  Edge instabilities are believed to be one of the possible causes of shear
banding in entangled polymeric fluids. Here, we investigate the effect of edge
disturbance on the shear-induced dynamics of well-entangled DNA solutions.
Using a custom high-aspect-ratio planar-Couette cell, we systematically measure
the velocity profiles of sheared DNA samples at different distances away from
the edge of the shear cell. Under a weak oscillatory shear with the
corresponding Weissenberg number (Wi) smaller than 1, where DNA solutions
exhibit linear velocity profiles with strong wall slip, the penetration depth
of the edge disturbance is on the order of the gap thickness of the shear cell,
consistent with the behavior of Newtonian fluids. However, under a strong
oscillatory shear with Wi > 1 that produces shear-banding flows, the
penetration depth is an order of magnitude larger than the gap thickness and
becomes spatially anisotropic. Moreover, we find that the shear-banding flows
persist deep inside the sheared sample, where the effect of edge disturbance
diminishes. Hence, our experiments demonstrate an abnormally long penetration
depth of edge disturbance and illustrate the bulk nature of shear-banding flows
of entangled polymeric fluids under time-dependent oscillatory shear.
