Simulation study of nonergodicity transitions : gelation in colloidal systems with short-range attractions

Computer simulations were used to study the gel transition occurring in colloidal systems with short-range attractions. A colloid-polymer mixture was modeled and the results were compared with mode coupling theory (MCT) expectations and with the results for other systems (hard-spheres system and Lennard-Jones system). The self-intermediate scattering function and the mean squared displacement were used as the main dynamical quantities. Two different colloid packing fractions have been studied. For the lower packing fraction, a-scaling holds and the wave-vector analysis of the correlation function shows that gelation is a regular nonergodicity transition within MCT. The leading mechanism for the novel nonergodicity transition is identified as the bond formation caused by the short-range attraction. The time scale and diffusion coefficient also show qualitatively the expected behavior, although different exponents are found for the power-law divergences of these two quantities. The non-Gaussian parameter was also studied and a very large correction to Gaussian behavior was found. The system with higher colloid packing fraction shows indications of a nearby high-order singularity, causing a scaling to fail, but the general expectations for nonergodicity transitions still hold