The sol-gel transition in gelatin viewed by diffusing colloidal probes

This paper reports a dynamic light scattering investigation of the sol-gel transition of gelatin solutions in the presence of latex spheres, which act as local probes for the viscoelasticity of the medium. The experimental procedure allows us to define a local gel time related to the size of the probes. In the sol state and in the gelling solutions, below the gel point the dependence of the apparent diffusion coefficients of the spheres on correlation rimes, shows the existence of a fast local diffusion coefficient and of a slow long-term diffusion coefficient, related to viscosity. The behaviour of the fast diffusion coefficient during the course of gelation was quite unexpected, as it remained constant until the moment when the network was formed, when it suddenly started to increase. We propose a model for the diffusion of spherical particles in a viscoelastic medium (Voigt model) by using the Langevin equation. The experimental results are compared to the model. This investigation shows evidence for the coupling between the cooperative movements of the gel network and the local diffusion of the spheres. It illustrates the diffusion of particles through a disordered medium which leads to non-brownian, hypodiffusive regimes.