Gravity-driven unstable interfacial patterns of two-phase flow in square microchannels

Gas–liquid two-phase flows in porous media are ubiquitous in numerous hydraulic and environmental applications. However, the mechanisms governing gravity-driven interfacial instability and the associated flow-regime transitions remain insufficiently explored. In this study, we experimentally investigate gravity-driven gas–liquid flow patterns in vertical square microchannels. The experimental results reveal that all observed flow regimes originate from a gravity-driven interfacial instability characterized by the accumulation, acceleration, and subsequent sliding of liquid columns. Five representative flow patterns are identified, namely bubbly, slug, bubbly–slug, falling film, and continuous liquid flows, whose occurrences are governed by the liquid injection rate, fluid properties, and channel dimensions. The liquid injection velocity mediates regime transitions by regulating the bubble length in the microchannel. A flow-pattern phase diagram is established in the space of the Weber number and the Bond number, and the corresponding regime boundaries are determined. These findings provide new insight into gravity-driven two-phase flow in confined pore spaces and provide guidance for predicting and controlling flow patterns in subsurface engineering applications such as geological CO2 sequestration and underground hydrogen storage.