Elastic secondary flows in sharp 90 degree micro-bends: A comparison of PEO and DNA solutions

The growth or absence of elastic secondary flows is documented for flows of dilute and semi-dilute polymer solutions in sharp 90 degrees micro-bends in channels of rectangular cross-section; secondary flows are not present for Newtonian flows under similar conditions. Flow visualization shows that a vortex is present in the inner, upstream corner of the bend and grows with increasing Reynolds (Re) and Weissenberg (Wi) numbers for flows of shear-thinning, semi-dilute polymeric solutions containing lambda-DNA (9.9x10(-7)< Re < 3.1x10(-2), 0.42 < Wi < 126) or high molecular weight poly(ethylene) oxide (PEO) (3.5x10(-4)< Re < 4.7x10(-3), 1.8 < Wi < 17.7). Rheological differences, likely due to differences in the flexibility of DNA and PEO, influence the degree of vortex enhancement with increasing Wi. The vortex is absent for flow of a dilute, non-shear-thinning PEO solution over a large Re and Wi range (3.3x10(-4)< Re < 1.6x10(-2), 1.1 < Wi < 52.8) that includes conditions where vortices are observed for the semi-dilute, shear-thinning solutions. Hence, shear-thinning appears to be central to the presence of an elastic secondary flow in this geometry.