  Many metastable complex fluids such as colloidal glasses and gels show
distinct nonlinear viscoelasticity with increasing oscillatory-strain
amplitude; the storage modulus decreases monotonically as the strain amplitude
increases whereas the loss modulus has a distinct peak before it decreases at
larger strains. We present a qualitative argument to explain this ubiquitous
behavior and use mode coupling theory (MCT) to confirm it. We compare
theoretical predictions to the measured nonlinear viscoelasticity in a dense
hard sphere colloidal suspensions; reasonable agreement is obtained. The
argument given here can be used to obtain new information about linear
viscoelasticity of metastable complex fluids from nonlinear strain
measurements.
