Flow characterization and mixing performance of weakly-shear-thinning fluid flows in a microfluidic oscillator

•Microfluidic oscillators with rounder carve-out are preferred for mixing purpose.•Unique flow patterns are found due to the non-Newtonian characteristics.•Nearly-Shear-thinning fluid flow leads to slower and weaker flow oscillation.•Microfluidic oscillator can be used for flow metering of the nearly-shear-thinning fluid.•The Strouhal number is one order of magnitude lower. In this study, the characteristics and mixing performance of weakly-shear-thinning fluid flows through a microfluidic oscillator are investigated. The effects of three different geometric parameters are investigated: the radius of the concave curvature, the feature of the conjunct channel, and the edge shape of the cavity. Using Carbopol 934 aqueous solution as the working fluid, we quantify the mixing performance with fluorescence imaging, identify the flow patterns from particle tracking technique, and determine the frequency of flow oscillation from the pressure signal at different flow rate. For the microfluidic oscillator, rounding the corner of the bluff body extends the arc length of the concave cavity and thus increases the Görtler vortex instability, resulting in an enhancement of mixing. For Carbopol 934 solution, this effect actually dominates over the influence of the conjunct geometry, albeit that a concave cavity with smaller radius of curvature is usually preferred. Comparing to the Newtonian fluid flows, passing the weakly-shear-thinning fluid though a microfluidic oscillator leads to a slower oscillation with smaller amplitude. This diminishes the impact of flow instability on the mixing performance, and mass transport of the weakly-shear-thinning fluid replies more heavily on the advection effect. Due to the weakly-shear-thinning characteristics, flow patterns of the Carbopol 934 solution fluid in microfluidic oscillators are subtly different. However, the spectrum analysis reveal that the oscillation frequency increases linearly with the inlet velocity and flow metering remains possible when weakly-shear-thinning fluid is applied. Herein, we establish the functional relationship between the flow rate and the frequency for the Carbopol 934 solution so that the detected frequency can be employed to accurately estimate the flow rate.