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'(omega), larger than the loss modulus, G "(omega), and with both G'(omega) and G "(omega) exhibiting only a very weak frequency dependence. Upon small but finite strains gamma < 0.45 the elastic modulus increases roughly exponentially with gamma(2). We explain the observed strain hardening with the highly nonlinear elastic response of the rigid backbone of the gel to elongational deformation.