Direct Observation of Polymer Chain Tumbling in Entangled Solutions Under Shear Flow

Topological constraints in entangled polymeric liquids dramatically influence chain dynamics under large, nonlinear deformations. A fundamental question in polymer physics focuses on how single polymer chains stretch and deform when highly entangled systems are subjected to strong shear flow. In this talk, I will discuss our recent work on single-molecule imaging of entangled polymer chains in transient and steady shear flow using a custom-designed shear flow device for fluorescence microscopy. Our results reveal striking dynamic heterogeneity in chain stretching and conformational dynamics during both startup and steady-state shear flow. Under strong shear flow (Wi > 1 based on the longest Rouse relaxation time), individual polymer chains frequently undergo end-over-end tumbling despite the highly entangled environment. Comparisons with polymer chain dynamics in dilute and semidilute unentangled solutions suggest that strong shear flow may induce local chain disentanglement, challenging conventional tube-based models of entangled polymer dynamics under nonlinear deformation. These findings highlight the need to consider the assumptions of existing theoretical frameworks and provide new insight into the nonlinear rheological response of entangled polymers in shear flow.