  When an ensemble of particles interact hydrodynamically, they generically
display large-scale transient structures such as swirls in sedimenting
particles [1], or colloidal strings in sheared suspensions [2]. Understanding
these nonequilibrium fluctuations is a very difficult problem, yet they are of
great importance for a wide range of processes including pigment deposition in
cosmetic or paint films, the transport of microfluidic droplets, ... All these
samples concern rigidly confined fluids, which we consider in this paper. We
address the collective dynamics of non-Brownian droplets cruising in a shallow
microchannel. We provide a comprehensive characterization of their
spatiotemporal density fluctuations. We show that density excitations freely
propagate at all scales, and in all directions even though the particles are
neither affected by potential forces nor by inertia. We introduce a theory
which quantitatively accounts for our experimental findings. By doing so we
demonstrate that the fluctuation spectrum of this nonequilibrium system is
shaped by the combination of truly long-range hydrodynamic interactions and
local collisions.
