1. Polymers and Complex Fluids, Department of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, U.K., Department of Physics, University of Patras, GR 26500, Greece, Department of Physics, Boston University, Boston, Massachusetts 02215, Institute Laué Langevin, BP 220, Grenoble, F-38043, France, and Department of Gastroenterology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215