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focusing technique [75, 80, 81] (from now on FF) uses the pressure gradientinduced by an outer fluid stream to focus a steady liquid meniscus, whichemits a microjet from its tip. In the selective withdrawal method [76, 77, 78,79], the energy necessary to expel the jet is originated by the viscous forceexerted by an external liquid current.

Cross-flowing streams produce droplets in the dripping regime, while co-flowing currents and extensional co-flows can work on both the dripping and

jetting modes [54]. Here, dripping is referred to as the regime in whichdroplets are produced from the pinching of the interface at distances fromthe emitting orifice of the order of its diameter. Droplets generated in thedripping mode exhibit a very high degree of monodispersity, and their sizecan even be controlled within a certain size range by mechanical excitation[30, 31, 36]. This occurs because the orifice diameter plays the role of a wavefilter, suppressing those wavelengths sufficiently different from the dominantone [61]. In many applications, dripping is caused by capillary waves which

move upstream while growing until they pinch off the interface at the orifice(absolute instability [82]). Under certain conditions, those waves are sweptaway by the fluid stream (convective instability [82]), preserving an intactliquid ligament (the jetting regime).

In the jetting regime, a jet long compared to its diameter forms andmoves downstream. During this motion, waves also grow on the interfacedriven by the surface tension force (the Rayleigh instability [14, 83]), butthey are convected downstream by the jet until the interface pinches off ata distance from the orifice much longer than its diameter. There, the jetbreaks up into droplets with diameters which commensurate with that ofthe jet [13]. In principle, the fact that the jet breaks up far away from the

orifice could expand the range of wavelengths leading to the jets breakup,which would considerably increase the polydispersity of the resulting drops.Nevertheless, relatively monodisperse collections of droplets can still be ob-tained from many jetting realizations. The dependence of the perturbationgrowth factor on the wavelength usually exhibits a sharp maximum, whichconstitutes a natural wave filter for the dominant perturbations. In mostapplications, there is a window of operational conditions within which jettingleads to a degree of monodispersity similar to that of dripping. This windowconstitutes a remarkable spot for technological and industrial developmentfor its unique combination of high productivity and control, amenable for anextra-control and enhanced monodispersity through acoustic excitation [84].Flow Focusing constitutes an important example in this regard (e.g. [85]),

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