Effect of volume fraction on droplet break-up in an emulsion flowing through a microfluidic constriction

This paper reports the effect of the droplet volume fraction on the breakup of droplets within an emulsion flowing as a two-dimensional monolayer through a tapered microchannel into a constriction. To obtain emulsions with different volume fractions, a concentrated emulsion with droplet volume fraction phi = 0.85 is injected into the channel and diluted on-chip by introducing an additional continuous phase at different flow rates. At a fixed flow rate, the breakup fraction decreases significantly when the droplet volume fraction phi decreases below 0.50. This result is consistent with our previous report showing that droplet breakup in the emulsion arises primarily from droplet-droplet interactions, which are expected to decrease significantly in dilute emulsions. Furthermore, an optimal location for the introduction of the additional continuous phase is identified to be approximately one to two droplet diameters upstream of the constriction. Away from this optimal location, the dilution of the emulsion is ineffective. Finally, we find that while a higher emulsion volume fraction packs more drops per unit volume, the propensity of the drops to undergo breakup limits droplet throughput if droplet integrity and assay accuracy are to be maintained. At a droplet breakup fraction of 0.10, diluting the emulsion 2.1 times from phi = 0.85 to phi = 0.40 increases the droplet throughput by similar to 1.5 times. Published under license by AIP Publishing.