Three-step colloidal gelation revealed by time-resolved x-ray photon correlation spectroscopy.

The gelation of PEGylated gold nanoparticles dispersed in a glycerol-water mixture is probed in situ by x-ray photon correlation spectroscopy. Following the evolution of structure and dynamics over 10 ⁴ s, a three-step gelation process is found. First, a simultaneous increase of the Ornstein-Zernike length ξ and slowdown of dynamics is characterized by an anomalous q-dependence of the relaxation times of τ ∝ q ^-6 and strongly stretched intermediate scattering functions. After the structure of the gel network has been established, evidenced by a constant ξ, the dynamics show aging during the second gelation step accompanied by a change toward ballistic dynamics with τ ∝ q ^-1 and compressed correlation functions. In the third step, aging continues after the arrest of particle motion. Our observations further suggest that gelation is characterized by stress release as evidenced by anisotropic dynamics once gelation sets in.