Shear microscopy of the ‘‘butterfly pattern’’ in polymer mixtures

The concentration fluctuations induced in a semidilute polymer solution by shear flow are directly visualized for the first time. The spatial structure of the ‘‘butterfly pattern’’ observed by light scattering is elucidated, being the result of hydrodynamic rippling waves which have a distribution of tilt angles. The minimal tilt angle depends on shear rate in a power law fashion, causing the butterfly to almost close upon itself at high shear rates. The concentration waves have a time scale related to a shear rate γ̇∼1 ${\sec }^{\mathrm{-}1}$ and periodicity λ∼10 μm, indicating a link to reptation processes.