Ripping fluids

Viscoelastic materials are known to exhibit both solid-like and fluid-like behavior under conditions of restricted flow or deformation. The Pennsylvania State University’s Andrew Belmonte and Joseph Gladden (now at the University of Mississippi) have done the first study of how one such material, a concentrated solution of surfactants and organic salt, responds to something more drastic: a blunt cutting tool. When a cylinder is slowly dragged through the material, the material flows smoothly around it, as a liquid normally does. At higher speeds, a small cylinder creates a cleft that lingers long enough for air bubbles to become trapped in the material, whereas a larger cylinder creates a tear with a jagged shape like what’s observed in torn sheets of rubber and plastic (see Physics Today, February 2007, page 33 ). In that case, however, the torn surface heals itself within a few hours. What underlies all this behavior is the aggregation of the surfactant molecules. Usually, and in their most familiar context as detergents, surfactants cluster together by their hydrophobic tails in spherical structures called micelles. But with the addition of the organic salt, the surfactant molecules reorganize into long, flexible tubes—worm-like micelles—again with their tails inward and hydrophilic heads pointing outward. The resulting gel-like material is held together not by covalent chemical bonds but by the much weaker interactions among the surfactant molecules. (J. R. Gladden, A. Belmonte, Phys. Rev. Lett. 98 , 224501, 2007 .)