  We investigate mixing effects on the glass state of binary colloidal
hard-sphere-like mixtures with large size asymmetry, at a constant volume
fraction phi = 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.
