FLUCTUATING BOND AGGREGATION - A NUMERICAL-SIMULATION OF NEUTRALLY-REACTED SILICA-GELS

Numerical simulations of gel formation using the newly developed Fluct uating Bond Aggregation (FBA) algorithm are presented. This algorithm allows possible cluster deformations during aggregation by considering a tuning bond flexibility parameter F. Three-dimensional computer sim ulations show that for large F, there is a well-defined threshold valu e of concentration c below which the realization of all intra-aggregat e bond possibilities prevents the formation of a gelling network. For c > c(g), a true sol-gel transition occurs at a characteristic time t( g), i.e. an infinite cluster (self connected through the boundary cond itions) appears. In contrast to the diffusion limited cluster-cluster aggregation (DLCA) model, t(g) does not increase as the box size incre ases. Comparison between numerical results and measurements of the gel ation time of silica gels prepared in container of various sizes sugge sts that gelation without addition of catalyst (neutrally-reacted) and under base catalyzed conditions corresponds to large and small F, res pectively. Moreover, for large F values, we calculate scattering inten sity curves for FBA gels and find that they agree with experimental SA NS data for neutrally-reacted silica aerogels.