Journal
JOURNAL OF RHEOLOGY
Volume 66, Issue 5, Pages 937-953Publisher
SOC RHEOLOGY
DOI: 10.1122/8.0000455
Keywords
thixotropy; antithixotropy; rheopexy; viscoelasticity; superposition rheometry; carbon black; colloidal suspension
Categories
Funding
- Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub - U.S. Department of Energy
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In this study, we conducted a detailed experimental investigation on the rheological dynamics of carbon black suspensions in mineral oil. We observed a peculiar antithixotropic response and verified its association with shear-induced structural changes using orthogonal superposition rheometry. We also found mechanical anisotropy in the suspension under shear using the same technique.
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 antithixotropic structural change rather than viscoelastic stress relaxation by using orthogonal superposition (OSP) rheometry to probe viscoelastic moduli during the step-down tests. The orthogonal storage and loss moduli are present, showing this two-timescale recovery then decay response, which demonstrates that this response is antithixotropic, 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, antithixotropic 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.
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