We investigate the mechanical properties of an over-jammed dispersion of
Carbopol-microgels in propylene glycol. This dispersion belongs to the class of yield
stress materials, which are materials that are solids when left unperturbed, yet flow
when the shear stress applied exceeds a certain threshold value.
The yielding behavior of materials is typically assessed in either steady shear
experiments exploring the flow behavior as a function of shear rate or in oscillatory
strain experiments exploring the oscillatory stress response as a function of applied
strain amplitude. In this work we address the question whether the maximal shear
rate or the maximal strain experienced during oscillation is the relevant parameter
governing yielding in oscillatory shear experiments.
Our oscillatory experiments exploring the stress response as a function of strain
amplitude reveal that yielding occurs in an intermediate range of strains between
two strain regimes, in which the stress response is respectively governed by strain
and strain rate. Beyond a critical strain defining the high strain regime the stress solely
depends on the maximum strain rate experienced during oscillation. This is denoted
by the direct correspondence of the stress values obtained in respectively oscillatory
and steady shear rate experiments. By contrast, in the regime denoting the low strain
regime the stress is a sole function of the strain.
Thus, yielding would be governed by both parameters when considering the
mechanical response. By contrast, an exploration of the restructuring dynamics at
the yield point indicates that structural rearrangements are a sole function of the
strain. Further adding to the contradiction we find that the dynamics sharply
increase at the yield point, while the mechanics smoothly evolve across the yielding
transition. This may indicate that structural relaxation and stress are not linearly
related.