Formation of bubbles and droplets in parallel, coupled flow-focusing geometries.

This paper describes the mechanism of formation of bubbles of nitrogen in water containing Tween 20 as a surfactant, and of droplets of water in hexadecane containing Span 80 as a surfactant. The study of these microfluidic systems compares two or four flow-focusing generators coupled through shared inlets, supplying the continuous phase, and through a common outlet channel. The processes that form bubbles in neighboring generators interact for a wide range of flow parameters; the formation of bubbles alternates in time and space, and the bubbles assemble into complex patterns in the outlet channel. The dynamics of formation of bubbles in these systems are stable for long time (at least 10 min). For a certain range of flow parameters, the coupled flow-focusing generators exhibit two stable modes of operation for a single set of flow parameters. The dynamics of formation of droplets of water in hexadecane by the coupled flow-focusing generators are simpler--the adjacent generators produce only monodisperse droplets over the entire range of flow parameters that are explored. These observations suggest that the mechanism of interaction between coupled flow-focusing generators relies on the compressibility of the dispersed phase (e.g., the gas or liquid), and on variations in pressure at the flow-focusing orifices induced by the breakup of bubbles or droplets.