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

TL;DR

This study investigates the complex thixotropic and anti-thixotropic behaviors of carbon black suspensions in mineral oil, revealing shear-induced structuring, anisotropy, and dynamics beyond standard models through detailed rheological experiments.

Contribution

It provides new experimental insights into the anisotropic and anti-thixotropic rheology of carbon black suspensions, challenging existing thixotropic models.

Findings

Long-time stress decay linked to anti-thixotropy

Detection of shear-induced anisotropic structuring

Observation of two-timescale viscoelastic response

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 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.