Anael Lemaitre - Non-structural aging in adhesive colloidal suspensions

This talk was part of the Thematic Programme on "Non-equilibrium Processes in Physics and Biology" held at the ESI August 19 -- October 11, 2024.

Moderately dense colloidal suspensions (or pastes) constitute a broad class of materials found in many areas, ranging from environmental systems (silts, clays), to industry (ceramics, drilling muds, slurries) and construction (cement), foodstuff, cosmetics, etc. They often display time-dependent rheological properties, such as a non-saturating logarithmic growth of their shear modulus and yield stress at rest -- a phenomenon called mechanical aging. Until recently, due to the influence of many studies focusing on structural aging in transparent colloidal model systems, this behavior was systematically attributed to the existence of slow, glass-like, structural dynamics; however, the conditions of transparency required in these studies to visualize structural dynamics also eliminate van der Waals attraction, which is otherwise one of the most pervasive forces at small scales. In contrast, real suspensions are often opaque and ionic: these two conditions set the stage for the existence of adhesive forces that drive the formation of interparticle adhesive contacts. In an experimental study combining rheometry, confocal microscopy, and particle-scale mechanical tests using laser tweezers [1,2|, we could demonstrate that, in such systems at moderate densities, macroscopic aging is driven by time-dependent processes that take place at the contact scale. In such systems, indeed, the formation of adhesive contacts freezes the microstructure within seconds of flow arrest, which rules out the structural aging scenario. However, we could demonstrate the existence of non-structural mechanical aging in two broadly different model systems: it results from the progressive increase of both the flexural modulus and the rolling limit of individual interparticle contacts. This finding point to the essential role of the flexural contact response in determining the macroscopic mechanical properties of such suspensions, and leads to identifying a constitutive relation between the time-dependent shear modulus and yield stress.



[1] Francesco Bonacci, Xavier Chateau, Eric M. Furst, Jennifer Fusier, Julie Goyon, and Anaël Lemaı̂tre. Contact and macroscopic ageing in colloidal suspensions. Nature Materials, 19(7):775–780, 2020.
[2] Francesco Bonacci, Xavier Chateau, Eric M. Furst, Julie Goyon, and Anaël Lemaı̂tre. Yield stress aging in attractive colloidal suspensions. Phys. Rev. Lett., 128(1):018003, January 2022.