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.

Colloidal systems are prized for their simplicity while nevertheless exhibiting a range of phenomena from phase transitions to collective behaviour reminiscent of that in biological systems.

Here we discuss two forms of yielding. The first is exhibited by depletion gels. Such colloid-polymer mixtures represent a remarkably simple system of “sticky spheres” whose equilibrium properties are very well understood. Yet out of equilibrium mysteries remain. Far-from equilibrium, colloidal gels age, and become stronger. This puts them in the class of materials like glasses whose properties are influenced by their slow exploration of the energy landscape [1]. Bizarrely, for a material which becomes stronger, colloidal gels can catastrophically fail. By introducing two new experimental imaging methodologies, we propose a microscopic mechanism for this counterintuitive phenomenon [2].

Our second example showcases active colloids as a model for yielding in more complex systems. Shear melting under extreme confinement has been studied for passive colloids [3]. In the case of active colloids [4], we find that the mechanism of yielding under shear is profoundly altered despite a strong geometric similarity to the passive case.

[1] Royall CP, Faers MA, Fussell SL and Hallett JE, J. Phys.: Condens. Matter, 33 453002 (2021).
[2] J Dong, F Turci, RL Jack, MA Faers and CP Royall J. Chem. Phys. 156 214907 (2022).
[3] Williams I, Oguz EC, Speck T, Bartlett P, Loewen H and Royall CP Nature Physics, 12 98–103 (2016).
[4] Mauleon-Amieva A, Allen MP, Liverpool TB and Royall CP, Sci. Adv. 9 eadf5144 (2023).