Formation of Multiphase Flow

When immiscible fluid streams are contacted at the inlet section of a microchannel network, the ultimate flow regime depends on the geometry of the microchannel, the flow rates and instabilities that occur at the fluid-fluid interface. In microfluidic systems, flow instabilities provide a passive means for co-flowing fluid streams to increase the interfacial area between them and form, e.g. by an unstable fluid interface that disintegrates into droplets or bubbles. Because of the low Reynolds numbers involved, viscous instabilities are very important At very high flow rates, however, inertial forces become influential as well. In the following, we discuss different instabilities that either lead to drop/bubble breakup or at least deform an initially flat fluid-fluid interface. Many important phenomena relate to classical work on the stability of unbounded viscous flows (see e.g. the textbooks by Drazin and Reid[56]and Chandrasekhar [57]). We will see, however, that flow confinement provides a number of new effects that are not yet fully understood and remain active research topics. 

Cases (a-c and e) are concerned with immiscible fluids case (d) considers two miscible fluid streams. 

With the aforementioned limitations in mind the number of different flow patterns may be reduced to the following four  

Bubbly flow. When the flow rate of the non-wetting phase is much lower than the one of the wetting phase, droplets/bubbles with diameters that are smaller than the microchaimel size are formed it. If stabilized by surfactants, these droplets or bubbles are stable for extended periods of time without coalescence, especially when the volume fraction of the disperse phase is small. 

Segmented flow. When the ratio of flow rates for the wetting to the non-wetting phases is close to unity, the dispersed phase forms droplets or bubbles that span most of the cross-section of the charmel. Two consecutive droplets confine the continuous liquid phase between them. 

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