Regardless if strained or compressed, the new material always expands.
CREDIT
Thomas Heine et al.
Abstract:
If you stretch an elastic band, it becomes thinner - a physical behavior that applies to most "common" materials. Since the 20th century, an opposite behavior has been known in materials research: The so-called auxetic (from ancient Greek auxetos, meaning 'stretchable') materials expand in the direction orthogonal to the strain. Likewise, they shrink when they are compressed. Scientifically, they are characterized by a negative Poisson's ratio. Probably the best known and oldest application of unusual Poisson's ratios is the bottle cork, which has a Poisson's ratio of zero. This has the effect that the cork can be put into the thinner neck of the bottle.