Self-diffusion and collective diffusion in a model viscoelastic system.
We use dynamic light scattering (DLS) and fluorescence recovery after pattern photobleaching (FRAPP) to investigate the dynamics of a model transient network made of an oil-in-water droplet microemulsion to which small amounts of a telechelic polymer are added. The DLS correlation functions exhibit three relaxation modes. The two first modes can be interpreted quantitatively in the frame of the classical De Gennes-Brochard theory of DLS in viscoelastic system. The third, slower mode is diffusive and arises from the ternary character (droplets, polymers, and water) of the system. By contrast, the pattern relaxation in FRAPP exhibits a single-, slow-exponential decay with a characteristic time proportional to the squared inverse scattering vector: the corresponding self-diffusion coefficient of the droplets is found to be close to the diffusion coefficient characterizing the third mode in DLS. We interpret these results in terms of the coupled relaxation of the concentration fluctuations of the polymers and the droplets.