(321bf) Gravitational Stability of Suspensions with a Depletion Attraction

In a gravitational field colloidal gels sediment or cream depending on the density difference between colloids and solvent. From the gravitational stress on a gel and the change in local colloid volume fraction, we find a stress (σ) - volume fraction (φ) relationship for the gel in the form of σ(φ) = α(φ-φ_g)/(φ_c-φ), where φ_g is the critical gel fraction,φ_c is the random close packing fraction, and α is a stiffness. We find that α is proportional to the inter-particle interaction. Compressional modulus of the gel is derived from the σ-φ relationship in a steady state. From this modulus, we are able to predict the total deformation of the gel and the final volume fraction profile in good agreement with experimental measurements. In combination with a nonlinear poroelastic model, we can depict the creaming/ sedimentation process. Our study thus offers a novel route for predicting long-term processes in colloidal gels; in particular, it represents a strategy towards predicting long-term stability of colloidal systems.