Numerical simulation of microdroplet formation in coflowing immiscible liquids

Monodisperse microdroplets can be formed when a liquid is injected via a capillary needle nozzle into another immiscible coflowing liquid in a microchannel. Such system has been applied in microfluidic devices to produce monodisperse microdroplets with controllable size. In this article, the drop formation in a coflowing system is simulated numerically using a front tracking/finite volume method to investigate the droplet formation mechanism. This numerical method solves a single set of Navier-Stokes equations for both liquid phases on a fixed Eulerian two-dimensional cylindrical coordinate mesh to account for the fluid flow dynamics. The front tracking method is applied to track the movement of the interface between the two immiscible liquids as well as the surface tension force. The simulation results demonstrate that the process of droplet formation in the coflowing immiscible liquids can be reasonably predicted. Two droplet generation modes (namely dripping and jetting), which have been observed in experiments, are successfully produced through the numerical simulations under certain flow conditions. Moreover, the effects of the continuous phase flow speed, viscosity, and the interface tension on the droplet size are investigated. The correlation of the non-dimensional droplet size (r(d)*) with the continuous phase flow parameters such as the Reynolds number (Re-o), Weber number (We(o)), Capillary number (Ca-o), and viscosity ratios (mu*) can be obtained as r(d)* proportional to Ca-o(-1/2) Re-o(-1/6) for the dripping mode and as r(d)* proportional to Ca(o)(1/3)We(o)(-1/2) mu*(-1/2) for the jetting mode from the current simulations. (c) 2007 American Institute of Chemical Engineers.