In an attempt to establish connections between classical rheology and aging in paste colloidal suspensions, we report in this paper a large set of experimental results on a given system. We have studied suspensions of polyethylene oxide-protected silica particles and performed classical rheology experiments that exhibit a very nonlinear behavior. We have then evidenced aging through stress relaxation as observed in various glassy systems, and finally show other manifestations of aging through various rheological experiments. Qualitative agreement between these experimental results and the predictions of a simple model suggests that the behavior observed experimentally is governed by the competition between aging and mechanically induced rejuvenation.

We introduce a rheological model to describe the low-frequency mechanical properties of systems near a fluid/paste transition. We propose a Landau-like expansion for the vicinity of this transition, treating the stress relaxation 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 the mechanical response of the system. We focus here on systems prepared by fluidification under a strong shear, on which mechanical measurments are performed (oscillatory rheology, stress relaxation, response to a steady shear rate), after a waiting time during which the system evolves on its own. PACS. 62.20.Fe Deformation and plasticity -83.20.-d Constitutive relations -83.50.Gd Nonlinear viscoelasticity

We study the rapid collapse of gels formed from strongly aggregating colloidal suspensions. This gravity-driven collapse is associated with the apparition of fractures in the bulk of the gels that provide an easy route to the gel-supernatant interface for the solvent and are the cause for the strong increase of the settling velocity. We propose a model that connects the apparition of a fracture in the gel to the settling velocity of the interface. This description takes into account the microscopic structure of the gel and is consistent with the experimental results.

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