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 (~10<sup>1</sup> mm/s) and is postulated to be easily integrated
into μTAS platforms due to its simple design.
Fluid flow in microfluidic systems can be achieved by electroosmosis (EO) pumping, with its own unique characteristics and advantages. In practice, multi-fluid (one fluid displacing another fluid) flows are frequently encountered. Understanding of multi-fluid EO flow associated with non-uniform liquid properties is of importance to precise flow control. This paper reports an EO-driven, two-fluid displacement flow in a microcapillary. The electrical current
monitoring method is adopted for investigating the dynamic flow response. The nonlinear change of the electrical current with time under a constant applied voltage is observed during the displacing processes. The theoretical and experimental results validate the hypothesis that the non-uniform zeta potential and electric field induce local pressure gradients in the two different fluids. This results in the deviation of the velocity flow profile from the ideal plug-like flow profile expected for EO flow. The model predictions agreed well with the experimental data when a low concentration fluid displaces a high concentration fluid, but not vice versa. The time of displacement, and thus the flow velocity, is found to be dependent on the displacing flow direction, which is hitherto not reported. The underlying mechanisms are postulated, but require further investigation.