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.