ABSTRACT
There are a number of soft materials that yield when subjected to a moderate shear stress. To a first approxi mation, the response of the material is elastic below some yield stress and plastic above. Some of these materials have a crystalline structure, and the yielding originates in the motion of dislocations [1-3]. But many other ma terials, such as concentrated emulsions and colloidal sus pensions, foams, and clays, have an amorphous structure and also exhibit a yield stress [4]. The origin of the yield stress in these glassy materials is not well understood.
