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.Comment: 9 pages, 8 figures, accepted in Soft Matte

Using confocal microscopy we investigate binary colloidal mixtures with large size asymmetry, in particular the formation of dynamically arrested states of the large spheres. The volume fraction of the system is kept constant, and as the concentration of small spheres is increased we observe a series of transitions of the large spheres to different arrested states: an attractive glass, a gel, and an asymmetric glass. These states are distinguished by the degree of dynamical arrest and the amount of structural and dynamical heterogeneity. The transitions between two different arrested states occur through melting and the formation of a fluid state. While a space-spanning network of bonded particles is found in both arrested and fluid states, only arrested states are characterized by the presence of a space-spanning network of dynamically arrested particles.

The linear and nonlinear rheological behavior of two rod-like particle suspensions as a function of concentration is studied using small amplitude oscillatory shear, steady shear and capillary breakup extensional rheometry.

We investigate the yielding and transition to flow of different colloidal glasses. Using a single model system, a binary mixture of colloidal hard spheres with different compositions and size ratios, we study single, double and asymmetric glasses, which differ in the degree of mobility of the small particles and the caging mechanisms of the large spheres. The rheological response following either a step to a constant shear rate or to a constant stress (creep) is measured and the two responses are quantitatively compared. Although the same steady state of flow is observed at long times, the transient responses in strain-and stress-controlled experiments differ significantly. To achieve yielding and a steady state of flow, less time and less energy input is required if a constant strain rate is applied. Moreover, larger strain rates or stresses result in faster yielding and flow, but require more total energy input. If a constant strain rate is applied, yielding and the transition to flow depend on the properties of the glass state, while much smaller differences are observed if a constant stress is applied. V

In this study, we present a comparative investigation of linear and branched wormlike micelles using two nonlinear rheological tools: orthogonal superposition rheology and large amplitude oscillatory shear (LAOS) rheology. The surfactants were a series of mixtures of octyl trimethyl ammonium bromide (C 8 TAB) and sodium oleate (NaOA). A transition from linear to branched wormlike micelles was obtained by either varying the relative ratio of NaOA to C 8 TAB at a fixed total surfactant concentration or by fixing the ratio of NaOA to C 8 TAB and varying the total surfactant concentration. Orthogonal superposition rheology imposes a small amplitude oscillatory shear strain over an orthogonally imposed shear flow to probe the effect of shear on the storage and loss modulus of the fluid. For both the linear and branched wormlike micelle solutions, the plateau modulus, the relaxation times, and the dynamic viscosity were all found to be sensitive to the strength of the orthogonally imposed shear rate. However, the nonlinear effects were much more pronounced for the case of the branched wormlike micelle solutions. This is likely due to a breakdown of the branched wormlike micelles under flow which can be inferred from the decrease in the plateau modulus and the subsequent increase in the calculated mesh size of the entangled micelle network with increasing orthogonal shear-rate. In the LAOS measurements, both the linear and branched wormlike micelles exhibited a qualitatively similar trend in the viscoelastic nonlinearities as the strain amplitude of the imposed oscillatory flow was increased. However, the strength of viscoelastic nonlinearities, both within an oscillatory cycle and with increasing strain amplitude, of the branched wormlike micelles was found to be significantly larger than those observed for the linear wormlike micelles. Additionally, at large strain amplitudes, the Lissajous-Bowditch plots of the branched wormlike micelle solutions were found to exhibit a secondary loop resulting from a negative elastic modulus. This is indicative of a stress overshoot and likely the result of a cyclical breakdown and reformation of the underlying wormlike micelle entangled structure during an oscillatory cycle. V