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The yielding behaviour of hard sphere glasses under large amplitude oscillatory shear has been studied by probing the interplay of Brownian motion and shear-induced diffusion at varying oscillation frequencies. Stress, structure and dynamics are followed by experimental rheology and Browian Dynamics simulations. Brownian motion assisted cage escape dominates at low frequencies while escape through shear-induced collisions at high ones, both related with a yielding peak in $G^{prime prime}$. At intermediate frequencies a novel, for HS glasses, double peak in $G^{prime prime}$ is revealed reflecting both mechanisms. At high frequencies and strain amplitudes a persistent structural anisotropy causes a stress drop within the cycle after strain reversal, while higher stress harmonics are minimized at certain strain amplitudes indicating an apparent harmonic response.
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There is growing evidence that the flow of driven amorphous solids is not homogeneous, even if the macroscopic stress is constant across the system. Via event driven molecular dynamics simulations of a hard sphere glass, we provide the first direct evidence for a correlation between the fluctuations of the local volume-fraction and the fluctuations of the local shear rate. Higher shear rates do preferentially occur at regions of lower density and vice versa. The temporal behavior of fluctuations is governed by a characteristic time scale, which, when measured in units of strain, is independent of shear rate in the investigated range. Interestingly, the correlation volume is also roughly constant for the same range of shear rates. A possible connection between these two observations is discussed.
We report results of dynamic light scattering measurements of the coherent intermediate scattering function (ISF) of glasses of hard spheres for several volume fractions and a range of scattering vectors around the primary maximum of the static structure factor. The ISF shows a clear crossover from an initial fast decay to a slower non-stationary decay. Ageing is quantified in several different ways. However, regardless of the method chosen, the perfect aged glass is approached in a power-law fashion. In particular, the coupling between the fast and slow decays, as measured by the degree of stretching of the ISF at the crossover, also decreases algebraically with waiting time. The non-stationarity of this coupling implies that even the fastest detectable processes are themselves non-stationary.
We study the flow of concentrated hard-sphere colloidal suspensions along smooth, non-stick 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.
The rheological response, in particular the non-linear response, to oscillatory shear is experimentally investigated in colloidal glasses. The glasses are highly concentrated binary hard-sphere mixtures with relatively large size disparities. For a size ratio of 0.2, a strong reduction of the normalized elastic moduli, the yield strain and stress and, for some samples, even melting of the glass to a fluid is observed upon addition of the second species. This is attributed to the more efficient packing, as indicated by the shift of random close packing to larger total volume fractions. This leads to an increase in free volume which favours cage deformations and hence a loosening of the cage. Cage deformations are also favoured by the structural heterogeneity introduced by the second species. For a limited parameter range, we furthermore found indications of two-step yielding, as has been reported previously for attractive glasses. In samples containing spheres with more comparable sizes, namely a size ratio of 0.38, the cage seems less distorted and structural heterogeneities on larger length scales seem to become important. The limited structural changes are reflected in only a small reduction of the moduli, yield strain and stress.
Glass forming liquids exhibit a rich phenomenology upon confinement. This is often related to the effects arising from wall-fluid interactions. Here we focus on the interesting limit where the separation of the confining walls becomes of the order of a few particle diameters. For a moderately polydisperse, densely packed hard-sphere fluid confined between two smooth hard walls, we show via event-driven molecular dynamics simulations the emergence of a multiple reentrant glass transition scenario upon a variation of the wall separation. Using thermodynamic relations, this reentrant phenomenon is shown to persist also under constant chemical potential. This allows straightforward experimental investigation and opens the way to a variety of applications in micro- and nanotechnology, where channel dimensions are comparable to the size of the contained particles. The results are in-line with theoretical predictions obtained by a combination of density functional theory and the mode-coupling theory of the glass transition.
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