops using an analytic model, and argue that cavitation caused by trapped shocks offers a distinct mechanism of erosion in high-speed impacts (>100 m/s).Comment: 4 page letter, 4 figure

Microgravity Experiment: Cavitation Studies Inside Water Drops

Poster session on microgravity experiments 2006

Direct effects of gravity on cavitation bubble collapse

We propose an experiment for studying the final stages of collapse of a single laser generated cavitation bubble in microgravity. Unlike previous investigations, the goal of the study is to examine the direct effects of gravity on the cavity collapse. In this paper we present ground-based research on these effects and outline a microgravity experiment destined for ESA Microgravity Research Campaign. The proposed experiment uses a focused laser to generate a highly spherical bubble in an extended water volume without disturbing the liquid and measures bubble rebound and shockwave intensity. Buoyancy forces being proportional to bubble volume, smaller bubbles are the least disturbed. Results show that as bubble size decreases, the part of bubble energy transformed into a shockwave increases, to the detriment of the rebound bubble and liquid jets

Water drops and cavitation bubbles in microgravity

We realized an experimental setup to study the dynamics of cavitation bubbles inside spherical water drops produced in microgravity (8th Student Parabolic Flight Campaign, European Space Agency ESA). Water volumes of up to 8 ml (25 mm diameter) were expelled through a specially designed injector tube. The latter contained a porous hydrophilic foam and was particularly coated to ensure a stable and attached water drop. Single cavitation bubbles were then generated by an electrical discharge inside the drop. The growth and collapse of the bubble were recorded using a high-speed visualization system (24’000 frames/s). Thereby, we could study for the first time the cavitation phenomenon in microgravity and observe the interactions between cavity and spherical free surface. In particular, the microjet/counterjet formation was visualized and significant counterjet broadening was found when passing from planar to spherical free surfaces

Secondary Cavitation and Shockwaves in Isolated Volumes

We studied spark-generated shockwaves propagating inside stable water drops produced in microgravity. The closed and isolated liquid geometry results in a unique confinement of shockwaves, since the latter bounce of the free surface. This setting results in an amplified form of secondary cavitation, and proved particularly useful to study the coupling between shockwaves and secondary cavitation. High-speed visualizations and 3D computer simulations reveal that focus zones in the shockwave propagation lead to a strongly increased density of secondary cavitation. Considering shockwave crossing and focussing may hence prove crucially useful to understand the important process of cavitation erosion