This property can be exploited to build a liquid mixer that has a fluid output with a highly accurate solute concentration. In these microfluidic mixers, two flows with different solute concentrations are brought into direct contact in a mixing chamber. The laminar flow within the chamber assures that the two liquid streams will not mix, however solute will travel between the flows by cross-stream diffusion. The amount of solute that diffuses from the concentrated stream to the dilute stream can be precisely controlled by controlling the time each fluid spends in the chamber before exiting. Computing these concentrations analytically proves difficult, however, because the viscosity of the carrier fluid is often heavily sensitive to the solute concentration, making the fluid flow problem bi-directionally coupled to the diffusion problem.

In this example, two fluids are brought into contact in a mixer 10 µm wide and 30 µm long, smaller than the thickness of a human hair. The fluids enter the device with a constant velocity of 0.1 m/s. The carrier fluid in this analysis is water, and the solute is a salt. The top stream enters the chamber pure, with no salt in solution, while the bottom stream enters with a solute concentration of 1 mol/m3.

The steady-state solution to this coupled fluid flow and convection-diffusion problem was solved using CFD and is shown above. The mixer has two outlets, the top of which has a controlled solute concentration of 0.212 mol/m3 and the bottom of which has a concentration of 0.783 mol/m3. Fluid streamlines are also plotted on the concentration graph to illustrate the lack of fluidic mixing in the diffusion chamber.