Numerical study of cluster formation in binary charged colloids

Formation of two-dimensional diamond-like colloidal crystals using layer-by-layer electrostatic self-assembly

We report here that a two-dimensional (2D) diamond-like structure of micron-sized colloidal particles can be obtained by layer-by-layer self-assembly. Positively and negatively charged silica particles, 1 μm in diameter, were used in the experiments. On a positively charged, flat glass substrate, the first layer of negatively charged particles was prepared to form a non-close-packed 2D crystal. Then the second and third layers were fabricated using electrostatic adsorption. The positions of adsorbed particles were controllable by tuning the zeta-potential of the particles and the salt concentration of the medium. The FDTD calculations show that the 2D diamond structures of particles with higher refractive index (titania) have an absorption band in the wavelength range corresponding to the photonic band gap of the 3D bulk crystal. We expect these findings to be useful for the fabrication of novel photonic materials.

Shear-Induced Heteroaggregation of Oppositely Charged Colloidal Particles

This paper investigates numerically the shear-induced aggregation of mixed populations of colloidal particles leading to the formation of clusters. Suspensions with different amounts of positively and negatively charged colloidal particles are simulated. To resolve the aggregation kinetics and structural properties of the formed clusters, we resort to a mixed deterministic-stochastic simulation method. The method is built on a combination of a Monte Carlo algorithm to sample a statistically expected sequence of encounter events between the suspended particles and a discrete element method built in the framework of Stokesian dynamics to simulate the encounters in a fully predictive manner. Results reveal a strong influence of the composition of the population on both the aggregation kinetics and the aggregate structure. In particular, we observe a size-stabilization phenomenon taking place in the suspension when the relative concentration of the majority particles lies in the range 80-85%; i.e., starting from primary particles, after a short growth period, we observed a cessation of aggregation. Inspection of the aggregate morphology shows that the formed clusters are composed of few minority particles placed in the inner region, while the aggregate surface is covered by majority particles, acting to provide a shielding effect against further growth.