Collective behavior of crowded drops in microfluidic systems

Droplet microfluidics, in which microdroplets serve as individual reactors, has enabled a wide range of high-throughput biochemical processes. Unlike solid wells typically used in current biochemical assays, droplets are subject to instability and can undergo breakup, especially under fast flow conditions. Although the physics of single drops has been studied extensively, the flow of crowded drops or concentrated emulsions - where droplet volume fraction exceeds ∼80% - is relatively unexplored in microfluidics. In this paper and the related invited lecture from the 74th Annual Meeting of the American Physical Society's Division of Fluid Dynamics, we describe the collective behavior of drops in a concentrated emulsion by tracking the dynamics and the fate of individual drops within the emulsion. At the slow flow limit of the concentrated emulsion, we observe an unexpected order, where the velocity of individual drops in the emulsion exhibits spatiotemporal periodicity. This periodicity is surprising from both fluid and solid mechanics points of view. We show the phenomenon can be explained by treating the emulsion as a soft crystal undergoing plasticity, in a nanoscale system comprising thousands of atoms as modeled by droplets. Our results represent a type of collective order which can have practical use in on-chip droplet manipulation. As the flow rate increases, the emulsion transitions from a solidlike to a liquidlike material, and the spatiotemporal order in the flow is lost. At the fast flow limit, droplet breakup starts to occur. We show that droplet breakup within the emulsion follows a probability distribution, in stark contrast to the deterministic behavior in classical single-drop studies. In addition to capillary number and viscosity ratio, breakup probability is governed by a confinement factor that measures drop size relative to a characteristic channel length. The breakup probability arises from the time-varying packing configuration of the drops. Replacing surfactant with nanoparticles as droplet stabilizers suppresses breakup and increases the throughput of droplet processing by >300%. Strategic placement of an obstacle suppresses breakup by >103-fold. Finally, we discuss recent progress in computation methods for recapitulating the flow of concentrated emulsions.