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Putting the squeeze on foam.

Putting the squeeze on foam

Stretch a rubber band, and the bandgets longer and thinner. Squeeze a sponge in one spot, and it bulges somewhere else. That's the way practically all spongy, porous materials behave. Now a biomedical engineer has created a flexible foam that does the opposite. His novel foam structures expand laterally when stretched and contract when squeezed.

"This whole business is a bit counterintuitive,'says Roderic Lakes of the University of Iowa in Iowa City. "A lot of people think it should be impossible to do that. However, once you have the idea, it's not all that difficult to execute.' His report appears in the Feb. 27 SCIENCE.

This foam's properties depend not onthe material's chemistry but on its structure. Each cell in the material looks somewhat like a distorted cube whose faces have been pushed in to give it the appearance of a collapsed, 24-sided polyhedron (see diagram). If each face had a little tab, then pulling tabs on opposite sides of the cube would also force the remaining sides to spring outward. The actual cell structure found in the new foam works in the same way but is much more convoluted and contains ribs that are bent and protrude into the cells.

So far, Lakes has found ways to makethese "reentrant' foams, as he calls them, from polyester, silicone rubber and even copper metal. In the case of polymers, the method involves squeezing the material while it's hot, then cooling it to freeze in the required structure.

The new foams turn out to be tougherand more resilient than conventional foams made from the same material. "It is anticipated,' says Lakes, "that reentrant foams may be used in such applications as sponges, robust shock-absorbing material, air filters and fasteners.'

Would such a foam make a good noveltyitem for kids to play with? Probably not, says Lakes. "Children don't have enough experience to think that if things are stretched they should always get skinnier.' They wouldn't find the new foam's behavior surprising.
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Title Annotation:flexible foam developed
Author:Peterson, Ivars
Publication:Science News
Date:Mar 14, 1987
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