Re-entrant Foam

Materials that expand in width when stretched in length are said to have a negative Poisson's ratio. Such materials are of interest as shoe insoles to reduce impact force in running. Most materials have a positive Poisson's ratio. Consider a stopper in a wine bottle. Rubber, with a postive Poisson's ratio, could not be used for this purpose because it would expand when compressed into the neck of the bottle and would jam. Cork, by contrast, with a Poisson's ratio of nearly zero, is ideal in this application.

Clicking a thumbnail on this web page (http://chemistry.beloit.edu/edetc/cineplex/foam/index.html) shows a movie of that demonstration.

Does the cross-section of these foam samples become wider or thinner during the stretching? The second sample has been heat treated while compressed. The explanation of this remarkable effect is that the combination of squeezing and heat causes the pores to, in effect, implode. Stretching in any direction will cause the entire imploded pore to open up, leading to the macroscopic effect of seeing a bulge in the cross-section when the sample is stretched.

Below are several other items that behave similarly. Click on the picture to see the movie.

An assemblage of Lego blocks.

A Hoberman Sphere sold in toy stores.

The Hoberman sculpture at the California Science Museum.

Climbing out of a Hoberman sphere.

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