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^'(ω), larger than the loss modulus, G^''(ω), and with both G^'(ω) and G^''(ω) exhibiting only a very weak frequency dependence. Upon small but finite strains γ<0.45 the elastic modulus increases roughly exponentially with γ². We explain the observed strain hardening with the highly nonlinear elastic response of the rigid backbone of the gel to elongational deformation.