Mixing and hydrodynamic analysis of a droplet in a planar serpentine micromixer

In this work we combined numerical simulation with molecular-diffusion effect, high-tempo micro-particle image velocimetry (μ-PIV), and probability distribution function (PDF) analysis to investigate the chaotic mixing and hydrodynamics inside a droplet moving through a planar serpentine micromixer (PSM). Robust solutions for the distributions of interface and concentration of the droplets were obtained via computational fluid dynamics. The simulated fluid patterns are consistent with those measured with μ-PIV, which serves as a powerful nonintrusive diagnostic approach to observe the droplets. Two mechanisms are proposed to enhance the performance of mixing in a PSM—the deformation of droplets and the asymmetric recirculation within the droplets. On introducing alternating cross sections into a winding channel, this specific design of PSM is found to amplify the fluid disturbance and maximum vorticity difference. Data show that the PDF of the vorticity fields is modified and the fraction with larger vorticity is increased. Accordingly, the PSM is capable of achieving a mixing index 90% within 700 μm (Re = 2), which is eight times better than for a straight microchannel. The results not only demonstrate explicitly the fluid patterns within the droplets but also provide significant insight into the factors dominating the mixing efficiency.