Capillary force mediated flow‐patterns and non‐monotonic pressure drop characteristics of oil‐water microflows

We report the capillary and frictional force mediated transitions of morphologies of an oil-water flow inside a microchannel using experiments and computational fluid dynamic simulations. A number of steady and time-periodic flow patterns were reported with the variations in the interfacial tension, exchange of inlets, flow ratio, and viscosity ratio of the phases. Transitions from slug to plug to droplet to stratified flow patterns were obtained by tuning the interfacial tension. Progressive reduction in the interfacial tension transformed big slugs into smaller plugs, plugs into droplets, and droplets into a stratified flow pattern. Interestingly, the simulations uncovered a non-monotonic and nonlinear reduction in pressure drop with the decrease in interfacial tension. The change in the pressure drop was correlated to the variation in the slug, plug, or droplet frequency of water at the outlet. The variations in the pressure drop were also associated with the transition from dripping to jetting of water droplet ejection near the channel inlet. Apart from the interfacial tension, the viscosity stratification across the phases was also found to play an important role in converting the slug flow patterns into smaller plugs or droplets. The study also reports the parametric space in which the droplet flow patterns could be obtained inside a microchannel tuning the flow and viscosity ratios of the phases alongside the interfacial tension. The reported transitions of flow patterns and the pressure drop characteristics can be of significance in improving the efficiency of future microfluidic devices.