Enhanced concentration fluctuations in polymer solutions under shear flow

Abstract

We report an elastic-light-scattering experiment for a semidilute polymer solution under a uniform laminar shear flow. The concentration fluctuations are greatly enhanced by shear flow and their structure is highly anisotropic in both the weak (\ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{}${\mathrm{\ensuremath{\tau}}}_{\mathit{d}}$\ensuremath{\ll}1) and strong (\ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{}${\mathrm{\ensuremath{\tau}}}_{\mathit{d}}$\ensuremath{\gg}1) shear regimes, where \ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{} is the shear rate and ${\mathrm{\ensuremath{\tau}}}_{\mathit{d}}$ is the longest relaxation time of the system. The anisotropy is qualitatively different from binary liquid mixtures under a shear flow as a result of the strong coupling between the concentration fluctuations and the hydrodynamic flow.