HE 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 millimeter long - was made by etching shapes into thin silicon sheets.

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 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 from University of Colorado at Boulder. "When we drive them with a voltage alternating a 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.

He 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.

Mark Spearing, currently testing micro turbines at the Massachusetts Institute of Technology, calls the fan "ingenious", but says the speeds achieved are "rather slow".

The Hindu

17th May 2001