Fluid dynamics of gas–liquid slug flow under the expansion effect in a microchannel

The expansion of bubbles in viscous fluids in microchannels is normally overlooked. The bubble dynamics in liquids with varying viscosities (1.15 ∼ 101.47 mPa·s) and contact angles (29.3 ∼ 137.6°) in a microchannel were investigated under various inlet pressure drops (8 ∼ 202 kPa). The findings indicate that bubble formation occurs within a squeezing-shearing regime over a wide Capillary number range of 0.0023–0.43. Interestingly, the wettability affects the bubble length rather than the bubble shape, and the poor wettability can hinders the decrease of length in higher viscosity fluids. Bubble expansion causes decreasing curvature radii of bubble caps, and non-linear rapid increases in its length and velocity along the microchannel. The bubble’s pressure–volume relation at the inlet and outlet confirms the validation of Boyle’s law in slug flow. A linear decline in pressure along the microchannel was deduced from this law. Further analysis suggests that besides the friction of the liquid slug, the pressure drop is influenced by the interface effect, film flow, and liquid circulation. Finally, a model for total pressure drop was developed, which effectively predicted the length and velocity of bubbles in the expansion process. This study offers valuable insights for a deeper understanding of bubble expansion behaviors in microchannels.