  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.
