The mechanism of flow in glassy materials is interrogated using
mechanical spectroscopy applied to model nearly hard sphere
colloidal glasses during flow. Superimposing a small amplitude
oscillatory motion orthogonal onto steady shear flow makes it
possible to directly evaluate the effect of a steady state flow on
the out-of-cage (α) relaxation as well as the in-cage motions. To
this end, the crossover frequency deduced from the viscoelastic
spectra is used as a direct measure of the inverse microstructural
relaxation time, during flow. The latter is found to scale
linearly with the rate of deformation. The microscopic mechanism
of flow can then be identified as a convective cage
release. Further insights are provided when the viscoelastic
spectra at different shear rates are shifted to scale the alpha
relaxation and produce a strain rate-orthogonal frequency
superposition, the colloidal analogue of time temperature
superposition in polymers with the flow strength playing the role
of temperature. Whereas the scaling works well for the α
relaxation, deviations are observed both at low and high
frequencies. Brownian dynamics simulations point to the origins of
these deviations; at high frequencies these are due to the
deformation of the cages which slows down the short-time
diffusion, while at low frequency, deviations are most probably
caused by some mild hydroclustering.

From: Alan R. Jacob, Andreas S. Poulos, Sunhyung Kim, Jan Vermant, and George Petekidis “Convective Cage Release in Model Colloidal Glasses”
Phys. Rev. Lett.,
115, 218301 (2015).