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purpose. These active methods involve moving parts and/or require explicitexternal actuation. A very useful alternative to this approach is the gener-ation of continuous droplet streams. This process essentially relies on theinterface deformation and pinching caused by the flow field [42]. The fluidmotion eventually triggers interfacial instabilities which yield the breakage

of the interface into a collection of droplets. This passive methodology isvery attractive because leads to large production rates and a high degree ofmonodispersity.

Continuous droplet streams are driven by surface tension and a combi-nation of external, thermal, chemical, and hydrodynamic forces. Differenttechniques have been proposed to generate such forces. Among the externalfields, one can distinguish the gravitational and electric forces. The produc-tion of droplets under the action of gravity is probably the simplest method,but is limited to relatively large sizes and low production rates [54, 56].Electrospray [57, 58, 59, 60, 61, 62] constitutes a remarkable class of tech-

niques which relies on the application of an electric field. It can produce fluidshapes of different types with sizes falling down to the nanometer scale [61].Both microdroplets [63, 38] and microemulsions [64] have also been producedby the combined action of dc and ac electric fields and a coflowing stream.Thermal effects induced by the energy of concentrated irradiation [65] andphysicochemical processes produced by surfactants [66, 67] are just two otherexamples of energy sources used to producing droplets in a continuous man-ner.

2.3. Continuous production of droplets by hydrodynamic means

Different purely hydrodynamic (mechanical) methods have been used to

form and break a fluid interface into drops. They can be grouped into threemain classes: cross-flowing systems, co-flowing streams, and extensional co-flows. In cross-flowing systems, two liquid streams intersect with each other,which generates stresses on the interface large enough to produce its pinching[68, 69, 42]. The production of droplets in co-flowing streams is essentiallycaused by the viscous stresses exerted on the interface by the outer stream[30, 31, 70, 71, 72, 73, 74]. Finally, extensional co-flows are referred to as thosein which the driving forces are produced by making the co-flowing streamcross an orifice located in front of the inner liquid source. This originatespressure gradients [75] and/or dilatational viscous stresses [76, 77, 78, 79]which stretch the interface until a fluid particle or ligament is obtained.One can mention two popular methods falling into this last class. The flow

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