Collective Behavior of Crowded Drops in Microfluidic Systems

TL;DR

This study investigates the collective dynamics of crowded droplet emulsions in microfluidic systems, revealing ordered flow at low speeds, a transition to disorder at higher flow rates, and probabilistic breakup behavior influenced by confinement.

Contribution

It provides new insights into the flow behavior, ordering, and breakup mechanisms of concentrated emulsions in microfluidics, highlighting the role of confinement and packing configurations.

Findings

Ordered spatiotemporal flow at slow speeds

Transition from solid-like to liquid-like behavior with increased flow

Droplet breakup probability depends on confinement and packing

Abstract

Droplet microfluidics, in which micro-droplets 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 percent, is relatively unexplored in microfluidics. In this article 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. As the flow rate increases, the emulsion transitions from a solid-like to a liquid-like 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, break-up 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. Finally, we discuss recent progress in computation methods for recapitulating the flow of concentrated emulsions.