Characterization of the mixing regimes of Newtonian fluid flows in asymmetrical T-shaped micromixers

• We characterize laminar flows in asymmetrical T-micromixers.
• Five different mixing flow regimes were identified.
• Asymmetry improves the mixing quality.
• Asymmetry reduces the transition Re between regimes.

Mixing at microscale is hindered by the difficulty in promoting turbulent flows. To overcome this, T-shaped micromixers, mainly with symmetrical inlets, have been used. Three micromixers with geometrically asymmetric inlets (weakly, moderately and strongly asymmetrical) and different heights are used herein to assess experimentally their mixing capacity for flows with Reynolds numbers (based on hydraulic diameter of the outlet channel) ranging from 50 to 310. The micromixers are manufactured with Polydimethylsiloxane by soft lithography. Images of the studied flows are acquired with a high-speed/high-resolution camera coupled to a microscope and digitally processed. The results allow identifying five different mixing regimes. The mixing efficiency for each regime is evaluated with two different parameters downstream the T-junction: one based on the local fluid concentration expressed by the gray-scale intensity of the corresponding pixel; the other, a complementary index, adds the potential diffusion mixing to the former through the calculation of the concentration gradients. The results show that the increase of the asymmetry level of the inlets improves the mixing quality and promotes the diminution of the transition Reynolds number between regimes. Additionally, the results evidence that the mixing regime characterized by a flow configuration with alternating layers of the two fluids is the most adequate one for a fast mixing since mass advection promotes a better mixing.