Structural arrest in chemical and colloidal gels

This chapter introduces a minimal model for gelling systems and examines the dynamic behavior by means of numerical simulations. At the gelation transition, a viscous liquid transforms to an elastic disordered solid. This corresponds to the formation of a spanning structure that makes the system able to bear stress. In polymer systems, the structure formation is due to chemical bonding, producing a polymerization process. The gelling system typically displays critical power law behavior in the viscoelastic response and slow dynamics. The relaxation functions show, at long times, a stretched exponential decay, and at the gel point the relaxation process becomes critically slow. This suggests a unifying picture for gelation phenomena, connecting classical gelation and recent results on colloidal systems. By varying the model parameters, the slow dynamics present a crossover from the classical polymer gelation to dynamics more typical of colloidal systems, with a glassy regime that is interpreted in terms of effective clusters.