NMR velocimetry studies of the steady-shear rheology of a concentrated hard-sphere colloidal system

Slip and flow of hard-sphere colloidal glasses

We study the flow of concentrated hard-sphere colloidal suspensions along smooth, nonstick walls using cone-plate rheometry and simultaneous confocal microscopy. In the glass regime, the global flow shows a transition from Herschel-Bulkley behavior at large shear rate to a characteristic Bingham slip response at small rates, absent for ergodic colloidal fluids. Imaging reveals both the "solid" microstructure during full slip and the local nature of the "slip to shear" transition. Both the local and global flow are described by a phenomenological model, and the associated Bingham slip parameters exhibit characteristic scaling with size and concentration of the hard spheres.

Rheophysics of pastes: a review of microscopic modelling approaches

Pastes are materials intermediate between solids and liquids which are of great practical interest as they keep the shape they have been given. Despite their various internal structures it is possible to draw up a generic rheophysical scheme from which one qualitatively understands, from a physical point of view, their main mechanical characteristics, solid regime, solid-liquid transition, liquid regime, thixotropy and aging. Here we review in detail these different properties as they are generally observed with most pasty materials and the attempts to describe them using microscopic structure-based theoretical models. For real systems a unified, qualitative, conceptual description is provided. For some model systems (, foams, colloidal gels…) there exist consistent microscopic approaches providing quantitative relationships between rheological parameters in the solid regime and physical parameters of the system. For the liquid regime and thixotropy the situation is more complex.

Yielding and flow in adhesive and nonadhesive concentrated emulsions

The nonlinear rheological response of soft glassy materials is addressed experimentally by focusing on concentrated emulsions where interdroplet attraction is tuned through varying the surfactant content. Velocity profiles are recorded using ultrasonic velocimetry simultaneously to global rheological data in the Couette geometry. Our data show that nonadhesive and adhesive emulsions have radically different flow behaviors in the vicinity of yielding: while the flow remains homogeneous in the nonadhesive emulsion and the Herschel-Bulkley model for a yield stress fluid describes the data very accurately, the adhesive system displays shear localization and does not follow a simple constitutive equation, suggesting that the mechanisms involved in yielding transitions are not universal.