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.
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We thank Y. Qiao, X. Ma, P. Agrawal, H. Chuang, G. Richards, and W. Sweeney for help in preparation of DNA samples, D. Giles for sample characterizations, S. L. Biswal for discussion of DNA-bridged dumbbell synthesis, C. Macosko for discussion of sample characterization, and B. Leahy for discussion of anisotropic diffusivity. This work was supported by the NSF-CBET 1700771.
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