Global network design for robust operation of microfluidic droplet generators with pressure-driven flow

Abstract

This study investigates the influence of both local generator design and global network architecture in improving the stability and operational performance of microfluidic droplet generators. We identify naturally occurring short-term and long-term oscillations that are related to changes in the flow of the two phases. Short-term oscillations are related to the creation of each droplet and are quantified by tracking droplet speed in the network. Long-term oscillations are caused by dynamic feedback associated with the periodic change in the hydrodynamic resistance of the network as droplets enter and exit the system. Our analysis identifies that these long-term oscillations are best quantified by measuring changes in droplet spacing rather than the conventional method of using droplet size. Furthermore, we find that these long-term oscillations have a periodicity that matches the residence time of droplets within the network. In combination with experiments, a simple compact model is developed to study these oscillations and guide the network design of droplet generators. As part of this analysis a set of design rules is developed to help improve overall generator performance using pressure-driven flow.