Particle and fluid transport during imbibition of strongly confined emulsions in parallel-wall channels.

We investigate capillary imbibition of a monodisperse emulsion into a high-aspect ratio microfluidic channel with the height h comparable to the droplet diameter d. For the confinement ratio d/h $=$ 1.2, the tightly confined droplets in the channel move more slowly compared to the average suspension velocity. Behind the meniscus that drives the imbibition there is a clear fluid region, separated from the suspension region by a sharp concentration front. The suspension exhibits strong density and particle velocity fluctuations, but on average the suspension domain remains uniform. For weaker confinement, d/h $=$ 0.65, the drop phase moves faster than the average suspension flow, resulting in the formation of a dynamically unstable high-concentration region near the meniscus. We describe the macroscopic suspension dynamics using linear transport equations for the particle-phase flux and suspension flux that are driven by the local pressure gradient. A dipolar particle interaction model explains the observed large density and velocity fluctuations in terms of the dynamics of elongated particle clusters with different orientation.