Investigation of microfluidic co-flow effects on step emulsification: Interfacial tension and flow velocities

Abstract

We propose a novel chip combining co-flow and step emulsification for flexible on-line control of droplet generation. Through this technique, we obtain controllable uniformly sized water in oil droplets. The droplet diameter is controlled between 101 and 1550 μm and the generation frequency ranges from 0.32 to 100 Hz when using the new chip. Compared with conventional step emulsification, droplet dimension is reduced by more than 50% and throughput is increased up to a factor of 50 by using continuous phase co-flow within a certain range in the chip. We also simulate the effects of interfacial tension and flow velocities on emulsification. As continuous phase velocity increases, co-flow gradually replaces the Laplace pressure difference as the driving mechanism in step emulsification. Compared with step-emulsification, the effects of interfacial tension on droplet size and generation frequency greatly change when continuous phase co-flow is added. Co-flow causes both droplet diameter and generation frequency to increase with increasing dispersed phase velocity. Meanwhile, the driving mechanism changes from co-flow to Laplace pressure difference as the dispersed phase velocity increases, because of the increase in pinch-off distance. In addition, droplet generation changes from dripping to jetting as dispersed phase velocity increases. These effects of continuous phase co-flow on step emulsification are significant, and this technique can be easily used for tunable and high-throughput droplet production.