Three-dimensional measurement of gas dissolution process in gas-liquid microchannel flow

The three-dimensional CO2 dissolution process through a gas-liquid interface in microfluidic devices was investigated experimentally, for the precise control of CO2 dissolution. The gas dissolution was evaluated by using confocal micron-resolution particle image velocimetry (micro-PIV) combined with laser induced fluorescence (LIF), which has the ability to measure the velocity and dissolved CO2 concentration distribution in a liquid flow field. The measurement system is based on the confocal microscope, which has excellent depth resolution and enables visualization of the three-dimensional distributions of velocity and dissolved CO2 concentration by rendering two-dimensional data. The device is comprised of a polydimethylsiloxane chip, whose microchannels were fabricated by using a cryogenic micromachining system. The width and depth of the liquid flow channel are larger than those of the gas flow channel. This is due to the need for decreasing the width of the gas-liquid interface and increasing the hydraulic diameter of the liquid channel, whose conditions generate a static gas-liquid interface. The experiments were performed for three different liquid flow conditions corresponding to Reynolds numbers of 1.0 × 10−2, 1.2 × 10−2 and 1.7 × 10−2, and the gas flow rate was set to be constant at 150 μL/min. The LIF measurements indicate that an increase in the Reynolds number yields a decrease in dissolved gas in the spanwise directions. Furthermore, molar fluxes by convection and diffusion were evaluated from the experimental data. The molar fluxes in the streamwise direction were at least 20 times as large as those in the spanwise and depthwise directions. This reveals that an increase in momentum transport in the spanwise and depthwise directions is an important factor for enhancing mass transfer in the gas-liquid microchannel flow.