We present a detailed comparison of the rheology of concentrated hard and soft-sphere suspensions using a variety of techniques including large-amplitude oscillatory shear (LAOS). While the soft spheres are jammed and exhibit permanent contact, the hard-sphere suspensions are below close packing where particle collisions lead to an effective modulus. Oscillatory shear measurements are used to determine the strain-dependent viscoelastic moduli and yield stress. A recent scheme is applied to interpret LAOS data in terms of a sequence of physical processes [Rogers et al., J. Rheol. 55, 435-458 (2011a)], revealing different characteristics of yielding, flow, and structural rejuvenation in the two systems. While for hard spheres, yielding and flow are governed by the breaking and rejuvenation of the nearest neighbor cage; for soft spheres, the particle compliance gives rise to a much more gradual yielding. We address the effect of particle softness directly by measuring the single-particle modulus with atomic force microscopy, and linking it to the suspension modulus via the pair correlation function determined by microscopy. V

No abstract

No abstract

We present an experimental investigation of the coiling of a filament of a yield stress fluid falling on a solid surface. We use two kinds of yield stress fluids, shaving foam and hair gel, and show that the coiling of the foam is similar to the coiling of an elastic rope. Two regimes of coiling (elastic and gravitational) are observed for the foam. Hair gel coiling, on the other hand, is more like the coiling of a liquid system; here we observe viscous and gravitational regimes. No inertial regime is observed for either system because of instabilities occurring at high flow rates or the breakup of the filament from large heights.

Glasses are lucrative engineering materials owing to their superior mechanical properties such as high strength and large elastic strain. A central question concerns incipient plasticity – the onset of permanent deformation – that is central to their relaxation, aging, yield and fracture. Here, we use an analogue of nano-indentation performed on a colloidal glass to obtain direct images of the incipient plasticity, allowing us to elucidate the onset of permanent deformation. We visualize the microscopic strain by following distorted nearest neighbor configurations, and observe a surprising hierarchical structure of deformation: at the onset of irreversible deformation, the strain acquires a robust fractal structure, and we measure its fractal dimension. These results give direct evidence that the onset of permanent deformation has the hallmarks of a critical point, in agreement with recent theoretical works.

We use an analog of nanoindentation on a colloidal glass to elucidate the incipient plastic deformation of glasses. By tracking the motion of the individual particles in three dimensions, we visualize the strain field and glass structure during the emerging deformation. At the onset of flow, we observe a power-law distribution of strain indicating strongly correlated deformation, and reflecting a critical state of the glass. At later stages, the strain acquires a Gaussian distribution, indicating that plastic events become uncorrelated. Investigation of the glass structure using both static and dynamic measures shows a weak correlation between the structure and the emerging strain distribution. These results indicate that the onset of plasticity is governed by strong power-law correlations of strain, weakly biased by the heterogeneous glass structure.

No abstract