The enhance mixing by a four-paddle rotor placed in the junction of a three-dimensional Y-shape micro-mixer, whose dimensions are miniaturized of order tens micrometers, was investigated by numerical simulation. The working conditions of the mixer are: low Reynolds (Re∼0.001), high Schmidt (Sc∼10E5) numbers, rotation speed of the rotor ranging from 100–800 RPM (round per minute). Flow visualization shows that the rotor created circular, helical flows in the region closed to the rotor and transversal velocity in the mixing channel. However, the latest decrease by a power law and is confined in downstream distance defined by the rotor radius scale, and produces effective diffusion process. The two priors build a considerable uniform concentration distribution compared with the without rotor case. Therefore, the effective mixing process by the rotor was observed in the junction and a small distance from the rotor in the mixing channel. However, the mixing improvement in the later region is not significant. Out of this region, the mixing behavior is the same as solely diffusion process based on the diffusion coefficient of the reagent. The process occurring during mixing of fluid in the junction bases on stretching by the motion of the rotor and on folding by the rotor shape. However, the folding mechanism seems to be disappeared at high RPM and has inconsiderable role at low RPM. The mixing efficiency on the cross sections located just after the rotor and located where the rotor has no further effect depends logarithmically on RPM. Overall view, the mixing efficiency of the rotor archives 60–90%. The fluid torque exercising on the rotor is well self-similar to the frequency of the paddle tip interval by a power law. The results of this study can be used for efficient design a micro-mixer.

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