Flash & Splash: 10 Years of research on bubble dynamics in variable gravity

Abstract: Although the collapse of a cavitation bubble has received a great deal of attention, there are still many open questions related to this spectacular phenomenon. It is well known that when a cavitation bubble collapses, it generates luminescence and shockwaves. In presence of anisotropic pressure field, an initially spherical bubble may deform and generate the so-called micro-jet. Here we review 10 years of research activity conducted at EPFL, in the frame of Flash&Splash project, on the dynamics of a cavitation bubble in still water under controlled pressure anisotropy. We present the experimental setup, which we have designed to generate highly spherical bubbles using a focused pulsed laser. The instrumentation involves high speed imaging, a needle hydrophone to capture the shockwave and an innovative optical method to measure the luminescence spectrum and estimate the bubble temperature at the final stage of the collapse. Another innovative aspect of our experiment is the use of ESA parabolic flights to allow for the variation of the pressure gradient. So far, we have participated in ten ESA microgravity campaigns and made several discoveries on the dynamics of cavitation bubbles. In the present lecture, we will first present the results related to bubble dynamics within a water drop to illustrate how shockwaves may initiate a secondary cavitation, which is thought to be the cause of erosion due to high speed impact of a liquid drop on a solid surface. We will then provide an experimental evidence of the gravity induced pressure gradient on the bubble collapse. We will also focus on our latest results related to bubble distortion and micro-jets formation under a variety of experimental conditions. In an attempt to unify the family of micro-jets, we introduce a universal anisotropy parameter xsi > 0, a dimensionless measure of the liquid momentum at the collapse point (Kelvin impulse). The observation made on hundreds of high-speed visualizations of bubbles collapsing near a solid surface, near a free surface or in variable gravity, allows for the identification of three distinct regimes of jets: weak, intermediate and strong. Weak jets (xsi < 10-3) are not visible and remain within the bubble throughout the collapse and rebound phases. Intermediate jets (10-3 < xsi < 0.1) pierce the opposite bubble wall at the end of the collapse and clearly emerge during the rebound. Strong jets (xsi > 0.1), which are thought to be the most erosive, pierce the bubble signi´Čücantly before the end of the collapse. The dynamics of the jets is analyzed through key parameters, such as the jet impact time, jet speed, bubble displacement, bubble volume at jet impact and vapor-jet volume. We have found that, when normalizing these parameters to dimensionless jet parameters, they all reduce to straightforward functions of xsi.

For further information and publications list, visit the following website: http://bubbles.epfl.ch

Ackowledgment : The work presented would not be possible without the significant contribution and the strong commitment of the Flash&Splash team : Danail Obreschkow, Philippe Kobel, Marc Tinguely, Nicolas Doszas, Aurel De Bosset and Outi Supponen.