5 research outputs found
Collective Diffusion of Colloidal Hard Rods in Smectic Liquid Crystals: Effect of Particle Anisotropy
We study the layer-to-layer diffusion in smectic-A liquid crystals of
colloidal hard rods with different length-to-diameter ratios using computer
simulations. The layered arrangement of the smectic phase yields a hopping-type
diffusion due to the presence of permanent barriers and transient cages.
Remarkably, we detect stringlike clusters composed of inter-layer rods moving
cooperatively along the nematic director. Furthermore, we find that the
structural relaxation in equilibrium smectic phases shows interesting
similarities with that of out-of-equilibrium supercooled liquids, although
there the particles are kinetically trapped in transient rather than permanent
cages. Additionally, at fixed packing fraction we find that the barrier height
increases with increasing particle anisotropy, and hence the dynamics is more
heterogeneous and non-Gaussian for longer rods, yielding a lower diffusion
coefficient along the nematic director and smaller clusters of inter-layer
particles that move less cooperatively. At fixed barrier height, the dynamics
becomes more non-Gaussian and heterogeneous for longer rods that move more
collectively giving rise to a higher diffusion coefficient along the nematic
director.Comment: 24 pages, 10 figure
Rotational averaging-out gravitational sedimentation of colloidal dispersions and phenomena
We report on the differences between colloidal systems left to evolve in the
earth's gravitational field and the same systems for which a slow continuous
rotation averaged out the effects of particle sedimentation on a distance scale
small compared to the particle size. Several systems of micron-sized colloidal
particles were studied: a hard sphere fluid, colloids interacting via
long-range electrostatic repulsions above the freezing volume fraction, an
oppositely charged colloidal system close to either gelation and/or
crystallization, colloids with a competing short-range depletion attraction and
a long-range electrostatic repulsion, colloidal dipolar chains, and colloidal
gold platelets under conditions where they formed stacks. Important differences
in the structure formation were observed between the experiments where the
particles were allowed to sediment and those where sedimentation was averaged
out. For instance, in the case of colloids interacting via long-range
electrostatic repulsions, an unusual sequence of
dilute-Fluid/dilute-Crystal/dense-Fluid/dense-Crystal phases was observed
throughout the suspension under the effect of gravity, related to the volume
fraction dependence of the colloidal interactions, whereas the system stayed
homogeneously crystallized with rotation. For the oppositely charged colloids,
a gel-like structure was found to collapse under the influence of gravity with
a few crystalline layers grown on top of the sediment, whereas when the
colloidal sedimentation was averaged out, the gel completely transformed into
crystallites that were oriented randomly throughout the sample. Rotational
averaging out gravitational sedimentation is an effective and cheap way to
estimate the importance of gravity for colloidal self-assembly processes.Comment: 13 pages, 13 figure