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.
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.