Effect of edge disturbance on shear banding in polymeric solutions

TL;DR

This study investigates how edge disturbances influence shear banding in entangled DNA solutions, revealing that shear-banding flows can extend deep into the sample, indicating their bulk nature under oscillatory shear.

Contribution

It provides experimental evidence of long-range edge disturbance effects and clarifies the bulk behavior of shear banding in polymeric fluids under oscillatory shear.

Findings

Edge disturbance penetration depth is comparable to the gap size at Wi<1.

At Wi>1, shear banding flows extend much deeper than the gap.

Shear banding persists deep inside the sample, unaffected by edge effects.

Abstract

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.