Non-linear dynamics of semi-dilute polydisperse polymer solutions in microfluidics: effects of flow geometry

The non-linear dynamics of a semi-dilute (c/c* = 15) polydisperse polyethylene oxide (PEO) solution in microfluidics are studied experimentally using benchmark contraction–expansion flow geometries with three contraction–expansion ratios (4:1:4, 8:1:8 and 16:1:16) and two narrow channel lengths (Lc/Dh = 53 and 5.3, where Lc is the length of the narrow channel and Dh is its hydraulic diameter). Complex flows over a range of elasticity numbers (El), Weissenberg numbers (Wi) and Reynolds numbers (Re) are characterized using micro-particle image velocimetry (μ-PIV) and pressure drop measurements. The evolution of vortex formation and dynamics has been visualized through a step-flow-rate experiment. Various flow dynamics regimes have been quantified and are presented in a Wi–Re diagram. The experimental results reveal that the contraction ratio can result in qualitatively different vortex dynamics of semi-dilute polymer solutions in microfluidics, whereas the length of the narrow channel Z. Li · X.-F. Yuan (B) Manchester Interdisciplinary Biocentre and School of Chemical Engineering and Analytical Science, University of Manchester, 131 Princess Street, Manchester, M1 7DN, UK e-mail: [email protected] S. J. Haward · J. A. Odell HH Wills Physics Laboratory, The University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK S. Yeates School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK merely affects the dynamics at a quantitative level. A single elasticity number, if defined by the size of the narrow channel, is not sufficient to account for the effects of contraction ratio on the non-linear vortex dynamics.