Pressure-driven flow focusing of two miscible liquids

Flow focusing consists in injecting a core liquid into another surrounding flowing sheath liquid. Here we investigate experimentally the influence of imposing pressure to generate coflow of two miscible liquids. We inject water in the central inlet of a cross-junction microfluidic device and different mixtures of glycerol-water in the two lateral inlets. A pressure generator is used to control the flows, and the established flow rates are monitored in both inlets. We draw a state diagram that delimits the regions of the coflow, the inner and outer back flows. We measure the width of the jet as a function of different control parameters: the inlet pressures, the flow rates, the viscosity contrast, and the channel aspect ratio. We show that the jet width can be controlled by tuning the internal to external pressure ratio solely, provided that the viscosity contrast is low. We discuss the possibility to use such a system to center particles in a channel.