  We report a detailed experimental study of peculiar thixotropic dynamics of
carbon black (CB, Vulcan XC-72) suspensions in mineral oil, specifically the
observation of sequential stress increase then decrease at a fixed shear rate
in a step-down test. We verify that such dynamics, though peculiar, come from a
true material response rather than experimental artifacts. We also reveal how
this long-time stress decay is associated with anti-thixotropy, rather than
viscoelasticity, by using orthogonal superposition (OSP) rheometry to probe
viscoelastic moduli during the step-down tests. The orthogonal storage and loss
modulus are present, showing this two-timescale recovery then decay response,
which demonstrates that this response is anti-thixotropic, and it involves
shear-induced structuring. We further show a mechanical anisotropy in the CB
suspension under shear using OSP. Based on the rheological results, a
microstructural schematic is proposed, considering qualitatively thixotropic
structure build-up, anti-thixotropic densification, and anisotropic structure
evolution. Our observation for these CB suspensions is outside the standard
paradigm of thixotropic structure-parameter models, and the elastic response
provides us with new insight into the transient dynamics of CB suspensions.
