Shear-banding and superdiffusivity in entangled polymer solutions

TL;DR

This study reveals shear banding and superdiffusive tracer dynamics in entangled DNA solutions under large amplitude oscillatory shear, linking microscopic particle motion to macroscopic shear profiles.

Contribution

It provides new insights into the microscopic origins of shear banding and superdiffusivity in entangled polymer solutions using high-resolution confocal rheometry.

Findings

Shear profiles transition from Newtonian to shear-banding with increasing Wi.

Tracer particles exhibit superdiffusivity and Levy-walk behavior under shear.

Localized chain disentanglement likely causes abnormal tracer dynamics.

Abstract

Using high-resolution confocal rheometry, we study the shear profiles of well-entangled DNA solutions under large amplitude oscillatory shear (LAOS) in a rectilinear planar shear cell. With increasing Weissenberg number (Wi), we observe successive transitions from normal Newtonian linear shear profiles to wall-slip dominant shear profiles and finally to shear-banding profiles at high Wi. To investigate the microscopic origin of the observed shear banding, we study the dynamics of micron-sized tracers embedded in DNA solutions. Surprisingly, tracer particles in the shear frame exhibit transient super-diffusivity and strong dynamic heterogeneity. The probability distribution functions of particle displacements follow a power-law scaling at large displacements, indicating a Levy-walk-type motion, reminiscent of tracer dynamics in entangled wormlike micelle solutions and sheared colloidal glasses. We further characterize the length and time scales associated with the abnormal dynamics of tracer particles. We hypothesize that the unusual particle dynamics arise from localized shear-induced chain disentanglement.