PMMH Laboratory

[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).

[3] Chao, Skipper, Royall, Henkes, Liverpool: Traveling strings of active dipolar colloids, Phys. Rev. Lett. 134 018302 (2025).

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

[1] Royall, Charbonneau, Dijkstra, Russo, Smallenburg, Speck, Valeriani: Colloidal Hard Spheres: Triumphs, Challenges and Mysteries, Rev. Mod. Phys. (2024)

[2] Zampetaki, Yang, Loewen, Royall: Dynamical Order and Many-Body Correlations in Zebrafish show that Three is a Crowd, Nature Commun (2024)

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 for fish, “three is a crowd” [4].

We use similar methods to analyse the collective behaviour of zebrafish. We show that trajectories of the fish can be mapped onto a modified Vicsek model with surprising accuracy [2]. Colloidal systems can be tuned and we show that the zebrafish can also be tuned through genetic modification.

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].

Until now, experiments with active colloids have been limited to (quasi) 2D systems, but we may 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

Modelling them is very different: understanding (active) colloids builds on a century of development of effective interactions between particles based on accurate physical models [1]. In contrast, for zebrafish one seeks a model reproducing their collective behaviour, without emphasis on why [2].

Understanding collective behaviour in active systems is greatly enhanced by model systems which can be studied in the laboratory. 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.

#### Come to PMMH laboratory: www.pmmh.espci.fr/Contact-357

#### Next Seminars
Décembre 5th : Francesca Borghi (U. Milan)
Décembre 12th : to be announced
December 19th : Lara Kohler (Max Planck, Dresden)

Full list at www.pmmh.espci.fr/Seminaires.

The PMMH seminar at 11:00 on Friday 28 November will be given by Paddy Royall of Gulliver, ESPCI.

Laboratory Active Matter in Three Dimensions:
from Colloids to Fish

Abstract below. Zoom link upon request.

Both examples highlight intricate hydrodynamic couplings in active systems and their potential consequences for the accumulation of microbial communities and the onset of biofilm formation.

Although in the absence of flow, agents accumulate at boundaries, shear also generates extended trapping phases of microswimmers in areas of the flow backbone, leading to prominent power-law tails in the exit-time distributions.

behaviors that are fundamentally different to swimming near rigid walls. In the second part, I will discuss the impact of flows on active transport through a disordered, porous channel.

In this talk, I will first discuss the hydrodynamic interactions of microswimmers with a nearby deformable boundary, like a biological membrane. Using a far-field description for the flows and a perturbation theory for small deformations, we find that elastohydrodynamic couplings can generate

Swimming microorganisms represent fascinating exemplars of non-equilibrium systems and display a range of unusual physical phenomena. These active agents often operate in complex environments, characterized by confining boundaries and flows, that can strongly modify their swimming dynamics.

#### Come to PMMH laboratory: www.pmmh.espci.fr/Contact-357

#### Coming Seminars

Nov 14: Romain Mari (LiPhy, Grenoble)
Nov 21: Christiana Mavroyiakoumou (Oxford, UK)
Nov 28: Paddy Royall (Gulliver)
Dec 5: Francesca Borghi (U. Milan)

Full list at www.pmmh.espci.fr/Seminaires.

The PMMH seminar on Friday 7 November at 11:00 will host Christian Kurzthaler (MPI Dresden).

Microbes in motion: from surface interactions to porous media

Abstract below. Zoom link upon request.

Division of Statistical and Nonlinear Physics Fellowship The American Physical Society is a nonprofit membership organization working to advance physics by fostering a vibrant, inclusive, and global community dedicated to science and society.

José BICO, formé à l'ESPCI et diplômé #Ingénieur en 1996 (#ESPCIAlum 111e promotion), professeur @espciparispsl.bsky.social et chercheur @pmmh-lab.bsky.social, a été élu Fellow de l'American Physical Society @apsphysics.bsky.social :

www.aps.org/funding-reco...

c/ @olivetree33.bsky.social

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Anke LINDNER & José BICO #ESPCIAlum élus Fellows de deux prestigieuses sociétés savantes !

Deux distinctions qui confirment l'envergure internationale du laboratoire de Physique & mécanique des milieux hétérogènes (PMMH) @pmmh-lab.bsky.social @espciparispsl.bsky.social @sorbonne-universite.fr
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Photo collage of research photos from PMMH

The 2025-2026 PMMH internships booklet is ready! https://www.pmmh.espci.fr/Internships-at-PMMH

"For outstanding contributions to elasto-capillary phenomena, elastic instabilities, and shape-morphing materials that combine creative experiments and elegant modeling."

https://www.aps.org/funding-recognition/aps-fellowship/dsnp-fellowship

We will show how this invariant may be used to quickly derive useful information on the equilibrium shapes of elastica in self-contact or in interaction with obstacles, sliding sleeves, force fields, and droplets.

This static-dynamic analogy allows us to write a quantity that is invariant along the elastic rod at equilibrium. For a pendulum, mechanical energy constant in time whereas, for a planar elastica, the sum of its curvature energy and its axial force will be uniform along the structure.

The static-dynamic analogy discovered by G. Kirchhoff shows that the statics of the planar elastica are equivalent to the dynamics of the pendulum. In this analogy, time and angular velocity are, for example, equivalent to arc length and curvatures.

To come to PMMH:
www.pmmh.espci.fr/Contact-357

Coming Seminars:

Oct 17: Marie Poulain-Zarcos (LMFA, Lyon)
Nov 7: Christina Kurzthaler (Max-Planck Dresden)
Nov 14: Romain Mari (LiPhy, Grenoble)
Nov 21: Christiana Mavroyiakoumou (Oxford, UK)

Full list at www.pmmh.espci.fr/Seminaires.

The PMMH seminar at 11am on Friday 10 Oct will be given by Florence Bertails-Descoubes (INRIA, Grenoble) and Sébastien Neukirch (Institut d'Alembert, Sorbonne Université). The talk will be in French.

Title: Elastic Rods made easy with a Noether invariant

Abstract follows. Zoom link upon request.

I will show that fracture toughness increases with the square root of the defect fraction.
This counterintuitive effect highlights how disorder, rather than weakening, can reinforce materials by arresting cracks.

fracture in heterogeneous materials, and the pinning of elastic lines.
In the second part, I shift from adhesive contacts to fracture in bulk disordered materials. Using numerical simulations of spring networks with randomly removed bonds,

—a phenomenon known as adhesion hysteresis.
In the first part of this talk, I will show that adhesion hysteresis originates from the pinning of the contact perimeter by surface roughness. This mechanism connects adhesion hysteresis to broader phenomena such as contact angle hysteresis,