  In this experimental study, we report on the mixing properties of interfacial
colloidal floaters (glass bubbles) by chemical and hydrodynamical currents
generated by self-propelled camphor disks swimming at the air-water interface.
Despite reaching a statistically stationary state for the glass bubbles
distribution, those floaters always remain only partially mixed. This
intermediate state results from a competition between (i) the mixing induced by
the disordered motion of many camphor swimmers and (ii) the unmixing promoted
by the chemical cloud attached to each individual self-propelled disk.
Mixing/unmixing is characterized globally using the standard deviation of
concentration and spectra, but also more locally by averaging the concentration
field around a swimmer. Besides the demixing process, the system develops a
"turbulent-like" concentration spectra, with a large-scale region, an inertial
regime and a Batchelor region. We show that unmixing is due to the Marangoni
flow around the camphor swimmers, and is associated to compressible effects.
