Dynamics of DNA-Bridged Dumbbells in Concentrated, Shear-Banding Polymer Solutions

Abstract

Although experimental evidence for shear-banding flows in concentrated polymer solutions has accumulated over the last 20 years, the origin of such shear-banding flows is still under heated debate. Experiments that probe the microscopic dynamics of shear-banding polymer solutions are still scarce. Here, using a custom-built high-resolution rheo-confocal shear cell, we experimentally study the dynamics of DNA-bridged colloidal dumbbells in the shear-banding flow of concentrated double-stranded DNA (dsDNA) solutions under large amplitude oscillatory shear. We synthesize dumbbells consisting of two spherical colloids linked by λ-DNA and track their 2D-projected configurations in sheared dsDNA solutions. We first confirm that the velocity profile of the concentrated dsDNA solutions is inhomogeneous at high Weissenberg numbers and exhibits strong shear banding with two distinct shear bands. We then measure the orientational distribution of the DNA-bridged dumbbells and investigate their translational and rotational dynamics within the two shear bands. The preferred alignment of the dumbbells along the flow direction in the high-shear-rate band suggests the dominant role of elastic stresses in that band. In contrast, a bimodal distribution of dumbbell orientations is observed in the low-shear-rate band, indicating more balanced contributions from both normal and elastic stresses. Furthermore, exclusively in the high-shear-rate band, we also find spatially localized correlated enhanced translational and rotational motions and a strong coupling between enhanced translation and chain extension. These unique conformational and dynamic features suggest shear-induced breakage of the local entanglement network in the high-shear band, which we postulate leads to puddles of low viscosity within an otherwise high-viscosity fluid. Together, our quantitative analyses of the spatially distinct dynamics of dsDNA-bridged dumbbells in coexisting shear bands provide important insights into the microscopic origin of shear-banding flows in concentrated polymer solutions.

Shear Banding in Entangled Polymers: Stress Plateau, Banding Location, and Lever Rule

“…The generality of shear banding in entangled polymer solutions was established in a series of experiments by S.-Q. Wang’s group − and other groups. − Wang et al attributed shear banding to “cohesive failure” or “elastic yielding” of the entangled polymers. However, several authors challenged the “elastic yielding” mechanism and instead suggested that the observed shear banding was due to a flow instability initiated at the edge of the apparatus or strong secondary flows. − …”

Microstructural evolution and reverse flow in shear-banding of entangled polymer melts

“…Despite advances, single-chain microscopy must be refined in order to overcome optical tracking challenges and simultaneously track multiple macromolecules on relevant time scales for use in dense polymeric fluids. [42][43][44] Recent experiments by Shin et al 45 on entangled polymeric solutions via fluorescent DNAbridged dumbbell probes have overcome some of the experimental issues. These experiments have revealed key details of the physical mechanisms leading to the emergence of shear bands, such as an inhomogeneous flow-induced disentanglement of the network leading to higher orientation within the fast band, which have largely confirmed the basic molecular mechanism of shear banding in entangled polymeric melts put forth by Mohagheghi and Khomami.…”