Unstable coalescence in the mixing of microcapillary suspended aqueous droplets in silicone oil

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 at microcapillary tips immersed in density matched silicone oil. It was found that there are two possible outcomes from a binary capiilary-suspended droplet interaction. The droplets may coalesce in a stable manner forming a permanent fluid bridge between opposing capillary tips. The droplets may, however, temporarily coalesce in a rapid and highly unstable manner resulting in the two fluid volumes combining into one droplet suspended from one capillary tip. The range of volumes that the system can combine is dramatically increased by suspending the host droplet from a larger diameter microcopillary. This ensures that necking and pinch-off occurs near the tip of the finer microcapillary thereby dispensing microfluidic samples in one direction. A surface energy criterion describing the phenomenon of unstable coalescence was found to agree well with experimental data extracted from high speed digital video and image processing.