Concentration fluctuations in sheared polymer solutions

When subjected to shear flow, polymer solutions can exhibit strongly enhanced concentration fluctuations, leading to an observed increase in light scattering with a characteristic angular dependence. This is attributable to the coupling of polymer concentration to the polymer elastic stress. We have developed a phenomenological model to study the polymer concentration fluctuations in the presence of flow. We use the two-fluid model (separate polymer and solvent flow fields) to obtain Langevin equations for concentration, velocity, and a defined polymer strain. The constitutive equation employed is a simplified Doi-Edwards model derived from reptation theory. We will present results for the structure factor under steady-state shear for scattering wave vectors in the q x q y and q x q z planes, where x=flow direction, y=gradient direction, and z=vorticity direction. The results are compared with scattering measurements. We have also studied the effects of a small oscillatory shear added to the steady shear