Mixing of two miscible liquids in T-shaped microdevices

Numerical simulations were performed to study the flow fields and mixing characteristics of liquid flows converging in a T-shaped micromixer, when the two inlet fluids are both water or water and ethanol. We showed that at smaller Reynolds number, Re < 100, mixing is controlled by transverse diffusion, and therefore by the residence times of each fluids. Accordingly, mixing ethanol and water is slightly easier than mixing water with water, due to the fact that, as ethanol is slightly more viscous than water and therefore it is slower, the residence time of water-ethanol mixtures is larger than that of the water-water case. On the other hand, at larger Reynolds number, mixing water and ethanol may take considerably longer, as the onset of engulfment is retarded and occurs at larger Reynolds number, namely increasing from Re ≅ 140 in the water-water case to Re ≅ 230 in the water-ethanol case. This is due to the fact that a water-ethanol mixture has a viscosity that is up to almost three times larger than that of water; therefore, at the confluence of the T-mixer, the water and the ethanol streams are separated by a quite viscous layer of a water-ethanol mixture, that hampers any vortex formation, thus retarding mixing.