Orthogonal superposition rheometry of carbon black suspensions - anisotropic thixotropy and anti-thixotropy

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 thixotropic stress increase then decrease at a fixed shear rate in a step-down test. We reveal how this long-time stress decay is associated with anti-thixotropy, also called rheopexy, rather than viscoelastic relaxation, by using orthogonal superposition (OSP) rheometry, where an oscillatory flow orthogonal to the shear flow is superposed on a rotational rheometer, to probe viscoelastic moduli during the thixotropic dynamics. The orthogonal elastic modulus is present, showing this two-timescale response, which demonstrates that this response involves shear-induced structuring that stores elastic energy. We further show a mechanical anisotropy in the CB suspension under shear using OSP. Therefore, a microstructural schematic is proposed, considering qualitatively thixotropic structure build-up, anti-thixotropic densification, and anisotropic log-rolling. 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 this two-timescale thixotropy of CB suspensions.