Aging and solid or liquid behavior in pastes

Abstract

We carried out systematic creep tests after different times of rest and over sufficiently long times with pasty materials of various internal structures in a Couette geometry. From an analysis of the data taking into account the inertia of the system and the heterogeneous distribution of stress, we show that: ͑i͒ for a stress below the yield stress these materials remain solid but undergo residual, irreversible deformations over long time which exhibit some trends typical of aging in glassy systems; ͑ii͒ as a result of thixotropy ͑or aging͒ in the solid regime the elastic modulus increases logarithmically with the time of rest; ͑iii͒ in the liquid regime the effective behavior of the material can be well represented by a truncated power-law model; ͑iv͒ a fundamental parameter of the solid-liquid transition is a critical effective shear rate ͑associated with the yield stress͒ below which the material cannot flow steadily.

NMR observation of water transfer between a cement paste and a porous medium

“…Indeed, in such a bentonite suspension, the clay particles are linked by strong colloidal interactions. The material has thus a highly thixotropic behaviour, resulting in an easy flow shortly after a strong blending, but if the material is left at rest for a sufficient time, it may exhibit a high yield stress or can develop shear-banding in some cases [33]. Here, the apparent yield stress of our paste was 50 Pa just after mixing, and 200 Pa after 30 min at rest.…”

Slow flows of yield stress fluids: yielding liquids or flowing solids?

“…Indeed let us consider a series of creep tests with a given yield stress material, consisting to apply a given stress to a sample and follow the deformation in time, then start again the same test at another stress level on the material placed in the same initial state. For a stress below a critical value we get a response typical of a solid [Cottrell 1964] (see Figure 1): the deformation first increases, then very rapidly tends to saturate and reach a plateau [Coussot et al 2006]. Remarkably, for some YSF, if we maintain the stress application for a long time, we may observe an extremely slow "flow", in fact a very slight increase of the apparent deformation [Lidon et al 2017].…”

Rheological behavior of acrylic paint blends based on polyaniline

“…Yield stress measurements have previously been used to indicate the thixotropy of a material as a function of time [39][40][41] . The Yield stress values of acrylic paint modified with PAni are shown in Figure 3.…”