  Depletion-induced aggregation of rods enhanced by clustering is observed to
produce a novel model of attractive pairs of rods separated by a line of
spheres in a quasi-2D, vertically-shaken, granular gas of rods and spheres. We
show that the stability of these peculiar granular aggregates increases as a
function of shaking intensity. Velocity distributions of spheres inside and
outside of a pair of rods trapping a line of spheres show a clear suppression
of the momentum acquired by the trapped spheres. The condensed phase formed
between the rods is caused by a clustering instability of the trapped spheres,
enhanced by a vertical guidance produced by the confining rods. The liberated
area corresponding to direct excluded-volume pairs and indirect
depletion-aggregated pairs is measured as a function of time. The stability of
rod pairs mediated by spheres reveals an attraction comparable in strength to
the one purely induced by depletion forces.
