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The rotor that spins on a bubble
[May 05, 2006]

The rotor that spins on a bubble

(New Scientist Via Thomson Dialog NewsEdge)WHAT do you get if you take a set of miniature silicon helicopter blades, drop them into a beaker of water and blast them with sound waves? A remote-controlled underwater "bubble rotor" that could be used to manipulate individual cells.

The rotor, developed by Daniel Attinger of Columbia University in New York, consists of a piece of silicon, 60 micrometres wide, cut into two crossed blades. It can be made to spin by placing it near a bubble of air in water and hitting the bubble with ultrasound waves.

Although the bubble rotor itself is 100 times bigger than ordinary cells, if you attach a carbon nanotube shaft to drill into individual cells, you can learn more about how they respond to stimuli, Attinger says. The rotor could also be used in microfabrication: hundreds of spinning shafts could be assembled to form a moving "carpet" that guides tiny mechanical parts around a surface.

Attinger and his colleagues discovered the bubble rotor by accident in 2004 while using ultrasound waves to make an underwater air bubble expand and contract. The team was attempting to recreate a 2003 experiment by Philippe Marmottant at CNRS-Universit Joseph Fourier in France, in which he showed that the process produces a doughnut-shaped whirlpool above the bubble. By creating this whirlpool near cells suspended in water it is possible to trap and stretch the cells so that the pores in their surface are wide enough to absorb certain drugs.

When Attinger repeated the experiment, he threw in tiny crushed plastic pieces to help him see the vortex more easily. One of these pieces, a round plastic disk, was much larger than the rest. He found that the creation of the whirlpool sucked this plastic disk towards the 18-micrometre wide bubble, where it stuck to the top and started rotating along with the whirlpool, just like a spinning dish balancing on the end of a stick.

He has since built a helicopter rotor with four prongs, which he has spun on the surface of a bubble. By varying the frequency of the ultrasound, he can control the speed at which the rotor turns, up to a maximum of 600 rotations per minute. Attinger will present his bubble rotor on 8 May at Nanotech 2006 in Boston.

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