Rheology and aging: A simple approach

We introduce a rheological model to describe the low-frequency mechanical p roperties of systems near a fluid/paste transition. We propose a Landau-lik e expansion for the vicinity of this transition, treating the stress relaxa tion rate as an order parameter. This leads to a formally simple model that allows us to describe the interplay between aging and non-linearities in t he mechanical response of the system. We focus here on systems prepared bq. fluidification under a strong shear, on which mechanical measurements are performed (oscillatory rheology, stress relaxation: response to a steady sh eer rate), after a waiting time during which the system evolves on its own.