Lee Tanner's Experiment in NiAl
Why does the pattern look tweedy? Why the cross-hatched stripes?
These materials are elastically anisotropic: it's easier to stretch them horizontally and vertically than to stretch them along a diagonal. (That's not so weird: after all, they're about to spontaneously change shape in one of those directions!) The elastic anisotropy of various shape-memory alloys varies from about a factor of 20 to a factor of 100.
One can show by a simple computation (equations don't work so well in Mosaic, though) that if we refuse to allow either bulk compression or diagonal stretches, the only allowed deformations are superpositions of two diagonal modulations. That is, if we make the elastic anisotropy infinite, tweed is the only way the material can wiggle!
James P. Sethna, email@example.com
Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).