Liquid bridge instability applied to microfluidics

This paper presents a new way of combining and mixing reagents within one droplet, which may then be used as a microfluidic biochemical reactor. This is made possible by coalescing aqueous droplets on opposing microcapillary tips immersed in density-matched silicone oil. It was found that there are two possible outcomes from a binary capillary-suspended droplet interaction. The droplets may coalesce to form a stable fluid bridge between opposing capillary tips. The droplets may, however, coalesce to form an unstable liquid bridge that quickly ruptures resulting in the two fluid volumes combining into one droplet suspended from a single capillary tip. The stability boundary that determines one outcome or the other was found to be related to a number of variables that describe the equilibrium shape of the liquid bridge interface. Suspending the host droplet from a larger diameter microcapillary dramatically increases the range of volumes that the system can combine by shifting the stability boundary. This ensures the desired effect of pinch-off near the tip of the finer microcapillary thereby dispensing microfluidic samples in one direction.[PUBLICATION ABSTRACT]