Strain Hardening of Fractal Colloidal Gels

We report on experiments on the rheology of gels formed by diffusion-limited aggregation of neutrally buoyant colloidal particles. These gels form very weak solids, with the elastic modulus, G^' (), larger than the loss modulus, G^'' (), and with both G^' () and G^'' () exhibiting only a very weak frequency dependence. Upon small but finite strains <0. 45 the elastic modulus increases roughly exponentially with ^2. We explain the observed strain hardening with the highly nonlinear elastic response of the rigid backbone of the gel to elongational deformation.