title: Confinement and viscosity ratio effect on droplet break-up in a concentrated emulsion flowing through a narrow constriction.
abstract: This paper describes the dimensionless groups that determine the break-up probability of droplets in a concentrated emulsion during its flow in a tapered microchannel consisting of a narrow constriction. Such channel geometry is commonly used in droplet microfluidics to investigate the content of droplets from a concentrated emulsion. In contrast to solid wells in multi-well plates, drops are metastable, and are prone to break-up which compromises the accuracy and the throughput of the assay. Unlike single drops, the break-up process in a concentrated emulsion is stochastic. Analysis of the behavior of a large number of drops (N > 5000) shows that the probability of break-up increases with applied flow rate, the size of the drops relative to the size of the constriction, and the viscosity ratio of the emulsion. This paper shows that the break-up probability collapses into a single curve when plotted as a function of the product of capillary number, viscosity ratio, and confinement factor defined as the un-deformed radius of the drop relative to the hydraulic radius of the constriction. Fundamentally, the results represent a critical step towards the understanding of the physics governing instability in concentrated emulsions. Practically, the results provide a direct guide for the rational design of microchannels and the choice of operation parameters to increase the throughput of the droplet interrogation step while preserving droplet integrity and assay accuracy.
authors.affiliation: Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA. sindy@stanford.edu.
authors.forename: Ya
authors.initials: Y
authors.lastname: Gai
authors.forename: Jian Wei
authors.initials: JW
authors.lastname: Khor
authors.forename: Sindy K Y
authors.initials: SK
authors.lastname: Tang