Flow patterns of solid in water in oil (S/W/O) compound droplets formation in a microfluidic device with perpendicular shear

Abstract The flow patterns and corresponding breakup dynamics of solid in water in oil compound droplets under perpendicular shear force were experimentally investigated in a microfluidic device. Three typical flow patterns, namely double-encapsulation, plugs, and cobbles, were found and be quantitatively distinguished by the capillary number ( Ca c ) of continuous phase and the flow rate ratio ( R ) of continuous phase to dispersed phase. It is found that, under all of flow regimes, the consecutive entry of solid particles leads δ 1 (the neck thickness along the center of the side channel) varying periodically. In the double-encapsulation regime, the neck thickness ( δ 2 ) between the first and second particle increases as the interface deformation recovers driven by the interfacial tension force, whereas the neck thickness ( δ 3 ) between the second and third particle decreases to break off because the neck has been stretched long to become thin enough. As for the cobbles regime, hydrostatic pressure difference still remains dominant, but not a shearing force. Specially, different from the W/O droplet formation, it suggests that various breakup dynamics are always caused by a non-free deformation of S/W interface and the large diameter of solid particle. Finally, the flow pattern diagrams dependent on Ca c and R are obtained.