  Aqueous suspensions of Laponite, a system composed of disk-like
nanoparticles, are found to develop optical birefringence over several days,
well after the suspensions solidified due to jamming. The optical anisotropy is
particularly enhanced near the air Laponite suspension interface over
length-scales of several millimetres, which is beyond five orders of magnitude
larger than the particle length scale, suggestive of large scale ordering
influenced by the interface. The orientational order increases with time and is
always greater for higher concentration of salt, higher concentration of
Laponite and higher temperature of the suspension. While weakly birefringent,
Laponite suspensions covered by paraffin oil do not show any enhancement in the
optical anisotropy near the interface compared to that in the bulk. We suggest
that the expedited structure formation near the air interface propagating
progressively inside the sample is responsible for the observed behaviour. We
discuss the observed nematic ordering in the context of glasslike and gellike
microstructure associated with aqueous Laponite suspensions.
