Glasses of dynamically asymmetric binary colloidal mixtures: Quiescent properties and dynamics under shear

Abstract

We investigate mixing effects on the glass state of binary colloidal hard-sphere-like mixtures with large size asymmetry, at a constant volume fraction φ = 0.61. The structure, dynamics and viscoelastic response as a function of mixing ratio reflect a transition between caging by one or the other component. The strongest effect of mixing is observed in systems dominated by caging of the large component. The possibility to pack a large number of small spheres in the free volume left by the large ones induces a pronounced deformation of the cage of the large spheres, which become increasingly delocalised. This results in faster dynamics and a strong reduction of the elastic modulus. When the relative volume fraction of small spheres exceeds that of large spheres, the small particles start to form their own cages, slowing down the dynamics and increasing the elastic modulus of the system. The large spheres become the minority and act as an impurity in the ordering beyond the first neighbour shell, i.e. the cage, and do not directly affect the particle organisation on the cage level. In such a system, when shear at constant rate is applied, melting of the glass is observed due to facilitated out-of-cage diffusion which is associated with structural anisotropy induced by shear.

Different mechanisms for dynamical arrest in largely asymmetric binary mixtures

“…We compare these findings with results obtained in previous work for δ = 0.38 and 0.2 [17][18][19] as well as previous experiments for δ = 0.1 [13] and theory predictions [11,12]. They are summarized in a state diagram as a function of composition x s and size ratio δ (Fig.…”

“…Very small values of x s , like x s = 0.01, were not investigated for δ = 0.2 and therefore the existence of an attractive glass cannot be excluded. The asymmetric glass state observed for [13], δ = 0.2 and 0.38 [17][18][19]. Different arrested states are identified in the present work: repulsive glass (•), attractive glass ( ), asymmetric glass ( ), and gel ( ).…”

“…Addition of a second component with a different size affects the glass transition [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. For moderate size disparities, δ = R s /R L 0.35, mixing particles with different radii R s and R L , respectively, results in a shift of the glass transition to larger total volume fractions φ, similar to the effect of polydispersity [8][9][10].…”

Transient dynamics during stress overshoots in binary colloidal glasses

“…Similar data were reported and discussed in detail in ref. 19. We thus only recall the main ndings.…”

“…19,20 For larger fractions of small particles, x s > 0.5, the long-time dynamics again slow down and particles continue to become increasingly localised in the cage of small particles. This transition in caging and the faster dynamics at intermediate compositions have been observed previously for the same d. 19 However, the acceleration of the dynamics in the present mixtures is much more pronounced than at larger d. 16,20,31,32,42,43 This could result from the melting of the cage of large spheres, which accompanies the glass-glass transition observed at x s ¼ 0.5 in our system. This appears to affect the particle dynamics more than the smaller cage polydispersity in mixtures of particles with more comparable sizes.…”