Shear thinning and tumbling dynamics of single polymers in the flow-gradient plane

The conformational changes imparted on single, flexible DNA polymers by a steady, simple shear flow were directly visualized in the flow-gradient plane. Two fluorescently stained DNA double-strand sizes of 22 mum and 80 mum in contour length were employed, and Wi values of up to 584 were probed (Wi = shear rate x longest polymer relaxation time). By exploitation of the linear proportionality between polymer density and its recorded image, the accessible radius of gyration tensor elements (G(ij)) were measured. Of those, the ensemble-averaged (G(22)) and (G(12)) were related to the bulk shear viscosity and first normal stress coefficient, respectively, via the Giesekus stress tensor. We found their respective behaviors to follow power-law decays of Wi (-0.52) and Wi (-1.28) at large Wi. Polymer dynamics were also investigated. Like rigid ellipsoids of revolution, polymers displayed a constant partition between positive and negative orientations irrespective of shear rate at Wi >> 1. Unlike them, however, polymers preferred positive orientations, spending there similar to 75% or their time vs 50% for rigid ellipsoids. End-over-end tumbling was observed, confirming a long-standing prediction and numerous single-chain computer simulation studies. The tumbling frequency followed Wi(0.62), and an equation was derived from simple advection and diffusion arguments to reproduce these observations.