Microchips inside a PC could one day be cooled by armies of microscopic fans actually grown on the surface of the chips and removing the need for a large, noisy cooling fan.
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 analysis devices.
The fan has eight blades, each less than half a millimetre long. It was made by etching shapes into thin silicon sheets. 鈥淵ou have to think in two dimensions when you design it, but know how it will turn out once it鈥檚 been folded into position,鈥� says Paul Kladitis of the University of Colorado at Boulder. 鈥淚t鈥檚 like being some kind of pop-up book artist.鈥�
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鈥淚鈥檓 impressed they got such an elaborate structure to rotate so well with integrated motors,鈥� says Kris Pister, who is working on insect-like micromachines at the University of California at Berkeley.
Friction drive
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. 鈥淭he solder wets the gold pads and pulls the wing up into a fan blade position,鈥� he says.
The fan is powered by a so-called 鈥渟cratch drive鈥�, which Kladitis鈥檚 colleague, Ryan Linderman, designed to nudge the fan around. At the hub end of the thin silicon plates are silicon 鈥渇eet鈥�. These rest on an insulating layer of silicon nitride that coats a silicon substrate beneath the fan (see diagram).
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.
鈥淲e used nine scratch plates in a circle like a merry-go-round,鈥� says Kladitis. 鈥淲hen we drive them with a voltage alternating at two kilohertz, we get speeds from 50 to 180 revolutions per minute.鈥� At three 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. 鈥淵ou 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,鈥� he says.
One million revs
Mark Spearing, currently testing microturbines at the Massachusetts Institute of Technology, calls the fan 鈥渋ngenious鈥�, but says the speeds achieved are 鈥渞ather slow鈥�.
鈥淲e鈥檙e striving for in excess of one million rpm in our motor,鈥� he says.
Spearing has other concerns too. 鈥淚 am not a big fan of frictional or sliding contacts in micro electro-mechanical devices,鈥� he says. 鈥淔riction and wear tend to be potential show-stoppers at these scales.鈥�