We identify dynamic and structural signatures which distinguish the gel and glass transitions in a colloidal model system of `sticky' spheres. Since gels are related to arrested phase separation, this presents a thermodynamic basis for gelation, and distinguishes it from vitrification. We base our findings on confocal microscopy experiments on colloid-polymer mixtures and confirm these with molecular dynamics simulations. Here we show that on crossing the phase-separation line there is a sharp change in the structural relaxation time and local structure, reflecting densification of the colloid-rich phase. Furthermore simulation reveals pressure discontinuities even at packing fractions of 0.59. These suggest gelation is first-order-like and occurs at much higher densities than previously thought: gels are not low-density networks but have a clear thermodynamic definition which is manifested in the path to dynamic arrest leading to a non-equilibrium state with distinct local structure. Approaching the glass transition, dynamics and structure change continuously.

Gelation and vitrification: The nature of ergodic-non-ergodic transitions in sticky spheres

244th National Meeting (2012)

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