Abstract

Complex fluids are often exhibit unusual rheological properties, reflecting the dynamics of the structures within the fluid. These dynamics are typically parameterized by the complex dynamic shear modulus, G*(co), which is a function of frequency, co. The real part of the modulus, G'(co), determines the magnitude of the in-phase response of the medium to an applied oscillatory shear, and thus reflects the elasticity or energy stored by the material. By contrast, the imaginary part of the modulus, G"(<u), determines the magnitude of the out-of-phase response of the medium to an applied oscillatory shear, and thus reflects the energy dissipated, or viscous loss of the material. The shear modulus of a complex fluid is critical in determining its mechanical properties; these are the properties that most directly affect the appearance and fundamental nature of the material. However, since the elastic modulus directly reflects the response of the material to a shear excitation, and since light scattering directly probes the response of the material to a longitudinal excitation, there has traditionally been a complete separation between the two properties; light scattering has probed the dynamics of the material which reflect the diffusional motion of the constituents, while their rheological properties are probed using mechanical methods that entail the direct application of a shear strain and the measurement of the resultant shear stress, or through the application of a shear stress and the measurement of the resultant shear strain. The connection of these mechanical properties with those measured with light scattering has traditionally been tenuous at best. However, in principle, one might expect a rather more direct relationship since both forms of excitation, longitudinal or transverse, are probing the response of the same material. In this paper, we show that there is a direct connection between these excitations, and introduce a technique by which dynamic light scattering can be used to directly probe the shear rheology of a complex fluid.

© 1996 Optical Society of America