  The rheological properties of highly concentrated suspensions of hard-sphere
particles are studied with particular reference to the rheological response of
shear induced crystals. Using practically monodisperse hard spheres, we prepare
shear induced crystals under oscillatory shear and examine their linear and
non-linear mechanical response in comparison with their glassy counterparts at
the same volume fraction. It is evident, that shear-induced crystallization
causes a significant drop in the elastic and viscous moduli due to structural
rearrangements that ease flow. For the same reason the critical (peak of G'')
and crossover (overlap of G' and G'') strain are smaller in the crystal
compared to the glass at the same volume fraction. When, however the distance
from the maximum packing in each state is taken into account the elastic
modulus of the crystal is found to be larger than the glass at the same free
volume suggesting a strengthened material due to long range order. Finally,
shear induced crystals counter-intuitively exhibit similar rheological ageing
to the glass (with a logarithmic increase of G'), indicating that the shear
induced structure is not at thermodynamic equilibrium.
