Effects of contraction ratio on non-linear dynamics of semi-dilute, highly polydisperse PAAm solutions in microfluidics

Highly non-linear flows of semi-dilute (3.3≤c/c*≤16.6) polyacrylamide (PAAm) aqueous solutions through three micro-fabricated 4:1:4, 8:1:8 and 16:1:16 contraction-expansion geometries have been investigated by micro-particle imaging velocimetry (μ-PIV) and pressure drop measurements. A set of quantitative data for PAAm solutions are obtained from: Gel Permeation Chromatography (GPC) analysis; rheometric characterization using Vilastic-3 and piezoelastic axial vibrator (PAV) with frequency up to 6650. Hz for linear viscoelasticity measurements, and using a microfluidic device for the measurement of the nonlinear shear viscosity. With three different flow geometries, and a wide range of Elasticity numbers (14.7≤El≤476.8) and Weissenberg numbers (1.4≤Wi≤131.7), different nonlinear flow regimes are identified and correlated to vortex growth mechanisms. Results of similar El flows were mapped in a contraction ratio - Wi diagram. We found that under a given flow geometry tuning El can either increase or decrease the stability of the nonlinear flow structures. In addition to Wi and Re, the viscoelastic flow phenomena are dependent on the contraction and aspect ratios of flow geometry, and also sensitive to the molecular weight distribution of polymer. These results are of significance to the further development of microfluidic technologies. © 2011 Elsevier B.V.