Necking and failure of a colloidal gel arm: signatures of yielding on different length scales

TL;DR

This study uses simulations to analyze how a colloidal gel arm undergoes necking and failure, revealing plastic flow and structural changes that signal a transition from solid-like to viscous fluid behavior.

Contribution

It provides a multi-scale analysis of gel arm failure, linking microscopic structure, local stresses, and continuum behavior during necking.

Findings

Necking involves plastic flow near the yield stress.

The gel arm's structure becomes less stable during failure.

Residual stresses decrease as the system transitions to a more fluid-like state.

Abstract

Colloidal gels consist of percolating networks of interconnected arms. Their mechanical properties depend on the individual arms, and on their collective behaviour. We use numerical simulations to pull on a single arm, built from a model colloidal gel-former with short-ranged attractive interactions. Under elongation, the arm breaks by a necking instability. We analyse this behaviour at three different length scales: a rheological continuum model of the whole arm; a microscopic analysis of the particle structure and dynamics; and the local stress tensor. Combining these different measurements gives a coherent picture of the necking and failure: the neck is characterised by plastic flow that occurs for stresses close to the arm's yield stress. The arm has an amorphous local structure and has large residual stresses from its initialisation. We find that neck formation is associated with increased plastic flow, a reduction in the stability of the local structure, and a reduction in the residual stresses; this indicates that {the} system loses its solid character and starts to behave more like a viscous fluid. We discuss the implications of these results for the modelling of gel dynamics.