We examine the spatial distribution of fluorescent-labeled charged polystyrene (PS) particles (particle volume fraction ϕ = 0.0001 and 0.001, diameter d = 183 and 333 nm) added to colloidal crystals of charged silica particles (ϕ = ϕ(s) = 0.035-0.05, d = 118 nm). At ϕ(s) = 0.05, the PS particles were almost randomly distributed in the volume-filling polycrystal structures before the grain growth process. Time-resolved confocal laser scanning microscopy observations reveal that the PS particles are swept to the grain boundaries of the colloidal silica crystals owing to grain boundary migration. PS particles with d = 2420 nm are not excluded from the silica crystals. We also examine influences of the impurities on the grain growth laws, such as the power law growth, size distribution, and existence of a time-independent distribution function of the scaled grain size.

We report that binary dispersions of like-charged colloidal particles with large charge asymmetry but similar size exhibit phase separation into crystal and fluid phases under very low salt conditions. This is unexpected because the effective colloid-colloid pair interactions are accurately described by a Yukawa model which is stable to demixing. We show that colloid-ion interactions provide an energetic driving force for phase separation, which is initiated by crystallization of one species

Uniformly shaped, charged colloidal particles dispersed in water form ordered "crystal" structures when the interaction between the particles is sufficiently strong. Herein, we report the behavior of "impurity" particles, whose sizes and/or charge numbers are different from those of the bulk, on addition to the charged colloidal crystals. These impurities were excluded from the crystals during the homogeneous crystallization, crystal grain growth, and unidirectional crystallization processes. Such systems will be useful as models for studying the refinement of materials and crystal defects.

Charged colloids form "crystal" structures, wherein the colloidal particles are regularly arranged as a result of strong electrostatic interparticle interactions. We have reported that silica colloids containing pyridine (Py) form colloidal crystals upon heating. Herein, we report that in Py + silica colloids including small numbers of fluorescent polystyrene (PS) particles, the PS particles are arranged in stripe patterns upon stepwise unidirectional crystallization when the temperature is increased by 1°C every 10 and 15 min. We think that the present stripe patterns are closely analogous to the thermally induced striations found in a number of crystalline materials.Charged colloidal particles self-assemble to form "crystal" structures when the electrostatic interactions among the particles are sufficiently strong. In the crystal states, the colloidal particles are regularly arranged in body-centered or face-centered cubic lattices. 14We have reported that colloidal silica containing pyridine (Py) exhibits thermally induced crystallization. 5,6 Silica particles have weakly acidic silanol groups (SiOH) on their surfaces, which partly dissociate to provide surface charges. On addition of Py, the dissociation of silanol groups is enhanced, resulting in an increase in the particle charge number Z, that is, the electrostatic interaction becomes stronger. Since dissociation of Py increases with temperature (T), the silica + Py colloids crystallize upon heating, under appropriate conditions. Furthermore, we reported that impurity particles added to the silica crystal are excluded from the crystal grains and accumulate in the crystal grain boundaries. 7 In particular, stripe patterns were observed in charged silica + polystyrene (PS) binary colloids after unidirectional crystallization driven by diffusion of Py. 812 This pattern formation was explainable in terms of a combination of (1) development of thin, lamellashaped crystal grains and (2) exclusion of the PS particles from the grains to the crystal grain boundaries.In the present study, we report the formation of another type of stripe pattern in silica + PS binary colloids, which is observed after unidirectional crystallization when the temperature is varied in a stepwise manner. We think that the stripe patterns observed here are closely analogous to the "striation" patterns observed in crystalline materials that results from the short-range composition fluctuations found in nearly all crystals. 13Aqueous dispersions of colloidal silica particles (KE-W10, Japan Catalyst Co., Ltd., Tokyo, Japan) were used after they were first purified by dialysis for four months and then purified using an ion-exchange method. The particle diameter, determined by the dynamic light-scattering method, was 105 nm. An aqueous dispersion of fluorescent PS particles (G300, Thermo Scientific Inc., Waltham, MA, USA, diameter: 333 nm) was purified using an ion-exchange method. In the binary colloids used for the experiments, the volume fractions of silica (º silica ) and PS particles (º...

We examine the spatial distributions of fluorescent-labeled polystyrene (PS) particles added to colloidal silica that resulted after unidirectional crystal growth due to diffusion of pyridine. We find that the PS particles are arranged in a stripe pattern with a submillimeter-sized spacing. The patterning mechanism is attributable to the formation of lamella-shaped crystal grains and exclusion of the PS particles to the crystal grain boundaries.