Coflowing and Hydrodynamic Focusing

One of the most salient applications of the tip streaming phenomenon is the production of monodisperse emulsions of micrometer droplets. The microdripping and microjetting modes can be realized in different microfluidic devices for this purpose. These devices can be grouped into three main classes [1]: crossflowing systems, coflowing streams, and hydrodynamic focusing. Only the last two classes correspond to an axisymmetric configuration. In this chapter, we review some relevant results obtained in this field. Specifically, we describe the coflowing, flow focusing, confined selective withdrawal, and selective withdrawal techniques. We present results for liquid-liquid and liquid-gas systems. We do not consider here the gaseous flow-focusing configuration (a liquid stream focused by a gaseous current), to which we devote two specific chapters. Two significant aspects of each configuration mentioned above are examined: the stability of the tip streaming flow and the scaling laws predicting the size of the jets and droplets. We present some preliminary numerical results on the effect of a soluble surfactant in liquid-liquid flow focusing. The chapter closes with some results about viscous entrainment and microjetting in a uniaxial extensional flow. This microjetting mode may lead to the stable production of arbitrarily thin fluid jets.