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° 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 λ -DNA ( 9.9 × 10 − 7 < Re < 3.1 × 10 − 2 , 0.42 < Wi < 126 ) or high molecular weight poly(ethylene) oxide (PEO) ( 3.5 × 10 − 4 < Re < 4.7 × 10 − 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 ( 3.3 × 10 − 4 < Re < 1.6 × 10 − 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.