Gelation of colloids leads to the formation of a percolated network spanning through the material. The structure of the network is closely linked to the mechanical properties of the resulting gel, especially the emergence of a yield stress. A mechanistic understanding of yielding of colloidal gels is of both fundamental and practical interest, especially in view of the widespread use of these materials. However, only a few rheo-confocal studies monitoring the evolution of the microstructure under shear focused on the yielding scenario. Their limited spatio-temporal resolution could, as yet, not provide a complete image of yielding at the colloidal network level. Especially, plastic rearrangement between particles before yielding, already evidenced by rheo-DLS experiments as precursors of yielding, still need to be directly observed.<br/><br/>We study the generation and the propagation of such basic plastic events in a model depletion gel of fluorescent PMMA-g-PHSA colloids. The microstructure of the gels is scanned at a rate of a 3D voxel every second thanks to a homemade ultrafast rheo-confocal. We mechanically solicitate the gels under constant shear stress and constant shear rate while image processing methods and particle tracking can be used to determine the local strain field in the material. We study the dynamics of the plastic events from the variations of this local strain field and link them to the ultimate yielding of the material as well as the initial gel microstructure obtained after gelation at different initial volume fractions and strengths of depletion interaction. From these observations, we would like to conclude about which physico-chemical parameter govern the flow of colloidal gel and how they could be used in the design of the material.

Probing plastic rearrangements in colloidal gels

ACS Spring 2022

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