Small wonder
on a table-top in Kazuo Hosokawa's laboratory is a small petri-dish filled with water. It contains a hundred or so small, flat silicon pieces. When Hosokawa shakes the dish, some of the little silicon pieces begin to clump together into something that looks similar to four-leaf clovers. To many, the thought of Hosokawa shaking his petri-dish may seem uninteresting and weird. But for him, this humble experiment is a step towards an ambitious goal. It would be nice, says Hosokawa, a mechanical engineer at the Institute of Physical and Chemical Research in Saitama, Japan, to be able to mix silicon and some metal bits and have them spontaneously assemble themselves into electronic components ( Discover , Vol 17, No 8).
Hosokawa and his colleagues at the University of Tokyo are still far from their original goal, but they have made an intriguing start using surface tension of water to compel silicon pieces to club together. Each piece of silicon shaped like a semicircle with a triangle protruding out on the flat side, is about 0.04-cm wide. The main aim of the researchers is to get four of these structures to link together, forming a clover shape. They have chosen this shape of the pieces for a good reason.
On a small scale, the surface tension of water is a force far more powerful than gravity. It acts on the silicon pieces somewhat like static electricity, making them stick together. But the amount of force it exerts depends on the shape of the object. Surface tension tends to minimise the surface area of the water in a confined space and one of the ways to minimise surface area is to minimise the deformation of the water surface around floating objects.
The water surface gets most deformed around sharp points. Hence, the researchers calculated that surface distortions would be minimised if the round edges of four linked pieces face outward, with the triangle on each piece pointing towards a common centre in the shape of a clover.
By gently shaking the petri-dish, Hosokawa and his colleagues have 30 out of 100 pieces spontaneously link up, forming clovers. They are trying to get better results by using pieces of several different shapes. "In the long run, the self assembling technique will help us to fabricate many microstructures at a high rate,' adds Hosokawa.