Numerical simulation of droplet generation mode transition of co-flowing liquids in micro-channel

Droplet formation in a co-flowing liquid system is simulated using the front tracking method to investigate the mechanism of droplet generation under the dripping and jetting modes, and the mode transition, which is critical in the design of micro-fluidic devices to produce mono-disperse droplets. The Navier-Stokes equations for the two liquid phases are solved numerically on a fixed Eulerian cylindrical coordinate mesh for the fluid flows, and the front tracking method is applied to track the movement of the interface between the two immiscible liquids as well as the calculation of surface tension. The simulations reasonably predict the droplet formation modes in the coflowing liquids and the detailed flow dynamics. Under certain critical scenarios, a slight change of the flow condition, e.g. outer liquid flow velocity, may cause abrupt mode transition, resulting in great change in the size of droplet produced. It is also found that the formation of a dispersed liquid cone-jet is the key phenomenon that relates to the droplet generation modes. The mode transition is caused upon the balance of the fluid flow inertia and the capillary pressure in the dispersed liquid cone-jet.