  We consider a system of spherical colloidal particles with a size
polydispersity and use a simple van der Waals description in order to study the
combined effect of both the polydispersity and the spatial non-uniformity
induced by a planar interface between a low-density fluid phase (enriched in
small particles) and a high-density fluid phase (enriched in large particles).
We find a strong adsorption of small particles at the interface, the latter
being broadened with respect to the monodisperse case. We also find that the
surface tension of the polydisperse system results from a competition between
the tendancy of the polydispersity to lower the surface tension and its
tendancy to raise the critical-point temperature (i.e. its tendancy to favor
phase separation) with the former tendancy winning at low temperatures and the
latter at the higher temperatures.
