269154 Scaling of Dispersed Phase Flow Transitions in a Microfluidic Flow Focusing Device
Microfluidic production of drops using a flow-focusing device is an integral component of many lab-on-chip technologies. Several studies have documented the different behaviors of dispersed phase in flow-focusing devices. These behaviors include break-up modes such as dripping, jetting, and a non-break-up mode involving co-flowing laminar streams. Despite the identification of these behaviors, the control parameters that govern the transitions between these behaviors are not fully known.
Recently, Utada et al.  have shown that the transition between dripping and jetting in a cylindrical microcapillary device is dictated by the capillary number (Ca) of the outer fluid and Weber number (We) of the inner fluid. In this study, we investigate whether these dimensionless numbers also govern the different behaviors observed in the more complex geometry of a flow-focusing device. We fabricated flow-focusing devices in polydimethyl(siloxane) with a square orifice of size 50μm. We varied the capillary number based on the outer fluid from ~10-4 – 0.1, and the capillary number based on the inner fluid from ~10-5 – 1. The viscosity ratio of the inner to outer fluid was varied from µi/µo= 0.03 – 100.
In the microfluidic flow-focusing device, for viscosity ratio ~ O(1), we find that the transition from dripping to co-flowing laminar streams occurs at Reynolds number, Re ~ O(1), where Re is based on the inertial force of the inner fluid to the viscous force of the outer fluid. The dripping to jetting transition is found to roughly occur at Weo ~ O(1), based on the outer fluid. For fluid viscosity ratios greater than or less than 1, initial results indicate that the transition between these different behaviors have a complex dependence on Re and Weo.
 A. S. Utada, A. Fernandez-Nieves, H. A. Stone, and D. A. Weitz, Physical Review Letters 99 (9), 4 (2007).