  Fluids made of two-dimensional hard particles with polygonal shapes may
stabilize symmetries which do not result directly from the particle shape. This
is due to the formation of clusters in the fluid. Entropy alone can drive these
effects, which represent a challenge for standard theories. In this article we
present a general density-functional theory for clustering effects in fluids of
hard particles in two dimensions. The theory combines a free-energy functional
of the angular distribution function with an association energy term which
qualitatively reproduces the clustering tendencies of the particles found in
Monte Carlo simulations. Application is made to a fluid of hard right-angled
triangles.
