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Scientists Spin Microscopic Particles with Laser Beams

Objects such as chromosomes and proteins are way too small to pick up or flip over using your fingers. But a team of Scottish researchers report in today's issue of Science that microscopic particles can be manipulated rather easily using laser beams. The idea of such optical tweezers is not new: a particle will become trapped in a light beam's path when some of the light refracts from the object, changing both the momentum of the light beam and the object. And this change will steer the object toward the area in the beam where the light is most intense and hold it there¿in essence creating a minuscule tractor beam. But the new, well, spin in this research is using optical tweezers to rotate objects.

To do so, the researchers used two specialized laser beams that combined to form a spiral. This moire-like spiral pattern, called interference, forms when light waves that are in step with each other merge, and those that are not cancel each other out. "The beauty of our technique is that we can dictate how far we want the spiral pattern to go around and at what speed," Kishan Dholakia of St. Andrews University explains. As a result, the scientists can control the rotation of chromosomes, enzymes and other tiny structures more freely and precisely. Dholakia adds that the technique "could be used to drive motors, mixers, centrifuges and other rotating parts in cheap, tiny automated technologies in the future."

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