Study On Two-Phase Flow Patterns And Droplet Formation In Microchannels;

In recent years, microfluidic technology has developed rapidly and been widely used. The multiphase flow, as the basis for research and application of microfluidics, has been attracted much attention. In this paper, two-phase flow patterns and droplet formation in microchannels were studied.The air-water two-phase flow patterns, including slug, annular and stratified flow, in vertical rectangle microchannels with the dimension of 40×160μm were investigated by means of laser image magnifying technique. A flow patterns map was constructed to obtain the transitions lines with the flow rates of both phases as the coordinates, and was compared with the results in the literature.A high-speed camera was used to observe the liquid-liquid two-phase flow patterns in the T and Y-junction microchannels. Cyclohexane and demonized water with 0.3% surfactant sodium dodecyl sulfate (SDS), were employed as the oil and water phase, respectively. Three main flow patterns were observed: droplet, slug and parallel flow. The transition lines of flow patterns were affected by microchannel size and the way of feeding. A flow patterns map was constructed with the apparent flow rates of both phases as the coordinates, and was compared with the existing reports.The processes and the mechanisms of droplet formation in the microchannels with head-on T-junctions and vertical-junction were studied using the high-speed camera. The effects of the geometry and size of microchannels, flow rates of both phases and physical properties (viscosity and interfacial tension) of fluids on droplet formation were investigated. In the slug flow regime, the droplet formation is mainly dominated by the accumulated pressure drop across the thread of the dispersed phase, and the size of formed droplet is proportional to the ratio of flow rates of dispersed/continuous phases. In the drop flow regime, the break-up of the droplet is mainly dominated by the shear force and interfacial tension, and the size of formed droplet has an exponential relation with capillary number. On the basis of interplay of the accumulated pressure drop and shear force, the correlations were proposed for predicting the size of formed droplet with different feeding ways, taking into account the capillary number Ca and the ratio of oil/water flow ratesφ, and the predicted values agreed well with the experimental data.