DIFFUSING-WAVE-SPECTROSCOPY MEASUREMENTS OF VISCOELASTICITY OF COMPLEX FLUIDS

We present a new use of dynamic light scattering that permits the dete rmination of the viscoelastic behavior of a complex fluid. By describi ng the motion of a scattering particle in a viscoelastic medium in ter ms of a generalized Langevin equation with a memory function, we relat e the time evolution of its mean-square displacement to the frequency- dependent storage and loss moduli of the medium. The utility of this t echnique is illustrated through the application of diffusing-wave spec troscopy to probe the viscoelastic behavior of two complex fluids. The properties of a concentrated suspension of colloidal particles intera cting as hard spheres are shown to be strongly influenced by the incip ient colloidal glass transition, which leads to an extended range of f requencies over which they behave like an elastic solid. Similar elast icity is observed in a compressed emulsion, resulting in this case fro m the additional interfacial energy of the deformed droplets. In both cases diffusing-wave spectroscopy is used to measure the frequency dep endence of the storage and loss moduli, and these results are compared with those from mechanical measurements. Besides providing a purely o ptical method for measuring mechanical properties, this technique prov ides new insight into the origin of the viscoelastic behavior. (C) 199 7 Optical Society of America.