The Beginning of a Colloidal Solid and the End of a Colloidal Solid

This talk deals with the beginning of a colloidal crystal, and the end of a colloidal gel. Crystallisation is among the most familiar physical phenomena of everyday life. One would imagine that it has been long understood. And yet there remain many basic unanswered questions, not least what is the rate of nucleation? In such general problems, model systems play a crucial role, as a benchmark of theory and computer simulation. Alas, the most studied material, colloidal hard spheres, exhibits a wild disagreement between experiment and numerical prediction of 20 orders of magnitude, which has been dubbed “the second-largest discrepancy in physics” [1]. Since the relevant experimental work was carried out in the last millennium, we expect that developments, such as particle-resolved studies may shed light on this phenomenon (Fig. 1a,b). With careful matching of state point between experiments and simulation, we do find agreement for the energy barriers to form pre-critical nuclei [2].

Colloidal gels are far-from equilibrium. As they age, gels stiffen due to coarsening of the network [3,4]. Remarkably, after a period of increasing strength, with no visible macroscopic change, gels undergo delayed catastrophic gravitational collapse (Fig. 1c). To understand  yielding in gels, we have developed an experimental system to determine forces between particles, which reveals the local stress tensor [5]. But why does macroscopic delayed collapse occur? To answer this, 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 which reveals the mechanism of this mysterious phenomenon [3].

[1] Royall CP et al, “Colloidal Hard Spheres: Triumphs, Challenges and Mysteries”, Rev. Mod. Phys. 96 045003 (2024).
[2] Kürten L, Castagnède A, Smallenburg F, Royall CP, “Narrowing down the cause of the hard-sphere nucleation discrepancy: The free energy of precritical nuclei is consistent with predictions”, Science Advances 12 eaec8906 (2026).
[3] Cheng R, et al. “Observing the mechanism of delayed collapse in colloidal gels: Yielding while becoming stronger”, Proc. Nat. Acad. Sci. 123 (20) (2026).
[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] J Dong, F Turci, RL Jack, MA Faers and CP Royall “Direct Imaging of Contacts and Forces in Colloidal Gels” J. Chem. Phys. 156 214907 (2022).

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