(140b) Droplet Formation by Collision of Two Aqueous Solutions in an Organic Phase and Application to Ag Particle Formation

In typical microreactors, two or more reactants were brought into contact in the microchannel. Since the characteristic length of the microchannel is in the order of 100 micrometers or less, mixing can be completed quickly. When microreactors are used for reaction crystallization, fast mixing leads to higher supersaturation. This may allow us to produce finer particles. A major problem associated with the crystallization in microchannel is fouling. Some nuclei formed at the interface between two reactant fluids adhere to the surface of the microchannel. Also some crystals deposit on the channel wall directly. The crystals on the wall then begin to grow in size and this eventually leads to fouling. In an attempt to suppress fouling, a co-axial microchannel has been proposed. By making the co-axial flow pattern, the interface appears to have a cylindrical shape and thus do not touch the channel wall. This prevents deposition on the channel wall. In the present study, a new microchannel device was proposed. The channel consists of main channel and two nozzles. Organic phase was fed to the main channel, and the reactants were forced into the stream of organic phase through the nozzles. The reactants fed to the main channel collides each other and form droplets, in which nucleation and growth take place. Since the nucleation takes place in the inner fluid, which hardly touches the channel wall, the problem of fouling can be relieved. The performance of the new microchannel device was evaluated experimentally. As a first example, the device was used to produce calcium carbonate particles, which are prone to deposit on the channel wall. It was confirmed that the new device shows smaller fouling rate as compared with the conventional microchannel device. A numerical simulation was carried out to examine the fluid behavior in the new device. The mixing rate within the droplet was exaluated for different channel configuration and flow rates. As a result, a strategy to enhance the mixing rate within the microchannel was proposed.