Geometrically-mediated snap-off of water-in-oil emulsion droplets in microfluidic flow focusing devices.

Microfluidic channel networks allow the control of flowing fluids within structures with length scales on the order of single or tens of micrometers (μm). This affords the opportunity to mix and separate fluids with fine precision and, in the case of immiscible multiphase flows, generate stable emulsions with well-controlled sizes and size distributions. It is generally well understood that emulsion droplet size can be regulated by carefully balancing capillary-associated parameters, such as relative fluid velocity, with the interfacial tension of the immiscible phases. Channel size and geometry, particularly that of the junction where fluids merge in microfluidic flow focusing (or "pinch flow") devices, has been shown to scale droplet size and bound the lower droplet size. Channel constrictions or "nozzles" are commonly employed to amplify the extensional flow at channel junctions, but their function has not been quantified and is, therefore, not well understood. This paper describes the use of geometry as a tunable parameter in microfluidic droplet generator design by focusing upon the effect of nozzle geometry (relative width, length and depth) upon droplet snap off behavior. Our results show that nozzle geometry can dramatically influence droplet size by shifting its snap-off position, an effect that can be anticipated by Raleigh-Plateau theory.