Water-in-oil droplet formation in a flow-focusing microsystem using pressure- and flow rate-driven pumps

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 2017-10, Vol.531, p.164-172
• Main Authors: Lignel, Sarah, Salsac, Anne-Virginie, Drelich, Audrey, Leclerc, Eric, Pezron, Isabelle
• Format: Article
• Language: English
• Subjects: Drop formation; Electronic–hydraulic analogy; Engineering Sciences; Flow-driven pump; Flow-focusing microsystem; Monodisperse emulsion; Pressure controller
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2017.07.065

[Display omitted] •We create monodisperse water-in-oil drops with a microfluidic flow-focusing system.•The inlet flow-conditions for drop formation are determined along with the regimes.•To compare flow- and pressure-driven results, a lumped-element model is derived.•A unique regime diagram pilots the emulsion production in the flow-focusing system.•The drop formation regimes are governed by the continuous phase capillary number. Microfluidic flow-focusing systems are simple and cheap devices to produce monodisperse emulsions. The objective of the study is to determine the flow conditions to create water-in-oil emulsions with flow- and pressure-driven techniques, the use of pressure controllers becoming more systematic, owing to their high precision and capability to generate flows within a large range of fluid properties. The challenge is to make the link between applied pressures and flow rates to be able to switch from pressure to flow-rate driven systems (or vice-versa). To reach this purpose, we have derived a simple model using the electronic–hydraulic analogy between fluid transport in microchannels and electron transport in electric circuit. Thanks to the model, we show that droplets are generated in both cases within exactly the same range of values of inlet-to-outlet pressure differences (and thus flow rates). A unique diagram governs the production of an emulsion in the flow-focusing system. There exists a minimal value of the continuous flow under which no drops form, the outlet channel being occupied by a pure oil flow for all the disperse flow conditions. Above this limiting value, drops may be generated but only in a narrow range of disperse flow rate/pressure, the outlet channel being filled by a pure water flow above it. The state diagram shows that the drop formation regimes are governed by the continuous phase capillary number. The unified results provide the conditions needed to produce water-in-oil emulsion droplets and prove the great potential of the model to compare flow-driven and pressure-driven microfluidic experiments.