  We investigate imbibition of a monodisperse emulsion into a low-aspect ratio
microfluidic channel with the height h comparable to the droplet diameter d.
For confinement ratio d/h = 1.2, the tightly confined disk-like droplets in the
channel move more slowly compared to the average suspension velocity. Behind
the meniscus that drives the imbibition, there is a droplet-free region,
separated from the suspension region by a sharp concentration front. The
suspension exhibits strong droplet density and velocity fluctuations, but on
average, the suspension domain remains uniform. For weaker confinement, d/h =
0.65, the spherical droplets move 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 orientations.
