  The yet virtually unexplored class of soft colloidal rods with small aspect
ratio is investigated and shown to exhibit a very rich phase and dynamic
behavior, spanning from liquid to nearly melt state. Instead of nematic order,
these short and soft nanocylinders alter their organization with increasing
concentration from isotropic liquid with random orientation to one with
preferred local orientation and eventually a multi-domain arrangement with
local orientational order. The latter gives rise to a kinetically suppressed
state akin to structural glass with detectable terminal relaxation, which, on
increasing concentration reveals features of hexagonally packed order as in
ordered block copolymers. The respective dynamic response comprises four
regimes, all above the overlapping concentration of 0.02 g/ml: I) from 0.03 to
0.1 g/mol the system undergoes a liquid-to-solid like transition with a
structural relaxation time that grows by four orders of magnitude. II) from 0.1
to 0.2 g/ml a dramatic slowing-down is observed and is accompanied by an
evolution from isotropic to multi-domain structure. III) between 0.2 and 0.6
g/mol the suspensions exhibit signatures of shell interpenetration and jamming,
with the colloidal plateau modulus depending linearly on concentration. IV) at
0.74 g/ml in the densely jammed state, the viscoelastic signature of
hexagonally packed cylinders from microphase-separated block copolymers is
detected. These properties set short and soft nanocylinders apart from long
colloidal rods (with large aspect ratio) and provide insights for fundamentally
understanding the physics in this intermediate soft colloidal regime, as well
as and for tailoring the flow properties of non-spherical soft colloids.
