Exclusion of Impurity Particles during Grain Growth in Charged Colloidal Crystals

We examine the spatial distribution of fluorescent-labeled charged polystyrene (PS) particles (particle volume fraction ϕ = 0.0001 and 0.001, diameter <i>d</i> = 183 and 333 nm) added to colloidal crystals of charged silica particles (ϕ = ϕ_s = 0.035–0.05, <i>d</i> = 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 <i>d</i> = 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

Controlled Clustering in Binary Charged Colloids by Adsorption of Ionic Surfactants

We report on the controlled clustering of oppositely charged colloidal particles by the adsorption of ionic surfactants, which tunes charge numbers <i>Z</i> of particles. In particular, we studied the heteroclustering of submicron-sized polystyrene (PS) and silica particles, both of which are negatively charged, in the presence of cetylpyridinium chloride (CPC), a cationic surfactant. The surfactant concentration <i>C</i>_surf was selected below the critical micelle concentration. As CPC molecules were adsorbed, <i>Z</i> values of the PS and silica particles decreased, inverting to positive when <i>C</i>_surf exceeded the isoelectric point <i>C</i>_iep. Hydrophobic PS particles exhibited much lower <i>C</i>_iep than hydrophilic silica particles. At <i>C</i>_surf valuess between their <i>C</i>_iep values, the particles were oppositely charged, and clustering was enabled. To explain the clustering behavior, we investigated adsorption isotherms of the CPC and screened-Coulomb-type pair potential. Expected applications of the present findings are the control of colloidal associations and construction of various particle types into heterogeneous colloidal clusters

Recrystallization and Zone Melting of Charged Colloids by Thermally Induced Crystallization

We examined the application of recrystallization and zone-melting crystallization methods, which have been used widely to fabricate large, high-purity crystals of atomic and molecular systems, to charged colloidal crystals. Our samples were aqueous dispersions of colloidal silica (with particle diameters of <i>d</i> = 108 or 121 nm and particle volume fractions of ϕ = 0.035–0.05) containing the weak base pyridine. The samples crystallized upon heating because of increases in the particle charge numbers, and they melted reversibly on cooling. During the recrystallization experiments, the polycrystalline colloids were partially melted in a Peltier cooling device and then were crystallized by stopping the cooling and allowing the system to return to ambient temperature. The zone-melting crystallization was carried out by melting a narrow zone (millimeter-sized in width) of the polycrystalline colloid samples and then moving the sample slowly over a cooling device to recrystallize the molten region. Using both methods, we fabricated a few centimeter-sized crystals, starting from millimeter-sized original polycrystals when the crystallization rates were sufficiently slow (33 μm/s). Furthermore, the optical quality of the colloidal crystals, such as the half-band widths of the diffraction peaks, was significantly improved. These methods were also useful for refining. Small amounts of impurity particles (fluorescent polystyrene particles, <i>d</i> = 333 nm, ϕ = 5 × 10^–5), added to the colloidal crystals, were excluded from the crystals when the crystallization rates were sufficiently slow (∼0.1 μm/s). We expect that the present findings will be useful for fabricating large, high-purity colloidal crystals

Thermoresponsive Colloidal Crystallization Using Adsorption of Ionic Surfactants

Thermoresponsive Colloidal Crystallization Using Adsorption of Ionic Surfactant