31/03/2023 – S. Manneville – Attractive, confined, and stirred: patterns and cracks in particulate networks

Séminaire IUSTI – 31 mars 2023 – 11h salle 250

Attractive, confined, and stirred: patterns and cracks in particulate networks

Sébastien Manneville – ENS, Lyon

Dilute dispersions of attractive colloids commonly self-assemble into space-spanning networks referred to as colloidal gels. With elastic moduli ranging typically from 1 Pa to 10^4 Pa, colloidal gels form an important class of soft solids, which applications range from food and cosmetics to construction materials as cement. One key feature of colloidal gels is the sensitivity of their microstructure to external deformation and flow. In particular, shear is known to break-up the particulate network and/or to induce aggregation and compaction of particulate clusters.
In this seminar, I will first address the effects of confined shear on dispersions of attractive particles. Experiments show that, when slowly sheared between two surfaces separated by a distance of the order of 10-100 particle sizes, an initially homogeneous carbon black gel gives way to a pattern of log-rolling flocs aligned along the vorticity direction [1]. This phenomenology is not only observed in colloidal systems but also in non-Brownian attractive particles. Based on numerical simulations, we interpret this striking behavior as the signature of an underlying flow instability triggered by perturbations in the particle concentration field.
We then show that the elasticity and the yielding behavior of colloidal gels can be modified by the application of high-intensity ultrasonic vibrations [2]. Combining rheology under vibrations with structural characterization provides evidence for micron-sized cracks within the gel network, which may or may not fully heal depending on the acoustic intensity. Ultrasonic vibrations also dramatically accelerate the onset of flow under creep. Such ultrasound-assisted yielding appears to be governed by an effective temperature that depends on the acoustic intensity.

[1] Varga et al., PNAS 116, 12193 (2019)
[2] Gibaud et al., Phys. Rev. X 10, 011028 (2020)