SHEAR MICROSCOPY OF THE BUTTERFLY PATTERN IN POLYMER MIXTURES

The concentration fluctuations induced in a semidilute polymer solutio n by shear flow are directly visualized for the first time. The spatia l structure of the ''butterfly pattern'' observed by light scattering is elucidated, being the result of hydrodynamic rippling waves which h ave a distribution of tilt angles. The minimal tilt angle depends on s hear rate in a power law fashion, causing the butterfly to almost clos e upon itself at high shear rates. The concentration waves have a time scale related to a shear rate gamma is similar to 1 sec-1 and periodi city lambda is similar to 10 mum, indicating a link to reptation proce sses.