Flow pattern diagrams of oil-water two-phase microflows and stable parallel flows obtained at low Reynolds numbers

- Flow pattern diagrams of oleic acid-water microflow were measured. The influences of surfactants, temperature, solid wall materials, and negative driving pressures on the flow pattern control were studied systematically.
- The liquid-solid interaction played an important role in manipulating the liquid-liquid interface. Driving the flow with a negative pressure was verified to be a simple, effective, and controllable method for producing liquid-liquid immiscible parallel flows in microchannels.
- Long and stable parallel flow was obtained at Re < 10−2 for both phases.

Immiscible liquid-liquid parallel microflows are useful in many applications. However, the immiscible two-phase flow behaves nonlinearly and it is a challenging task to control and stabilize the liquid-liquid interface. Oleic acid-water immiscible two-phase flow in microchannels was studied in the present work. The flow pattern diagrams were measured. Four different flow patterns, namely the single-phase flow of the aqueous phase, the droplet flow with the organic droplet dispersed in water, the parallel flow, and the single-phase flow of the oil phase were identified. Special attention was paid on transitions between the parallel flow and the other patterns of flow. Parallel flow formed under a proper balance between the driving force, the friction resistance, and the interfacial tension. The liquid-solid interaction as well as the liquid-liquid interaction played an important role in manipulating the liquid-liquid interface. Adding surfactants in the aqueous phase and raising the temperature altered both the liquid-liquid interfacial tension and the liquid-solid interaction. This led to a rather complicated transition behavior between the parallel flow and the droplet flow. The surface states of the solid walls were found to be the dominant condition in controlling the flow patterns in many cases. By applying a certain level of vacuum at the outlet of the channels, the first layer of water attached to the solid walls became lower in density and thicker in thickness. The flow resistance for both phases were remarkably lowered. Stable parallel flows at the Reynolds numbers of the oil and the aqueous phases as low as 1 × 10−5 and 8 × 10−3, respectively, have been obtained without any chemical modifications of the solid surfaces, additional channel structures, or surfactants. A straight interface as long as several to tens of millimeters between the two immiscible liquid flows was successfully established and maintained stably by hours. Driving the flow with a negative pressure was verified to be a simple, effective, and controllable method for producing liquid-liquid immiscible parallel flows. It can be applied to various solvents and solutes and has potential in various applications, especially at low Reynolds numbers.