Gas-Liquid Two-Phase Flow in Hot Embossed Square Microchannels

An experimental study was conducted to investigate the characteristics of gas-liquid two-phase flow in 200 μm square microchannels thermoformed in polymer chips. Polymer microfluidic chips were replicated using hot embossing of poly(methyl methacrylate) (PMMA) with micromachined brass mold inserts. The thermoformed microchannels in polymer chips typically had greater surface roughnesses compared to microchannels etched in the silicon substrate. Two more different polymer chips, a direct micromachined PMMA chip and a chip hot embossed with a LIGA nickel mold insert, were fabricated to compare surface characteristics of the sidewalls and bottoms of fabricated microchannels. Deionized water and dry air were injected separately into the chips at superficial velocities of j_L = 0.005 – 0.11 m/s for the liquid and j_G = 0.003 – 16.67 m/s for the gas. Capillary bubbly, plug, plug-annular, annular, and dry flows were observed in the microchannels. Two-phase flow pattern maps and transitions between flow regimes were determined for fixed values of the homogeneous liquid fraction defined as β_L = Q_L/(Q_L + Q_G) where Q_L and Q_G are the liquid and gas flow rates, and the liquid Weber number fraction defined as γ_L = We_L/(We_L + We_G) where We_L and We_G are the liquid and gas Weber number. The surface roughness in submicron range showed minor effect in comparison with the previous work in terms of the gas-liquid two-phase flow patterns and transitions between flow regimes. Dimensionless bubble sizes scaled by the width of observation microchannel were plotted against the homogeneous liquid fraction (β_L). A scaling law for the bubble length developed for the previous work with T-junctions was applicable to the present work used the cross junction for generation of segmented flow. With a fixed value of the fitting parameter, scaling law showed a good agreement with the experimental data. Deviation of the scaled bubble length from predicted bubble length line and irregularity of bubble length with a fixed homogeneous liquid fraction increased with higher gas flow rates.