8 research outputs found
Dislocations and Vacancies in Two-Dimensional Mixed Crystals of Spheres and Dimers
In colloidal crystals of spheres, dislocation motion is unrestricted. On the other hand, recent studies of relaxation in crystals of colloidal dimer particles have demonstrated that the dislocation dynamics in such crystals are reminiscent of glassy systems. The observed glassy dynamics arise as a result of dislocation cages formed by certain dimer orientations. In the current study, we use experiments and simulations to investigate the transition that arises when a pure sphere crystal is doped with an increasing concentration of dimers. Specifically, we focus on both dislocation caging and vacancy motion. Interestingly, we find that any nonzero fraction of dimers introduces finite dislocation cages, suggesting that glassy dynamics are present for any mixed crystal. However, we have also identified a vacancy-mediated uncaging mechanism for releasing dislocations from their cages. This mechanism is dependent on vacancy diffusion, which slows by orders of magnitude as the dimer concentration is increased. We propose that in mixed crystals with low dimer concentrations vacancy diffusion is fast enough to uncage dislocations and delay the onset of glassy dislocation dynamics
Restricted Dislocation Motion in Crystals of Colloidal Dimer Particles
At high area fractions, monolayers of colloidal dimer particles form a
degenerate crystal (DC) structure in which the particle lobes occupy triangular
lattice sites while the particles are oriented randomly along any of the three
lattice directions. We report that dislocation glide in DCs is blocked by
certain particle orientations. The mean number of lattice constants between
such obstacles is 4.6 +/- 0.2 in experimentally observed DC grains and 6.18 +/-
0.01 in simulated monocrystalline DCs. Dislocation propagation beyond these
obstacles is observed to proceed through dislocation reactions. We estimate
that the energetic cost of dislocation pair separation via such reactions in an
otherwise defect free DC grows linearly with final separation, hinting that the
material properties of DCs may be dramatically different from those of 2-D
crystals of spheres
Glassy Dislocation Dynamics in 2D Colloidal Dimer Crystals
Although glassy relaxation is typically associated with disorder, here we report on a new type of glassy dynamics relating to dislocations within 2D crystals of colloidal dimers. Previous studies have demonstrated that dislocation motion in dimer crystals is restricted by certain particle orientations. Here, we drag an optically trapped particle through such dimer crystals, creating dislocations. We find a two-stage relaxation response where initially dislocations glide until encountering particles that cage their motion. Subsequent relaxation occurs logarithmically slowly through a second process where dislocations hop between caged configurations. Finally, in simulations of sheared dimer crystals, the dislocation mean squared displacement displays a caging plateau typical of glassy dynamics. Together, these results reveal a novel glassy system within a colloidal crystal
Synthesis and Assembly of Nonspherical Hollow Silica Colloids Under Confinement
Hard peanut-shaped colloids were synthesized and organized into a degenerate crystal (DC), a phase previously observed only in simulations. In this structure, particle lobes tile a triangular lattice while their orientations uniformly populate the three underlying crystalline directions
Directed self-assembly of spherical caps via confinement
In this work we use Monte Carlo simulations to study the phase behavior of
spherical caps confined between two parallel hard walls separated by a distance
H. The particle model consists of a hard sphere of diameter \sigma cut off by a
plane at a height \chi, and it is loosely based on mushroom cap-shaped
particles whose phase behavior was recently studied experimentally [E. K. Riley
and C. M. Liddell, Langmuir, 26, 11648 (2010)]. The geometry of the particles
is characterized by the reduced height \chi^* = \chi/\sigma, such that the
model extrapolates between hard spheres for \chi^* \leftarrow 1 and infinitely
thin hard platelets for \chi^* \letfarrow 0. Three different particle shapes
are investigated: (a) three-quarter height spherical caps (\chi^* = 3/4), (b)
one-half height spherical caps or hemispheres (\chi^* = 1/2), and (c)
one-quarter height spherical caps (\chi^* = 1/4). These three models are used
to rationalize the effect of particle shape, obtained by cutting off spheres at
different heights, on the entropy-driven self-assembly of the particles under
strong confinements; i.e., for 1 < H/\chi < 2.5. As H is varied, a sequence of
crystal structures are observed, including some having similar symmetry as that
of the structures observed in confined hard spheres on account of the remaining
spherical surface in the particles, but with additional features on account of
the particle shapes having intrinsic anisotropy and orientational degrees of
freedom. The \chi^* = 3/4 system is found to exhibit a phase diagram that is
most similar to the one obtained experimentally for the confined mushroom
cap-shaped colloidal particles under. A qualitative global phase diagram is
constructed that helps reveal the interrelations among different phases for all
the particle shapes and confinements studied.Comment: 35 Pages, 14 Figures, 1 Appendi