Timescale and Spatial Distribution of Local Plastic Events in a Two-Dimensional Microfluidic Crystal

When a microfluidic crystal consisting of a concentrated emulsion flows in a convergent channel, the boundary conditions enforce a sequence of droplet rearrangements (T1 events). At low flow rates, these T1 events are periodic in space and time, giving rise to a surprisingly ordered flow pattern. At high flow rates, this order was lost. To understand the transition from order to disorder, we examined the timescale and spatial distribution of T1 events during the flow of a monolayer of monodisperse droplets within a concentrated emulsion confined in a convergent tapered microchannel. We show that the duration of a single T1 event consists of three distinct regimes, with two transitions upon an increase in applied strain rate. The first transition involves a change in the dynamics of the thin film formed between the emulsion drops, the second transition entails the emulsion transitioning from a solid to liquid-like state. Our results are used to understand the relationship between macroscopic properties and microscopic flow structures of the emulsion, as well as guiding the design of flow control elements in microfluidic devices.