Mixing processes of two liquids were investigated by visualizing the mixing when they were simultaneously injected in a micro-mixer with lithographically fabricated Y-shape flow paths, and the mixing phenomena was analyzed in detail. To visualize the mixing, flows were observed by an optical microscope, and a clearly detectable chemical reaction was utilized. As the two liquids, a transparent aqueous solution of a strong alkali and a phenolphthalein ethanol solution were used. When they were simultaneously injected in Y-shape flow paths of a micro-mixer, they flowed at first in parallel along the joined path as laminar flows. This is because the Reynolds’ number became very small caused by the narrow flow-path widths of 50-100 μm. However, because two liquids were always contacted at the boundary, they were gradually mixed by diffusion, and the color of the mixed parts changed to vivid red. For this reason, it was able to measure the diffusion distance from the flow path center. Because the flow speeds were much faster than the diffusion speeds, the area colored in red did not depend on the time but depended on the distance from the joint point. It was known that the distance from the joint point corresponded to the time for mixing the liquids by the diffusion. It was clarified that the diffusion distance x was proportional to the square root of the diffusion time t or the distance from the joint point. The calculated diffusion coefficient D was (0.87-1.00)×10-9 m2/s.
A new method to print patterns with very sharp vertical side walls in thick negative resist SU-8 was investigated. In addition, the technology was swiftly applied to fabrication of microfluidic devices. At first, 50-μm line-and-space patterns were printed using SU-8 with a thickness of 100 μm. When the best focal position for the thin resist film with a thickness of approximately 1 μm is defined as the focus origin, vertical sidewalls were obtained at defocus positions of between +2,400 μm and +3,000 μm. The profiles became the best at the defocus of +2,400 μm. Here, the plus defocus means that wafers were lowered far from the lens. It was considered that the excellent profiles were obtained because the light intensity decrease caused by the absorption in the resist was just balanced with the degradation and extent of positional light-intensity distribution caused by the intentional defocus. Using the technology, various flow- path patterns of microfluidic devices were successfully fabricated.