Microfluidic Configurations for Producing Tip Streaming

As described in the previous chapter, tip streaming can be produced by accelerating the fluid in the tip of a mother drop. Stresses of hydrodynamic or electrohydrodynamic nature drive this acceleration to the extent of overcoming the resistance offered by the capillary and viscous forces. This phenomenon has modest practical (technological) applications due to the intrinsic unsteady character of the process. Besides, the experimenter has limited control over the flow outcome (for instance, the size of the emitted droplets), which can be achieved only by fixing the flow rate at which the dispersed phase is ejected. When the droplet is attached to a capillary and fed at the appropriate flow rate, the system eventually adopts either the microdripping or microjetting mode. Microdripping periodically emits tiny droplets of almost equal size from the droplet tip, while microjetting steadily ejects a thin, long, fluid thread. Microdripping and microjetting can produce droplets with the desired morphology, size, and electrical charge. This chapter describes the axisymmetric microfluidic configurations used to produce microdripping and microjetting. Specifically, we consider the cone-jet mode of electrospray and the coflowing, flow focusing, and confined selective withdrawal configurations. The characteristics of the selective withdrawal and electrified films are also discussed. We introduce and explain the meaning of the dimensionless numbers characterizing the corresponding flows. The numerical and experimental results will be discussed in the following chapters.