Problem: Microfluidic mixers produce highly accurate solute mixing between fluid channels by cross-stream diffusion in laminar viscous dominated flow. What is the appropriate mixer configuration to achieve a desired mixing between pure and salt water channels?

Approach: We developed a coupled CFD species transfer model of the mixer in COMSOL. The model has bi-directional coupling due to the dependence of the viscosity of the carrier fluid on the solute concentration. In the microfluidic mixer shown, the upper channel has pure water and the lower channel has water with a salt solute concentration of 1 mol/m3.

Solution: The mixer model permitted design of the mixing channel to reach the targeted concentration. The mixer in this example is 10 µm wide and 30 µm long, smaller than the thickness of a human hair, and the fluids enter the device with a constant velocity of 0.1 m/s.

Salt concentration in microfluidic mixer