Cristóvão Sousa Dias (U of Lisbon) Self-organization dynamics and mechanical properties of limited-valence functionalized particles

Network fluids are structured fluids, consisting of chains and branches, product of limited valence interactions. They can be precursors of low-density liquids, photonic crystals, and other materials of enhanced physical properties. Prototypical examples are networks of patchy colloids, where the limited valence results from highly directional pairwise interactions. Here, we computationally study the relaxation dynamics of these networks. In their kinetic route towards thermodynamically stable structures, for low enough temperatures, they self-organize into intermediate (mesoscopic) structures that are much larger than the individual particles and become the relevant units for the dynamics. A temperature driven crossover is observed from exponential to scale-free relaxation dynamics. We show that the typical coarsening dynamics at the coexistence phase is hindered by the dominant dynamics of three-bond particles that falls into the random percolation universality class.

This scale-free relaxation indicates a structural transition that could, not only affect the dynamics, but also the onset of elasticity in the gel structure. The identification of the necessary conditions for the emergence of elasticity in a gel is among the most fundamental challenges in gelation. Recent confocal microscopy experiments suggest that a colloidal gel is mechanically stable if particles with at least six neighbors percolate. However, due to the lack of control over valence in experiments, it is not clear if this is the necessary condition. Here, we address this question using the same model system of limited-valence particles. Combining numerical simulations, oscillatory rheology, and a percolation analysis, we find that the onset of elasticity coincides with percolation of particles with three or more connections. We show that the emergence of elasticity results from a mixture of bending and non-bending interactions, which provides insight into the elasticity of low-valence colloidal gels.