Liquid-liquid flow patterns and slug hydrodynamics in square microchannels of cross-shaped junctions

- Flow regimes were mapped for square microchannels of cross-shaped junctions.
- Smaller channel size expands the tubing regime and narrows the dripping regime.
- Effects of physical properties on flow regime transitions were revealed.
- Slug length can be scaled with the Capillary number of the continuous phase.
- Slug velocity linearly depends on the average flow velocity of the mixture.

Flow patterns for water-butanol, water-toluene, water-hexane, water-oil and water/glycerol (weight ratio 60:40) mixture-oil two-phase flows were visualized in the cross-shaped junctions of three square glass microchannels with hydraulic diameters of 200 µm, 400 µm and 600 µm. The aqueous phase is the continuous phase contacting the channel walls while the organic phase is the dispersed phase in the experiments. Three main flow pattern groups were observed, including the tubing/threading regime group, the dripping regime and the jetting regime. The flow regimes were mapped based on the Capillary number of the continuous phase and the Weber number of the dispersed phase. The flow rate ratio and the Capillary number of the dispersed phase were also employed to present flow patterns. The effects of hydraulic diameter of the square microchannels, flow rates, and physical properties, e.g., the interfacial tension and the viscosities of the aqueous and organic phases on flow pattern transitions were clarified. Besides, in the dripping regime, the dimensionless slug length can be scaled as a function of the Capillary number of the continuous phase for cross-shaped junctions. The slug velocity is linearly dependent on the average flow velocity in the dripping regime.