Response of shear in bulk orientations of charged DNA rods: Taylor- and gradient-banding

Abstract

Shear-induced instabilities leading to various kinds of inhomogeneous flow profiles play an important role in the processing of complex fluids, ranging from polymeric materials to various types of biological systems. In previously studied systems, either Taylor banding, or gradient banding, or fracture is observed. In the present work we study a system for which all instabilities occur in orientation textures (OTs), and where Taylor banding occurs simultaneously with gradient banding. The system here consists of crowded suspensions of long and thin DNA-based rods (at a low ionic strength of 0.16 mM salt), where the applied shear rate is systematically varied, for concentrations well below and above the glass-transition concentration (12.4 mg ml−1). To simultaneously measure the velocity profile along the gradient direction, in fracture and gradient banding, the optical cell is placed in a specially designed heterodyne light scattering set up, where the scattering volume can be scanned across the cell gap. The results confirm that Taylor bands and gradient banding occur in the concentration of DNA rods and applied shear-rates (35–80 s−1). Taylor bands clearly show the flow access in vorticity-direction, while the gradient banding is rearranged as thick rolling flows of OTs, at the middle shear-rate (50 s−1). The observations can be then useful to facilitate other biological complex fluids and the glass-forming liquids.