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Chiral Active Matter

Some self-propelled agents are not symmetrical and may have a preferential direction.

Fish school

Fish can exhibit a collective behavior by forming different structures such as schooling, milling or swarming.

publications

The pressure dependence of the discharge properties in a capacitively coupled oxygen discharge

Published in Journal of Applied Physics, 2015

We use the one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 to explore the evolution of the charged particle density profiles, electron heating mechanism, and the electron energy probability function (EEPF) in a capacitively coupled oxygen discharge with pressure in the pressure range of 10–500 mTorr. We find that at higher pressure (50–500 mTorr) the electron heating occurs mainly in the sheath region, and detachment by the metastable singlet molecule $O_2(a^1\Delta_g)$ has a significant influence on the electron heating process. At a low pressure (10 mTorr), Ohmic heating in the bulk plasma (the electronegative core) dominates, and detachment by $O_2(a^1\Delta_g)$ has only a small influence on the heating process. Thus at low pressure, the EEPF is convex and as the pressure is increased the number of low energy electrons increases and the number of higher energy electrons (>10 eV) decreases, and the EEPF develops a concave shape or becomes bi-Maxwellian.

Recommended citation: Gudmundsson, J. T., & Ventéjou, B. (2015). "The pressure dependence of the discharge properties in a capacitively coupled oxygen discharge".Journal of Applied Physics, 118(15)
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Progressive quenching: Globally coupled model

Published in Physical Review E, 2018

We study the processes in which fluctuating elements of a system are progressively fixed (quenched) while keeping the interaction with the remaining unfixed elements. If the interaction is global among Ising spin elements and if the unfixed part is reequilibrated each time after fixing an element, the evolution of a large system is martingale about the equilibrium spin value of the unfixed spins. Due to this property the system starting from the critical point yields the final magnetization, whose distribution shows non-Gaussian and slow transient behavior with the system size.

Recommended citation: Ventéjou, B., & Sekimoto, K. (2018)."Progressive quenching: Globally coupled model". Physical Review E, 97(6), 062150.
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Susceptibility of Orientationally-Ordered Active Matter to Chirality Disorder

Published in Physical review letters, 2021

We investigate the susceptibility of long-range ordered phases of two-dimensional dry aligning active matter to population disorder, taken in the form of a distribution of intrinsic individual chiralities. Using a combination of particle-level models and hydrodynamic theories derived from them, we show that while in finite systems all ordered phases resist a finite amount of such chirality disorder, the homogeneous ones (polar flocks and active nematics) are unstable to any amount of disorder in the infinite-size limit. On the other hand, we find that the inhomogeneous solutions of the coexistence phase (bands) may resist a finite amount of chirality disorder even asymptotically.

Recommended citation: Ventejou, B., Chaté, H., Montagne, R., & Shi, X. Q. (2021). "Susceptibility of orientationally ordered active matter to chirality disorder". Physical review letters, 127(23), 238001.
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Behavioral transition of a fish school in a crowded environment

Published in Physical Review E, 2024

In open water, social fish gather to form schools, in which fish generally align with each other. In this work, we study how this social behavior evolves when perturbed by artificial obstacles. We measure the behavior of a group of zebrafish in the presence of a periodic array of pillars. When the pillar density is low, the fish regroup with a typical interdistance and a well-polarized state with parallel orientations, similarly to their behavior in open-water conditions. Above a critical density of pillars, their social interactions, which are mostly based on vision, are screened and the fish spread randomly through the aquarium, orienting themselves along the free axes of the pillar lattice. The abrupt transition from natural to artificial orientation happens when the pillar interdistance is comparable to the social distance of the fish, i.e., their most probable interdistance. We develop a stochastic model of the relative orientation between fish pairs, taking into account alignment, antialignment, and tumbling, from a distribution biased by the environment. This model provides a good description of the experimental probability distribution of the relative orientation between the fish and captures the behavioral transition. Using the model to fit the experimental data provides qualitative information on the evolution of cognitive parameters, such as the alignment or the tumbling rates, as the pillar density increases. At high pillar density, we find that the artificial environment imposes its geometrical constraints to the fish school, drastically increasing the tumbling rate.

Recommended citation: Ventéjou, B., Magniez--Papillon, I., Bertin, E., Peyla, P., & Dupont, A. (2024). "Behavioral transition of a fish school in a crowded environment ". Physical Review E, 109(6), 064403.
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Condensation and Synchronization in Aligning Chiral Active Matter

Published in Physical Review Letters, 2024

We show that spontaneous density segregation in dense systems of aligning circle swimmers is a condensation phenomenon at odds with the phase separation scenarios usually observed in two-dimensional active matter. The condensates, which take the form of vortices or rotating polar packets, can absorb a finite fraction of the particles in the system, and keep a finite or slowly growing size as their mass increases. Our results are obtained both at particle and continuous levels. We consider both ferromagnetic and nematic alignment, and both identical and disordered chiralities. Condensation implies synchronization, even though our systems are in 2D and bear strictly local interactions. We propose a phenomenological theory based on observed mechanisms that accounts qualitatively for our results.

Recommended citation: Wang, Y., Ventéjou, B., Chaté, H., Shi, X. (2024). "Condensation and Synchronization in Aligning Chiral Active Matter ". Physical Review iLetters, 133(25), 258302.
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Universal Scaling Laws for a Generic Swimmer Model

Published in Physical Review Letters, 2025

We introduce a minimal model of a swimmer without body deformation based on force and torque dipoles which allows accurate and efficient 3D Navier-Stokes calculations. Our model can reproduce swimmer propulsion for a large range of Reynolds numbers and generate wake vortices in the inertial regime, reminiscent of the flow generated by the flapping tail of real fish. We perform a numerical exploration of the model from low to high Reynolds numbers and obtain universal laws using scaling arguments. Collecting data from a wide variety of microswimmers, we show that our theoretical scaling laws compare very well with experimental swimming performances across the different hydrodynamic regimes, from Stokes to turbulent flows. The simple design of our generic swimmer model paves the way for efficient large-scale simulations of hundreds of individuals, crucial for understanding collective effects within assemblies of aquatic animals.

Recommended citation: Ventéjou, B., Métivet, T., Dupont, A., & Peyla, P. (2025). "Universal Scaling Laws for a Generic Swimmer Model". Physical Review Letters, 134(13), 134002.
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Hydrodynamics of Confined Swimming: From Stokes flow to intermediate inertial regime

Published in JFM, 2026

We investigate the dynamics of a swimmer in a confined environment. Our findings show that the transition from the Stokes regime to the intermediate intertial regime can be tuned solely by adjusting the swimmers confinement. The crossover between the two is governed by the formation of a boundary layer and occurs when the confinement length scale becomes larger than the Blasius boundary layer thickness. We further support these results with an analytical study, which shows qualitative agreement with the numerical simulations.

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