In recent years, much effort has been done to understand the physical mechanisms that lead to colloidal gelation. Some groups claim that the frustration of the spinodal decomposition might be understood as a kind of universal route for the formation of colloidal gels in short-ranged attractive colloids. However, recent experimental evidence pointed out that such route may be a consequence of the competition between gravitational forces and the attractive ones between colloidal particles. Furthermore, it was found that there is a critical and effective gravitational Péclet number that clearly indicates the onset of phase separation-induced gelation. Nevertheless, this important observation has not been studied in detail at the molecular level. Then, to have a better understanding of the gravitational effects on the formation of colloidal gels, in this contribution, we have performed extensive Monte Carlo computer simulations of sticky particles under the influence of a gravitational field. The Asakura-Oosawa potential, which describes the depletion-induced attraction between colloids immersed in a bath of non-adsorbing polymers, was used to model the forces between colloidal particles. We systematically analyze the effects that gravity has on the percolation, the cluster size distribution and the fractal dimension of the resulting aggregates. From this analysis and from the determination of those states where the critical gravitational Péclet number occurs, we have built a state diagram in terms of the strength of the attractive forces and the bare Péclet number. We thus report the boundary where the onset of gelation might be associated to gravity.

Effect of the gravitational field on the gelation in sticky colloidal dispersions

ACS Spring 2022

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