[youtube]http://www.youtube.com/watch?v=f2XQ97XHjVw[/youtube]
Cornstarch is famous as a shear-thickening fluid, leading to both sober and amusing experiments. If you were trying to transport a block that continuously excreted a thin layer of cornstarch, the harder you pulled it the more frictional force you would feel. Snails use the exact opposite strategy: they excrete a shear-thinning fluid that allows them to move even though they’re never actually in direct contact with the ground.
Shear-thickening fluids are also used in prototype bulletproof fabrics.
The normal model of friction at an interface posits that static friction perfectly counteracts any force applied to an object, up to the maximum value fsmax= µs N; then friction jumps to the smaller value fk= µk N and remains constant as long as the object is moving. This is equivalent to saying that the interface acts like an infinitely strong shear-thickening film up to the yield force of µs N, instantly shear thins to µk N and thereafter is shear neutral. This analogy is imperfect, however, because the behavior of a real shear-thinning or -thickening fluid depends strongly on its depth, while there is no equivalent idea of the depth associated with µs and µk.