Measurement of diffusion coefficients by observing mixing of two liquids simultaneously injected in a micro-mixer

Mixing of two liquids injected simultaneously in a micro-mixer with a simple Y-shape flow path was clearly visualized by utilizing a chemical reaction of phenolphthalein to a strong alkali. When a thin aqueous solution of tetra methyl ammonium hydroxide and a thin ethanol solution of phenolphthalein were mixed in a micro-mixer, the color of mixed parts changed from transparent to red. For this reason, the mixed parts were clearly distinguished. Because the flow paths were narrow and Reynolds number was small, flows were laminar, and were not mixed easily. However, it was clarified that the laminar parallel flows of two liquids were gradually mixed by diffusion. By analyzing the differential equation of diffusion mixing, a simple relationship between the time and the diffusion distance was obtained. When this relationship was fitted to the mixing experiments, the fitting curves were well assigned to the experimental results. The diffusion coefficients calculated using the fitting curves almost agreed with data shown in literatures. Thus, two liquid mixing in a micro-mixer was clarified specifically. © 2018 The Institute of Electrical Engineers of Japan.