New insights on carbon black suspension rheology -- anisotropic thixotropy and anti-thixotropy

Abstract

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 shearinduced 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.

Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation

“…In particular, ξ s is smaller than the cluster size ξ c , indicating that the clusters interpenetrate each other. Such cluster interpenetration has recently been suggested in carbon black gels to interpret step down shear rate rheology experiments [53]. In our case, for lower shear rate intensity γ0 prior to flow cessation, the ratio ξ c /ξ s increases, i.e., the clusters become more interpenetrated, accounting for the reinforcement of the gel elasticity.…”

Interpenetration of fractal clusters drives elasticity in colloidal gels formed upon flow cessation

“…Such cluster interpenetration has recently been suggested in carbon black gels to interpret stepdown shear rate rheology experiments. 56 In our case, for lower shear rate intensity _ g 0 prior to flow cessation, the ratio x c /x s increases, i.e., the clusters become more interpenetrated, accounting for the reinforcement of the gel elasticity. We therefore hypothesize that the gel elasticity is related to the cluster interpenetration, increasing the elasticity of the gel network as compared to the case where clusters would be packed in a random close-packing configuration [Fig.…”

Study on Thixotropy of Mastic Asphalt Binder and Asphalt Mastic

“…Thixotropy is an important research content of the rheology of dispersion systems, which has important theoretical research significance and engineering application value [14][15][16][17][18].…”