Innovation 3310: Levitation: Droplets Surfing On Sound Waves, 2013. sciencedaily/releases/2013/07/130716140702.htm “… Researchers are able to make objects such as particles and liquid droplets fly in mid-air by letting them ride on acoustic waves. For the first time, they have been able to also control the movement of objects, merge droplets, letting them react chemically or biologically and even rotate a toothpick in the air.” “A toothpick floating in mid-air without any support -- this may sound like it involves hidden threads, magnets or other sleight-of-hand tricks from magicians. But the actual trick … is based on acoustic waves. Despite the appearance of ‘magic,’ he and his colleagues realized and controlled the planar movement of floating objects in air, regardless of their properties, involving no sorcery but science. This is not simply an amusing trick: moving objects such as particles or droplets of a liquid freely in mid-air makes it possible to investigate processes while avoiding any disruptive contact with a surface. For instance, some chemical reactions and biological processes are compromised by surfaces, and certain substances disintegrate on contact with a surface.” “Until now, scientists have been able to generate such a "contact-free" levitational state only with the help of magnets, electrical fields or in liquids with the help of buoyancy. These methods, however, limit the selection of materials that can be handled.” “With acoustic waves, in contrast, it is possible to levitate various objects regardless of their properties. The limiting factor is the maximum diameter of the object, which must correspond to half the wavelength of the acoustic wave being used. An object reaches the stationary levitated state when all the forces acting on it are in equilibrium. In other words, the force of gravity that pulls the object in one direction is counteracted by an equally large force in the opposite direction. This force comes from the acoustic wave, which the researchers generate as a standing wave between an emitter and a reflector that reverberate the acoustic waves. The force of the acoustic wave pushes against the object and thus prevents it from falling due to gravity. It is conceptually similar to the air jet from a fan that keeps a ping-pong ball in the air.” “The knowledge that acoustic waves can exert a force -- the acoustic radiation pressure effect -- on an object to keep it in suspension was discovered more than 100 years ago. Until now, however, no one had been successful in controlling the motion of objects riding on acoustic waves in mid-air. Foresti achieved this goal by switching on multiple emitter-reflector modules in parallel next to each other. He varied the acoustic waves from module to module in order to transfer particles or droplets of liquid from one module to the next.” “This method of moving levitated objects could have a wide range of possible applications … With this technique, researchers can [move and] mix tiny amounts of substances and liquids in a step by step manner without any chemical changes arising due to contact with a surface.”
Posted on: Wed, 17 Jul 2013 13:23:15 +0000
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