A Three-Dimensional Micromixer Using Oblique Embedded Ridges

Abstract

A micromixer is one of the most significant components in a microfluidic system. A three-dimensional micromixer was developed with advantages of high efficiency, simple fabrication, easy integration, and ease of mass production. The designed principle is based on the concepts of splitting-recombination and chaotic advection. A numerical model of this micromixer was established to characterize the mixing performance for different parameters. A critical Reynolds number (Re) was obtained from the simulation results. When the Re number is smaller than the critical value, the fluid mixing is mainly dependent on the mechanism of splitting-recombination, therefore, the length of the channel capable of complete mixing (complete mixing length) increases as the Re number increases. When the Re number is larger than the critical value, the fluid mixing is dominated by chaotic advection, and the complete mixing length decreases as the Re number increases. For normal fluids, a complete mixing length of 500 µm can be achieved at a very small Re number of 0.007 and increases to 2400 µm as the Re number increases to the critical value of 4.7. As the Re number keep increasing and passes the critical Re number, the complete mixing length continues to descend to 650 µm at the Re number of 66.7. For hard-to-mix fluids (generally referring to fluids with high viscosity and low diffusion coefficient, which are difficult to mix), even though no evidence of strong chaotic advection is presented in the simulation, the micromixer can still achieve a complete mixing length of 2550 µm. The mixing performance of the micromixer was also verified by experiments. The experimental results showed a consistent trend with the numerical simulation results, which both climb upward when the Re number is around 0.007 (flow rate of 0.03 μm/min) to around 10 (flow rate of 50 μm/min), then descend when the Re number is around 13.3 (flow rate of 60 µm/min).

Keywords: chaotic advection; complete mixing; micromixer; vortex transverse flow.

Figure 1

(a) Structure and schematic diagram of the micromixer, the dimensions are in millimeters; (b) schematic diagram of design concept with streamlines of fluids in the micromixer.

Figure 2

Schematic diagram of design concept with procedures of fluid mixing illustrated through mixing patterns.

Figure 3

(a) Fabrication procedure of the micromixer using PDMS; (b) experiment setup.

Figure 4

(a) The mixing index of each cross-sectional plane in the micromixer at different Re numbers for the normal fluid; (b) complete mixing lengths versus Re number in the micromixer for the normal fluid.

Figure 5

The mass fraction contours of cross-section planes from simulation result at (a) Re number of 4.7 and (b) Re number of 66.7 for the normal fluid (mesh size 10 μm).

Figure 6

The various mixing indexes of cross-sectional plane versus mixing length at different Re numbers for the hard-to-mix-fluid.

Figure 7

Experimental results of mixing performance of (a) phenolphthalein-NaOH and (b) purple–green dye at various flow rates.

Figure 8

(a) Comparison of experimental and simulation results at a flow rate of 3 µL/min; (b) Comparison the trend of curve of complete mixing lengths between experimental and simulation results in the micromixer at variation of Re numbers.