From annular to surface bubbles: unraveling wettability-controlled flow patterns and governing mechanisms in microchannels

• A normalized gas-phase morphology parameter is proposed, which effectively mitigates data scattering in flow-pattern mapping across a wide wettability range (40°-150°). • A wettability-dependent flow-pattern spectrum is systematically revealed, spanning from annular to surface-attached bubbles, including transitional and coexisting regimes, far broader than typically observed in experiments. • The mechanism of bubble dynamics regulated by wettability is elucidated, demonstrating how three-phase contact-line dynamics are modulated to control neck collapse timing and flow-pattern stability. Numerical simulations of gas-liquid two-phase flow in a cross-focusing microchannel were performed to systematically investigate the effect of wall wettability (40° to 150°) on gas-flow-pattern formation. By introducing a Weber number defined with the minimum gas-neck width and deriving a normalized gas-phase morphology parameter, an empirical correlation between inlet conditions and gas-phase morphology was established, which enabled the construction of a comprehensive flow-pattern map. This approach effectively mitigates the data scattering observed in prior studies when characterizing bubble morphology. Our findings demonstrate that hydrophilic to weakly hydrophobic channels support a wide variety of flow regimes, extending beyond commonly observed annular, slug, and bubbly flows to include surface-attached bubble flow, multiple transitional patterns, and the coexistence of regimes, thereby providing a more complete picture than typical experimental observations. In contrast, superhydrophobic channels produce only limited patterns under high-pressure conditions, confirming the dominant role of wettability in regulating gas-phase evolution. Further analysis reveals that increased wall hydrophobicity linearly shortens the gas neck collapse time, with this trend more pronounced at higher liquid flow rates. However, on superhydrophobic surfaces, expansion of the three-phase contact line leads to a larger initial characteristic length, which delays the subsequent neck contraction process. This work elucidates the mechanism by which wall wettability governs gas flow pattern formation and stability through its control over three-phase contact line dynamics. The resulting flow pattern maps for microchannels with tailored wettability provide a theoretical foundation for active bubble control in microfluidic applications, offering insights for the design of advanced microfluidic devices.