Monodispersed water-in-oil emulsions prepared with semi-metal microfluidic EDGE systems

Monodispersed water-in-oil emulsions were prepared with EDGE (Edge based Droplet GEneration) systems, which generate many droplets simultaneously from one junction. The devices (with plateau height of 1.0 μm) were coated with Cu and CuNi having the same hydrophobicity but different surface roughness. Emulsification was performed by using water as dispersed phase and oils with different viscosities (hexadecane, decane, hexane and sunflower oil) as continuous phases; lecithin, polyglycerol polyricinoleate (PGPR) and span80 were used as emulsifiers. The roughness affected the emulsification behaviour significantly. The smoother Cu surface exhibited droplet formation over the entire length of the droplet formation unit, while the rougher CuNi surface showed non-uniform filling of the plateau and much lower droplet formation frequency. In spite of this different behaviour, monodispersed droplets (CV <10 %) were produced by both systems (with span80 and PGPR), with a size six times the plateau height (d _avg ≈ 6.0 μm). The droplet size decreased with increasing viscosity ratio and remained constant above some critical value. The emulsification process was stable over a wider range of pressures as previously found for silicon-based systems. The amount of PGPR influenced the pressure stability, but the system could be used effectively, while with lecithin and span80 the stable pressure range was very small. The pressure and viscosity stability of these semi-metal systems with rough surfaces show that the EDGE system has potential for practical applications, especially since overall productivity is not affected.