A major difference between a solid and liquid is that if you move a knife through a solid, the cleft portions stay cleft, whereas in a liquid the two parts flow back together. Almost always, however, nature provides materials and processes that don’t quite fit into such neat categories.
Joseph Gladden (Univ Mississippi) and Andrew Belmonte (Penn State) have contrived an experiment in which a cylinder is dragged through a a mixture of water, soap, and certain salts. At small drag speeds, the material-a viscoelastic gel-like substance which is a fluid at these temperatures-does indeed close back on itself, as a liquid normally does.
(Other viscoelastic substances include blood clots, the earth's mantle, toothpaste, and gelatin.) At higher speeds, the cylinder creates more of a cleft and the material is slower to “heal” itself. At still higher velocities, the fluid acts like a solid, at least for a while; it is ripped into several parts, with separate surfaces, which take as long as a few hours to close up (see figures at http://www.aip.org/png/2007/279.htm), and it exhibits various “cracks” emanating from the cylinder’s wake.
Gladden (email@example.com, 662-915-7428) says that the phase diagram (cylinder speed versus cylinder diameter) for the fluid displays three regions: flow, modest tearing, and outright ripping. Mapping out this phase diagram should help in understanding other phenomena involving viscoelastic materials, Gladden says. (Physical Review Letters, upcoming article