Efficient mixing enhancement by orthogonal injection of shear-thinning fluids in a micro serpentine channel at low Reynolds numbers

Viscoelastic fluids have been recently proposed to promote mixing effect in microfluidic systems, where limited success has been obtained. However, the flow and mixing mechanism of a viscoelastic fluid is still poorly understood, particularly regarding the roles of injection on mixing at low Reynolds numbers. In this work, an efficient mixer by orthogonal injection into a primary flow is proposed. The mixing behaviors of shear-thinning fluids in a serpentine channel with an orthogonal injection are investigated experimentally. Dye visualization and concomitant statistical analysis are conducted to quantitatively characterize the mixing performance and to reveal the flow kinematics. Enhanced mixing is achieved just over a short effective mixing length. The probability distribution functions (PDFs) analysis shows that the mixing efficiency, defined by the normalized concentration of dye intensity (Gan et al., 2007), can be significantly improved from 22% for the Newtonian fluids to 69% and 76% by using a shear-thinning fluid with polymer concentrations of 25 and 50 ppm, respectively. The decay exponents of shear-thinning fluids plateau at −3.4 indicates the occurrence of elastic instability. The relative-change-ratio fluctuations illustrates that the observation area can generate sufficient elastic stress to induce flow instabilities, resulting in effective mixing in the channel.