Analyzing flow behavior of shear-thinning fluids in a planar abrupt contraction/expansion microfluidic geometry

We compare the flow behavior of several shear-thinning fluids of xanthan gum (XG) and polyethylene oxide (PEO) with a relatively similar shear-thinning index (similar to 0.34-0.50) in a microfluidic planar contraction/expansion geometry. We explore vortex formation and growth near the abrupt constriction over a wide range of Re (0.0046 less than or similar to Re less than or similar to 3.7) and effective Weissenberg (134 <= Wi(eff) less than or similar to 9381). An important aspect of the present study is careful rheological characterization to determine the relaxation time of aqueous solutions. From flow visualization experiments, we observed that corner vortices form in the weakly elastic aqueous solutions upstream of the constriction. In the PEO solution, a Newtonian-like behavior (with no corner vortex) was observed up to Re = 0.0067, and the vortex length remained relatively unchanged up to much higher flow rates (Re = 0.4). The vortex growth mechanism was observed at higher flow rates. When Re >= 1 the flow became time dependent and chaotic. In xanthan gum solutions, the initial vortex appeared at Re as low as 0.0046, and the vortex length grew by increasing the Re, and Wi(eff). Interestingly, at similar Re, the vortex was much longer than the one in the PEO solution and the flow remained stable over the entire range of flow rates studied. We believe fluid elasticity is central in the vortex formation and growth. The onset of vortex formation and growth shifted to lower Re and Wi by increasing the elasticity number (El = Wi/Re), revealing the importance of fluid elasticity over inertia in the formation of upstream corner vortices. While chaotic flow instabilities can offer advantages in micromixing, heat, and mass transfer by employing elastic non-Newtonian fluids, instabilities have to be prevented in many other applications, including microrheometry, inkjet printing, and roll coating. Understanding the onset and mechanisms of flow instabilities can shed light on designing more efficient industrial processes and optimal products.