Background – At part-time level, I engage as mission specialist and coordinator in microgravity research conducted aboard parabolic flights with the European Space Agency ESA. This project began with an ESA student contest, which our team of four students won in 2004. The scientific yield of our first experiment (published in Physical Review Letters) convinced us to continue this type of research and seek substantial funding. Over many parabolic flights and a growing list of publications, our team became the world-leading group on cavitation research in microgravity. The objective of this research is to understand the collapse of so-called cavitation bubbles – small vapor bubbles in a liquid that form when the liquid is depressurized below the evaporation point. Cavitation bubbles are generally unstable and collapse within micro- to milliseconds, invisible to the naked eye. This fast collapse can harm industrial hydraulic systems (e.g., turbines and ship propellers) by causing erosion, vibration and loss of efficiency. On the good side, cavitation is sometimes applied beneficially in medicine, food technology and microfluidics. Whether good or bad, the collapse of cavitation bubbles remains poorly understood. During this collapse, the initial energy of the bubble is absorbed by a rebound bubble, liquid jets, shock waves, and thermal effects (heat, light and chemical reactions); however, the relative importance of these energy channels remains cumbersome.
Objective – The central aim (published here) of our research is to measure precisely where the energy of a collapsing cavitation bubble goes as a function of the liquid properties, bubble shape and geometric boundary conditions. To achieve this aim it helps tremendously to remove gravitational forces from the system, thus simplifying the case to the spherically symmetric collapse of a bubble in an isotropic pressure field.
Experiment – The experimental setup consists of a transparent test-chamber filled with approximately five liters of demineralized water. A single cavitation bubble can be generated at the centre of this test-chamber using a focussed laser pulse. The growth, collapse and rebound of the bubble (see figure below) is then recorded using a high-speed camera taking up to a million frames a second. In addition, the experiment is fitted with a spectrometer to characterize the luminescent pulse at the collapse stage, and hydrophones to capture the related shock wave. A list of auxiliary sensors controls all experimental conditions. This experiment is installed inside the aircraft performing the parabolic flight manoeuvres. In flight, cavitation bubbles are generated in normal gravity (1g), microgravity (~0g) and hyper gravity (1-1.8g).
Results – Our experiment revealed for the first time a gravity-driven jet of collapsing cavitation bubbles in a normal (1g) gravitational field (see figure above and movie below). Based on these precision measurements, we uncovered several scaling laws predicting the prominence of the erosive jet (Obreschkow et al. 2011, featured on the cover page of Physical Review Letters), shock and rebound (Tinguely et al. 2012) during the collapse of a cavitation bubble.
In addition to studying the collapse of cavitation bubbles inside extended volumes of water, we also addressed the case of cavitation inside drops. This study led to the discovery of a new mechanisms by which drops, such as rain drops, can erode hard surfaces (read more here).
Team & Resources – Our team is an international collaboration between the Ecole Polytechnique Federale de Lausanne (EFPL) and the University of Western Australia (UWA) – both listed in the top 150 universities of the current Shanghai ranking. For the continuation of the project, we have secured a solid financial basis, comprising a CHF 183,000 grant for a PhD thesis started on 1 Nov 2013, an AUD 17,200 travel grant start-ing on 1 Jan 2014 and about CHF 15,000 of industrial funds. The PhD candidate at EPFL is dedicating a full-time-equivalent (FTE) of 100% to the project, while receiving substantial support from flight-experienced academic staff: Dr. Mohamed Farhat (EPFL) at 30% FTE, Dr. Philippe Kobel (EPFL) at 20% FTE, Prof. Danail Obreschkow (UWA) at 20% FTE and external science and engineering support from Drs. Nicolas Dorsaz and Marc Tinguely. The flight campaigns are typically attended by 1-2 students and 3-5 academic staff, who all get to fly in microgravity.