SCIENCE AND ENGINEERING NEWS
The hum of a computer fan could soon be history. In future, the microchips inside a PC could be cooled by armies of microscopic fans actually grown on the surface of the chips – removing the need for a large, noisy cooling system.
The inventors of the microfan, which is small enough to sit on the head of a pin, speculate that it could also be used to propel tiny flying machines or pump chemicals around lab-on-a-chip devices for analysis. The fan, which has eight blades-each less than half a millimetre long-was made by etching shapes into thin silicon sheets. “You have to think in two dimensions when you design it, but know how it will turn out once it’s been folded into position,” says Paul Kladitis of the University of Colorado at Boulder. “It’s like being some kind of pop-up book artist.”
Each fan blade is connected to a central hub by a hinge. To pull the flat blades up into position, Kladitis deposited gold pads on either side of the hinge. He then dropped a small blob of solder onto the gold pads. Surface tension between the solder and gold pads raised the blade. “The solder wets the gold pads and pulls the wing up into a fan blade position,” he says.
The fan is powered by a so-called “scratch drive”, which nudges it around. At the end of thin silicon plates attached to the hub are silicon “feet”. These rest on an insulating layer of silicon nitride that coats a silicon substrate beneath the fan. To drive the fan, the scratch plate and the silicon base are connected to an alternating power supply. The difference in electrical potential between the scratch plate and the base produces cycling electrostatic forces that rapidly pull the scratch plate down onto the insulator coating and up again. Each time the scratch plate bows downward, the foot pushes against the insulator and nudges the fan around.
“We used nine scratch plates in a circle like a merry-go-round,” says Kladitis. “When we drive them with a voltage alternating at 2 kilohertz, we get speeds from 50 to 180 revolutions per minute.” At 3 kilohertz, an electrostatically induced wobble in the scratch plate makes the foot push in the opposite direction, driving the fan into reverse at 100 rpm.
Kladitis says the fan could be used to pump chemicals around microchip-based chemistry labs. “You could also put this fan right next to an electronic component in a computer to cool it, or even use it as some kind of micro-vehicle propulsion system,” he says.
“I’m impressed they got such an elaborate structure to rotate so well with integrated motors,” says Kris Pister, who is working on insect-like micromachines at the University of California at Berkeley. Mark Spearing, currently testing microturbines at the Massachusetts Institute of Technology, calls the fan “ingenious”, but says the speeds achieved are “rather slow”. “We’re striving for in excess of 1 million rpm in our motor,” he says. Spearing has other concerns too. “I am not a big fan of frictional or sliding contacts in micro electro-mechanical devices,” he says. “Friction and wear tend to be potential show-stoppers at these scales.”