An image-driven drop-on-demand system

Abstract

This paper introduces a microfluidic system whose purpose is to precisely and quantitatively characterize and control the mass transport between aqueous drops in a continuous oil background. The system consists of an image-based drop-on-demand microfluidic device based on programmable PDMS pneumatic valves. It consists of a closed feedback loop that uses real time image analysis to measure and generate a pre-set number of droplets one at a time with prescribed volume and composition in a specified sequence. Drops generated on-chip are exported off-chip into glass capillaries. In addition, the system allows the control of the spacing distance between adjacent drops, which is a key parameter regulating the rate of material transport between drops. Three studies illustrating the versatility of the device are presented: (1) Synchronization of chemical oscillations in networks of droplets containing the Belousov–Zhabotinsky reaction as a function of differences in drop volume. (2) Studies of the nucleation and stability of metastable amorphous calcium carbonate as a function of the spacing and composition of reacting drops. (3) Measurements of the exchange of water between drops immersed in oil as a function of added surfactant to address the question of whether micelles facilitate transport of salt between drops.