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. Applications Flow focusing may be applied in the food, medicine, pharmaceutical, cosmetic, photographic and environmental industry, among other potential uses. The production of compound particles is an important field: drug encapsulation, dye-labeled particles and multiple-core particles can be cited. Other applications include flow cytometry and microfluidic circuits. Contrast agent such as droplets and Microbubbles can be produced in flow focusing microfluidics device. References Flow cytometry Fluid dynamics