The Strange Case of Delayed Gel Collapse

Figure: Macroscopic collapse and microscopic stress in gels. (a) Delayed collapse. After a period of no visible change, colloidal gels can collapse catastrophically. Here the particles are light so the gel creams (sediments upwards). (b-e) Identifying the forces between particles α and β. (c) Particle channel. (d) Contact (force) channel. (e) Combined two channel image showing contacts in a gel.

Colloidal suspensions are prized as model condensed matter systems with simple interactions. Colloids exhibit phase behaviour akin to molecular systems, such as crystals and glasses whose processes can be followed at the microscopic level by tracking the particle coordinates with confocal microscopy. [1]. They are also very important both industrially and in a biological context. The equilibrium behaviour of colloidal “sticky spheres” is well understood [2]. Yet out of equilibrium, mysteries remain. Far-from equilibrium, sticky spheres can form a gel, whose solidity is due to rigid minimum energy clusters [3]. As gels age, they stiffen due to coarsening of the gel network [4]. Remarkably, after a period of increasing strength, with no visible macroscopic change, gels undergo delayed catastrophic gravitational collapse.

We can study the microscopic mechanisms of yielding using both experiments and computer simulation [4,5]. We have also developed an experimental system to determine forces between particles, which reveals the local stress tensor [6]. However, such microscopic measures of yielding cannot address the challenge of macroscopic samples which undergo delayed collapse after a period where nothing seems to happen for weeks or even years.

We have developed a new technique to address this needle-in-haystack problem: multiscale confocal microscopy. Here we image an entire bulk sample with microscopic resolution. We reveal the nature of precursors to the sudden collapse. These are channels of solvent which form at the top of the gel, overcome the yield stress and make their way through the bulk of the material. This leads ultimately to a runaway increase in flow rate and gel collapse [7].

[1] Royall CP, Charbonneau P, Dijkstra M, Russo J, Smallenburg F, Speck T and Valeriani C. “Colloidal Hard Spheres: Triumphs, Challenges and Mysteries”, Rev. Mod. Phys. 96 045003 (2024).

[2] Royall CP, Faers MA, Fussell SL and Hallett JE, J. Phys.: Condens. Matter, 33 453002 (2021).

[3] Royall CP, Williams SR, Ohtsuka, T and Tanaka H, “Direct observation of a local structural mechanism for dynamic arrest”, Nature Materials 7, 556-561, (2008).

[4] H Bhaumik, JE Hallett, TB Liverpool, RL Jack and CP Royall, “Cyclically sheared colloidal gels: structural change and delayed failure time”, Soft Matter 21 8555-8568 (2025).

[5] Thijssen, K, Liverpool TB, Royall CP, Jack RL, “Necking and failure of a colloidal gel arm: signatures of yielding on different length scales”, Soft Matter 19 7412-7428 (2023).

[6] Dong J, Turci F, Jack RL, Faers MA and Royall CP, “Direct Imaging of Contacts and Forces in Colloidal Gels” J. Chem. Phys. 156 214907 (2022).

[7] Cheng R, Faers MA, Turci F, Mauleon Amieva A, Liverpool TB, Jack RL and Royall CP, “Observing the mechanism of delayed collapse in colloidal gels: yielding while becoming stronger”, accepted in Proc. Nat. Acad. Sci. (2026).