Plastic rearrangements in colloidal gels investigated by LAOS and LS-Echo

We investigate the yielding behavior of colloid-polymer gels with intermediate volume fraction, which are subjected to large amplitude oscillatory shear, using rheology and light scattering echo (LS-Echo). Particular attention is given to the anharmonic contributions to the stress response and the characteristic timescale and extent of plastic rearrangements. Yielding is already observed at small strain amplitudes c0 1%, where the network of interconnected clusters starts to break up and irreversible particle rearrangements are first observed. However, only at considerably larger strain amplitudes, c0 100%, the network is completely disrupted and small clusters or individual particles flow. This complex yielding behavior is reflected in different regimes of in-cycle yielding, which were extracted from the analysis of the anharmonic contributions to the stress. In the range of strain amplitudes where bond breaking starts, in-cycle strain hardening and shear thickening are observed, with the strain hardening possibly caused by the shear-induced formation of more compact clusters. When the network is broken down, in-cycle shear thinning is observed with increasing strain amplitude, but still together with strain hardening. Both, the elastic and viscous nonlinear contributions, decrease with increasing strain amplitude, indicating the progressive fluidization of the system, consistent with the faster plastic rearrangements observed in LS-Echo. Two-distinct frequency-dependent regimes for the initial yielding at small strains are present: At small to moderate oscillation frequencies, the first yield strain increases with increasing frequency, while it decreases at large frequencies. This might be associated with the timescale for bond breaking, namely, the Brownian diffusion time at small frequencies and the inverse of the oscillation frequency at large frequencies.