RHEOLOGY OF COMPLEX FLUIDS MEASURED BY DYNAMIC LIGHT-SCATTERING

We introduce a method for using dynamic light scattering to measure th e frequency-dependent linear viscoelastic moduli of complex fluids. Th e technique exploits the fluctuation dissipation theorem, which relate s the relaxation of thermal excitations of a probe particle to the vis coelastic properties of the surrounding medium. The relaxation of the thermal excitations of probe particles are determined by measuring the time evolution of the mean square displacement using dynamic light sc attering. A Langevin equation with a time-dependent damping term is us ed to relate this mean square displacement to the dynamic shear modulu s of the medium. This method probes the linear viscoelastic moduli ove r a much larger frequency range than traditional mechanical means, and in particular, easily extends their measurement to much higher freque ncies.