1 3 Microfluid Nanofluid (2016) 20:40 DOI 10.1007/s10404-016-1708-3 RESEARCH PAPER Study of the geometry in a 3D flow-focusing device Elena Castro-Hernández 1 · Maarten P. Kok 2 · Michel Versluis 2 · David Fernandez Rivas 3 Received: 7 July 2015 / Accepted: 10 January 2016 © Springer-Verlag Berlin Heidelberg 2016 1 Introduction In many practical applications, such as in the food industry, fine chemicals, and pharmaceutical production, a reduction in the droplet size in emulsions is desired, but it is limited by the conventional emulsification techniques. Currently, rotor–stator homogenizers used for this purpose produce large emulsion droplets in the order of 100 μ m. Develop- ing equipment with dimensions larger than available for research purposes is an ongoing challenge the industry faces (Schroën et al. 2015). For example, the complexities in producing monodisperse emulsions and nanosized drop- lets with low energy requirements in cross-flow membrane emulsification are due to the morphology of the membrane (wetting properties, pore size, porosity, and pore shape) and defined by the desired droplet sizes and applied pres- sure (Gijsbertsen-Abrahamse 2004). The ultimate goal for industrial-scale emulsification is the control on a single geometrical parameter that could then be used to generate a range of droplets size with a single device. Microfluidic devices are a potential solution since their use has been demonstrated in the production of homogeneous monomer droplets and emulsions, for chemistry and biology applica- tions with low energy input, and high throughputs in the order of L / h (Nisisako and Torii 2008; Sugiura et al. 2001; Engl et al. 2008). There are two extensively used types of devices for the production of droplets in microfluidics : (1) co-flow devices where two immiscible fluids flow in parallel (Utada et al. 2007; Marín et al. 2009; Castro-Hernández et al. 2009) and (2) flow-focusing devices in which both streams flow through a constriction with a length of the same order as its width (Gañán-Calvo 1998; Anna et al. 2003; Cubaud and Mason 2008; Garstecki et al. 2005; Lee et al. 2009). In cross-flow emulsification, uniform droplets of 2–10 times, Abstract We present a numerical and experimental study on a non-planar three-dimensional design of a microfluidic flow-focusing device for the well-controlled generation of monodisperse micron-sized droplets. Three relevant geo- metric parameters were identified: the distance between the inner inlet channel and the outlet channel, the width of the outlet channel, and its length. Simulation data extracted from a full parameter study and finite element simulations yielded four optimum designs that were then fabricated using soft lithography techniques. Under the predicted operating conditions, micro-droplets of a size of ∼1 μm in diameter are obtained from a channel 50 μm in width. This work represents an important breakthrough in the practi- cal use of flow-focusing devices delivering a ratio of con- striction to droplet size of 50 times, with the advantage of reduced clogging of the micro-channel, greatly improving the control and reliability of the device. Keywords Flow-focusing · Microfluidics · Jet · Micro-droplets * Elena Castro-Hernández elenacastro@us.es Michel Versluis m.versluis@utwente.nl 1 Área de Mecánica de Fluidos, Departamento de Ingeniería Aeroespacial y Mecánica de Fluidos, Universidad de Sevilla, Avenida de los Descubrimientos s/n, 41092 Seville, Spain 2 Physics of Fluids Group, MESA+ Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands 3 Mesoscale Chemical Systems, MESA+ Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands