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14 February 2012 Multiple-stream flow and mixing of dissimilar polymeric solutions in abrupt microfluidic contraction/expansion geometries
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In simple microfluidic contraction/expansion geometry, even a dilute polymeric solution is able to exhibit large upstream corner vortices and unstable entry flow patterns at high enough deformation rate (Deborah Number > 200). We have previously demonstrated a similar concept on multiple-stream flow of dissimilar viscoelastic solutions in planar microdevices containing abrupt contraction. Using the same test-vehicle, here we attempt to show that the elasticity ratio between two solutions plays an important role in entire flow kinematics (both upstream and downstream of a contraction) and thus the enhanced mixing of the two solutions. That is the upstream's stretching dynamics induced by the converging flow and the downstream's relaxation events are not exclusively responsible for the multi-stream flow kinematics but the elasticity ratio is also equally important. In this paper, the necessity of elasticity ratio for convective flow instability and the associated enhanced mixing were demonstrated experimentally. Our results show that the magnitude of the viscoelastically induced flow instability can be directly correlated to the energy discontinuity at the stream-stream interfaces at downstream of a contraction. These findings lay the foundation for optimizing the desired mixing quality via viscoelastic flow instability with negligible diffusion and inertial effects. This type of mixing can be achieved over short mixing length at relatively fast flow velocities (~101 mm/s) and is postulated to be easily integrated into μTAS platforms due to its simple design.
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Hiong Yap Gan and Yee Cheong Lam "Multiple-stream flow and mixing of dissimilar polymeric solutions in abrupt microfluidic contraction/expansion geometries", Proc. SPIE 8251, Microfluidics, BioMEMS, and Medical Microsystems X, 82510U (14 February 2012);

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