Effect of edge disturbance on shear banding in polymeric solutions

Edge-induced instabilities have been suggested as one of the possible causes of experimentally observed shear-banding in well-entangled polymer solutions/melts. Using a high-aspect-ratio planar-Couette shear cell, we study the penetration length (L) of edge disturbance and the development of bulk shear profiles in highly entangled DNA solutions by measuring shear profiles while varying the locations from near-edge to center of the shearing plates. Under a weak oscillatory shear flow, where the corresponding Weissenberg number (Wi) > 1 and the DNA solutions display linear shear profiles with strong wall-slip, we find that L is comparable to the gap (H) between the plates. On contrary, under a stronger oscillatory shear (Wi > 1) that produces shear-banding profiles, L is an order-of-magnitude larger than H and the region of the stabilized shear-banding profiles becomes anisotropic. Moreover, a well-developed shear-banding profile persists farther away from the edge, where the edge effect is negligible, which implies its true bulk nature. Our results clarify a surprisingly long penetration of the edge disturbance and the bulk nature of shear-banding flows in entangled polymeric solutions under large amplitude oscillatory shear.