We demonstrated a liquid/liquid and a gas/liquid two-phase crossing flow in glass microchips. A 250-microm-wide microchannel for aqueous-phase flow was fabricated on a top glass plate. Then, as a way to utilize the surface energy difference for stable phase confluence and separation, a 250-microm-wide microchannel for organic-phase (or gas-phase) flow was fabricated on a bottom glass plate and the wall was chemically modified by octadecylsilane (ODS) group. The top and bottom plates were sealed only by pressure. A microchannel pattern was designed so that the two phases made contact at the crossing point of the straight microchannels. The crossing point was observed with an optical microscope. Results showed that the ODS modification of the microchannel wall clearly improved stability of the interface between the two fluids. Pressure difference between fluids was measured and the interface of water and nitrobenzene was stable for the pressure difference from +300 Pa to -200 Pa. The pressure drop in a countercurrent flow configuration was also estimated, and the pressure difference required to realize the counter current flow was less than the allowable pressure range. Finally, we discussed the advantages of utilizing this approach.
An integrated multireactor system for 2 x 2 parallel organic synthesis has been developed on a single glass microchip. Three-dimensional channel circuits in the chip were fabricated by laminating three glass plate layers. The fabrication method is a straightforward extension of the conventional one, and topological equivalence for any three-dimensional circuits can be constructed easily with it. 2 x 2 phase-transfer amide formation reactions, which constitute a simple model for combinatorial synthesis, were successfully carried out on the microchip, and the integrity of the three-dimensional circuits was confirmed. Combinatorial chemistry with multi-microreactors, in conjunction with a high-throughput screening method based on micro-TAS technologies, is expected to provide an efficient tool for drug discovery.
A capillarity restricted modification method for microchannel surfaces was developed for gas--liquid microchemical operations in microchips. In this method, a microstructure combining shallow and deep microchannels and the principle of capillarity were utilized for chemical modification of a restricted area of a microchannel. A hydrophobic--hydrophilic patterning in microchannels was prepared as an example for guiding gas and liquid flows along the respective microchannels. Validity of the patterning was confirmed by measuring aqueous flow leak pressure from the hydrophilic microchannel to the hydrophobic one. The leak pressure of 7.7-1.1 kPa agreed well with that predicted theoretically from the Young-Laplace equation for the microchannel depth of 8.6-39 microm. In an experiment to demonstrate usefulness and effectiveness of the method, an air bubble was first introduced into the hydrophilic microchannel and purged from the hydrophobic-hydrophilic patterned microchannels. Next, the patterning structure was applied to remove dissolved oxygen by contacting the aqueous flow with a nitrogen flow. The concentration of dissolved oxygen decreased with contact time, and its time course agreed well with numerical simulation. These demonstrations showed that the proposed patterning method can be used in general microfluidic gas-liquid operations.
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