text: The basic principle consists of a continuous phase fluid (focusing or sheath fluid) flanking or surrounding the dispersed phase (focused or core fluid), so as to give rise to droplet or bubble break-off in the vicinity of an orifice through which both fluids are extruded. The principle may be extended to two or more coaxial fluids; gases and liquids may be combined; and, depending on the geometry of the feed tube and orifices, the flow pattern may be cylindrical or planar. Both cylindrical and planar flow focusing have led to a variety of developments (see also the works of Peter Walzal).
  A flow focusing device consists of a pressure chamber pressurized with a continuous focusing fluid supply. Inside, one or more focused fluids are injected through a capillary feed tube whose extremity opens up in front of a small orifice, linking the pressure chamber with the exterior ambient. The focusing fluid stream moulds the fluid meniscus into a cusp giving rise to a steady micro or nano-jet exiting the chamber through the orifice; the jet size is much smaller than the exit orifice, thus precluding any contact (which may lead to unwanted deposition or reaction). Capillary instability breaks up the steady jet into homogeneous droplets or bubbles.
  The feed tube may be composed of two or more concentric needles and different immiscible liquids or gases to be injected, leading to compound drops. On being suitably cured, such drops may lead to multilayer microcapsules with multiple shells of controllable thickness. Flow focusing ensures an extremely fast as well as controlled production of up to millions of droplets per second as the jet breaks up.
  The role of the tangential viscous stress is essential in establishing a steady meniscus shape in flow focusing, as illustrated in the case of a simple liquid jet surrounded by a gas. In the absence of a sufficiently strong tangential stress, a round-apex meniscus is obtained. Both the inner liquid and the external gas flows would exhibit stagnation regions around the round apex. The surface tension stress σ/D would be simply balanced by an appropriate pressure jump across the interface. If one slowly pushes a liquid flow rate Q, the system would spit intermittently the excess of liquid to recover the round-apex equilibrium shape. However, when the tangential stress is sufficiently vigorous compared to σ /D, the surface can be deformed into a steady tapering shape, which allows the continuous and smooth acceleration of the liquid under the combined actions of the pressure drop ΔP and the tangential viscous stress τs on the liquid surface.