Marangoni Experiment in Space (MEIS-2)
Suspended between two solid disks, a liquid bridge extends, exquisitely formed in the microgravity environment of the International Space Station (ISS). This is the kind of fluid physics experiment that Dr. Robert (Bob) Thirsk performed as a crewmember of the long-term mission aboard the International Space Station, Expedition 20/21.
Liquid bridges are one method used to form semiconductor crystals, widely used in electronics. Semiconductors can be found in technology like the microprocessors that manage complex programs and drive graphics displays in our computers, the microchips that control our cell phones, and the transistors that amplify our stereos. Fabrication of semiconductors is an imperfect process, and many materials scientists and fluid physicists are currently doing research to improve the process. Dr. Masahiro Kawaji of the University of Toronto is interested in the physical processes underlying crystal formation, and he is part of a science team who tries to shed light on the role of “Marangoni” convection in crystal formation; this mysterious convection is difficult to study on Earth, so the scientists are going to the International Space Station to do their work.
As scheduled, in the early summer of 2009, Bob Thirsk set up the Marangoni Experiment in Space 2 (MEIS-2) apparatus and inserted it into a rack of the Japan Aerospace Exploration Agency’s (JAXA) Fluid Physics Experiment Facility (FPEF) located onboard the ISS. Dr. Thirsk received Mission Specialist training in Japan to facilitate his work in the Japanese Kibo module. He was responsible to set up the experiment and ensured that everything was running smoothly.
A liquid bridge was suspended on either side of two solid disks, one cold and the other hot. The liquid is exposed to gas on its sides, unlike on Earth, where liquids are generally exposed to a solid interface, such as a pot containing water. The temperature gradients, which result between the top and bottom surfaces, apply a surface-tension (Marangoni) driven convection that energizes and moves the molecules. This type of convection can be studied because buoyancy-driven convection is muted. Buoyancy-driven convection can be observed in a boiling pot of water. Warm molecules become less dense and rush to the surface, soon to be replaced by other cooler molecules that are heating up. This begins a rising and falling cycle propelled by gravity. By setting the experiment in near weightlessness, buoyancy-driven convection can be eliminated from the data. This will allow the scientists to isolate the effects of Marangoni convection, providing a clearer picture of how instabilities form in the solid-liquid interface, knowledge that can be applied to future semiconductor crystal production.
Though the MEIS-2 apparatus is 100% Japanese, Canadian scientist Masahiro Kawaji has made a significant contribution. He has developed a technique to measure the velocity of the liquid molecules. By adding a photosensitive dye to the liquid and following the dark traces formed using a video camera, he can measure the swirling brew’s velocity. Using a numerical model, he is able to determine the effects of the movement of liquid on the stability of the growth process in the liquid bridge.
Dr. Kawaji further contributed to MEIS-2 by studying the effects that the space station’s vibrations had on the liquid bridge. This was to ensure that the behaviours observed could not be attributed to external vibrations.
The results from the Marangoni experiments using the Fluid Physics Experiment Facility on the ISS could be used to develop higher quality, more efficiently produced semiconductor crystals. This could mean faster, better electronics for a lower price. So when you purchase that next-generation game console or DVD player, you might want to consider giving a nod to the scientists and astronauts of the International Space Station.
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