Seminars

Laboratory Active Matter in Three Dimensions : from Colloids to Fish

Understanding of collective behaviour in active systems is massively
enhanced by model systems which can be interrogated in the laboratory.
Here we consider two very different examples. Active colloids in 3d and
zebrafish. We approach each with the same philosophy of studying
trajectories with simple models. Modelling the two is profoundly
different : our understanding of (active) colloids builds on over a
century of development of effective interactions between the particles
based on accurate physical models [1]. Conversely in the case of the
zebrafish, one seeks a model which reproduces their collective
behaviour, with no emphasis on “why” this is so [2].

Until now, experiments with active colloids have been largely limited to
(quasi) 2d systems, but may reasonably expect that moving to 3d will
bring new phenomena. Here we introduce a 3d active colloidal system of
Janus particles in an AC electric field, which is active in the plane
perpendicular to the field and acquires a dipolar interaction parallel
to the field, and exhibits new phenomena such as travelling strings [3],
a wildly fluctuating labyrinth and phonon-like behaviour, all of which
are forbidden in the passive analogue [4].

We use similar methods to analyse the collective behaviour of zebrafish.
We show that the trajectories of the fish can be mapped onto a modified
Vicsek model with surprising accuracy [2]. Colloidal systems can be
tuned, and here we show that the zebrafish can also be tuned through
genetic modification. The mutant fish are described by the same Vicsek
model, but occupy a different state space. Finally, we show that that
the system size dependence of zebrafish shows little change once the
system has three fish when interpreted through suitable order
parameters, so with the fish, “three is a crowd” [4].

[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] Zampetaki A, Yang, Y, Loewen, H and Royall CP, “Dynamical Order and
Many-Body Correlations in Zebrafish show that Three is a Crowd”, Nature
Commun, 15 2591 (2024).

[3] Chao X, Skipper K, Royall CP, Henkes S, Liverpool TB, “Traveling
strings of active dipolar colloids”, Phys. Rev. Lett. 134 018302 (2025).

[4] Sakaï N, Skipper K, Moore FJ, Russo J and Royall CP, “Active Dipolar
Colloids in Three Dimensions : Non–Equilibrium Structure and Re-entrant
Dynamics”, Soft Matter, 21 5204 (2025).

[5] Yang Y, Turci F, Kague E, Hammond CL, Russo J and Royall CP,
“Dominating Lengthscales of Zebrafish Collective Behaviour” PLOS Comp.
Biol. 18 e1009394 (2020).