Direct measurements of viscoelastic flows of DNA in a 2:1 abrupt planar micro-contraction

Direct measurement techniques are employed to quantify the kinematics of DNA flows in a 2:1 planar abrupt micro-contraction. Flow through micro-contractions subjects the fluid to large spatial gradients in velocity, leading to viscoelastic effects. This study investigates the flow of semi-dilute λ-DNA solutions in a 2:1 micro-contraction for low Reynolds numbers (6.0 × 10−7 < Re < 9.8 × 10−2) and high Weissenberg numbers (0.8 < Wi < 629). We thus access high elasticity number flows (6.4 × 103 < El = Wi/Re < 1.4 × 106); flows with El > 105 have been previously unexplored in experiments. Video microscopy and streak images reveal highly elastic behavior evidenced by the presence of large, stable vortices symmetric about the centerline in the corners of the contraction. Digital particle image velocimetry (DPIV) is used to obtain high resolution, quantitative measurements of the velocity fields and in particular the vortex growth in this micro-contraction flow. We believe these are among the first measurements of this type in a viscoelastic microflow and the first involving DNA solutions. Knowledge of the fundamental physics that governs viscoelastic microflows will have a profound impact on optimization of lab-on-a-chip systems incorporating macromolecular flows.