Rapid micromixer via ferrofluids

Performances of a micromixer based on ferrofluids are predicted numerically. A permanent magnet is used to induce transient interactive flows between a water-based ferrofluid and water. The external magnetic field causes the ferrofluid to expand significantly and uniformly toward miscible water, associated with a great number of extremely fine fingering structures on the interface in the upstream and downstream regions of the microchannel. These pronounced fingering patterns, which mimic the experimental observations of Wen et al. (2009), increase the mixing interfacial length dramatically. Along with the dominant diffusion effects occurring around the circumferential regions of the fine finger structures, the mixing efficiency increases significantly. The mixing efficiency can be as high as 95% within 2.0 s and a distance of 3.0 mm from the inlet of the mixing channel, when the applied peak magnetic field is 145.8 Oe. The proposed mixing scheme not only provides an excellent mixing, even in simple microchannel, but also can be easily applied to lab-on-a-chip applications with an external permanent magnet.