Visualizing shear-induced structures in carbon black gels by tomo-rheoscopy

TL;DR

This study uses in situ X-ray tomo-rheoscopy to visualize and understand how shear history influences the mesoscale structure and mechanical properties of carbon black gels, revealing large-scale heterogeneities linked to flow memory.

Contribution

It demonstrates the ability of X-ray tomo-rheoscopy to directly image mesoscale structures in colloidal gels and links shear history to large-scale structural heterogeneities affecting elasticity.

Findings

Low-shear reinforcement correlates with increased mesoscale correlation length.

High-shear strengthening occurs without mesoscale reorganization.

Flow memory involves mesoscale structural organization beyond local particle rearrangements.

Abstract

Suspensions of attractive particles form space-spanning networks that endow the suspension with solid-like behavior at rest. The microstructure of these colloidal gels depends sensitively on the shear history and on the path followed across the sol-gel transition, resulting in viscoelastic properties that can be tuned by shear. Here, we report in situ X-ray tomo-rheoscopy experiments on carbon black gels whose elastic properties exhibit a non-monotonic dependence on the shear intensity applied prior to flow cessation. By directly imaging the gel microstructure under a well-controlled rheological protocol, we reveal the emergence of pronounced structural heterogeneities extending from tens to hundreds of microns -- length scales far larger than those accessible by conventional scattering techniques such as Ultra-Small Angle X-ray Scattering. In particular, we show that only the low-shear reinforcement of elasticity correlates with a growing mesoscale correlation length, while high-shear strengthening occurs without detectable mesoscale reorganization. These observations demonstrate that flow memory in colloidal gels is not solely governed by local particle rearrangements, but is also encoded in a mesoscale structural organization extending up to 100 times the particle size. More broadly, this work highlights the power of X-ray tomo-rheoscopy to uncover large-scale structural signatures of flow history in soft materials, opening new perspectives to tailor their mechanical properties.