Experimental and CFD modeling of fluid mixing in sinusoidal microchannels with different phase shift between side walls

In the present work, a passive model of a micromixer with sinusoidal side walls, a convergent-divergent cross section and a T-shape entrance was experimentally fabricated and modeled. The main aim of this modeling was to conduct a study on the Dean and separation vortices created inside the sinusoidal microchannels with a convergent-divergent cross section. To fabricate the microchannels, CO2 laser micromachining was utilized and the fluid mixing pattern is observed using a digital microscope imaging system. Also, computational fluid dynamics was applied with the finite element method to solve Navier-Stokes equations and the diffusion-convection mode in inlet Reynolds numbers of 0.2-75. Numerically obtained results were in reasonable agreement with experimental data. According to the previous studies, phase shift and wavelength of side walls are important parameters in designing sinusoidal microchannels. An increase of phase shift between side walls of microchannels leads the cross section being convergent-divergent. Results also show that at an inlet Reynolds number of <20 the molecular diffusion is the dominant mixing factor and the mixing index extent is nearly identical in all designs. For higher inlet Reynolds numbers (>20), secondary flow is the main factor of mixing. Noticeably, mixing index drastically depends on phase shift ( phi) and wavelength of side walls (.) such that the best mixing can be observed in phi = 3 pi/ 4 and at a wavelength to amplitude ratio of 3.3. Likewise, the maximum pressure drop is reported at f = p. Therefore, the sinusoidal microchannel with phase shifts between pi/ 2 and 3p/ 4 is the best microchannel for biological and chemical analysis, for which a mixing index value higher than 90% and a pressure drop less than 12 kPa is reported.