Matthieu Mangeat, PhD

[21] S. Chatterjee, M. Karmakar, M. Mangeat, H. Rieger, and R. Paul, Stability of discrete-symmetry flocks: sandwich state, traveling domains and motility-induced pinning, submitted (2025). [links][arXiv:2507.08187] [movies] [pdf] [abstract]Abstract: Polar flocks in discrete active systems are often assumed to be robust, yet recent studies reveal their fragility under both imposed and spontaneous fluctuations. Here, we revisit the four-state active Potts model (APM) and show that its globally ordered phase is metastable across a broad swath of parameter space. Small counter-propagating droplets disrupt the flocking phase by inducing a persistent sandwich state, where the droplet-induced opposite-polarity lane remains embedded within the original flock, particularly pronounced at low noise, influenced by spatial anisotropy. In contrast, small transversely propagating droplets, when introduced into the flock, can trigger complete phase reversal due to their alignment orthogonal to the dominant flow and their enhanced persistence. At low diffusion and strong self-propulsion, such transverse droplets also emerge spontaneously, fragmenting the flock into multiple traveling domains and giving rise to a short-range order (SRO) regime. We further identify a motility-induced pinning (MIP) transition in small diffusion and low-temperature regimes when particles of opposite polarity interact, flip their state, hop, and pin an interface. Our comprehensive phase diagrams, encompassing full reversal, sandwich coexistence, stripe bands, SRO, and MIP, delineate how thermal fluctuations, self-propulsion strength, and diffusion govern flock stability in discrete active matter systems.

[20] A. K. Dutta, M. Mangeat, H. Rieger, R. Paul, and S. Chatterjee, Stability of flocking in the reciprocal two-species Vicsek model: Effects of relative population, motility, and noise, submitted (2025). [links][arXiv:2504.13709] [gitHub] [movies] [pdf] [abstract]Abstract: Natural flocks need to cope with various forms of heterogeneities, for instance, their composition, motility, interaction, or environmental factors. Here, we study the effects of such heterogeneities on the flocking dynamics of the reciprocal two-species Vicsek model [PRE 107, 024607 (2023)], which comprises two groups of self-propelled agents with anti-aligning inter-species interactions and exhibits either parallel or anti-parallel flocking states. The parallel and anti-parallel flocking states vanish upon reducing the size of one group, and the system transitions to a single-species flock of the majority species. At sufficiently low noise (or high density), the minority species can exhibit collective behavior, anti-aligning with the liquid state of the majority species. Unequal self-propulsion speeds of the two species strongly encourage anti-parallel flocking over parallel flocking. However, when activity landscapes with region-dependent motilities are introduced, parallel flocking is retained if the faster region is given more space, highlighting the role of environmental constraints. Under noise heterogeneity, the colder species (subjected to lower noise) attain higher band velocity compared to the hotter one, temporarily disrupting any parallel flocking, which is subsequently restored. These findings collectively reveal how different forms of heterogeneity, both intrinsic and environmental, can qualitatively reshape flocking behavior in this class of reciprocal two-species models.

[19] M. Mangeat, S. Chatterjee, J. D. Noh, and H. Rieger, Emergent complex phases in a discrete flocking model with reciprocal and non-reciprocal interactions, Commun. Phys. 8, 186 (2025). [links][doi:10.1038/s42005-025-02098-x] [arXiv:2412.02501] [gitHub] [movies] [pdf] [abstract]Abstract: There is growing interest in multi-species active matter systems with reciprocal and non-reciprocal interactions. While such interactions have been explored in continuous symmetry models, less is known about multi-species discrete-symmetry systems. To address this, we study the two-species active Ising model (TSAIM), a discrete counterpart of the two-species Vicsek model. Our investigation explores both inter-species reciprocal and non-reciprocal interactions, along with the possibility of species interconversion. In the reciprocal TSAIM, we observe the emergence of a high-density parallel flocking state, a feature not seen in previous flocking models. With species interconversion, the TSAIM corresponds to an active extension of the Ashkin-Teller model and exhibits rich state diagrams. In the non-reciprocal TSAIM, a run-and-chase dynamics emerge. We also find that the system is metastable due to droplet excitation and exhibits spontaneous motility-induced interface pinning. A hydrodynamic theory validates our numerical simulations and confirms the phase diagrams.

[18] M. Mangeat, S. Chakraborty, A. Wysocki, and H. Rieger, Stationary particle currents in sedimenting active matter wetting a wall, Phys. Rev. E, 109, 014616 (2024). [links][doi:10.1103/PhysRevE.109.014616] [arXiv:2309.09714] [gitHub] [pdf] [abstract]Abstract: Recently it was predicted, on the basis of a lattice gas model, that scalar active matter in a gravitational field would rise against gravity up a confining wall or inside a thin capillary - in spite of repulsive particle-wall interactions [PRL 124, 048001 (2020)]. In this paper we confirm this prediction with sedimenting active Brownian particles (ABPs) in a box numerically and elucidate the mechanism leading to the formation of a meniscus rising above the bulk of the sedimentation region. The height of the meniscus increases with the activity of the system, algebraically with the Péclet number. The formation of the meniscus is determined by a stationary circular particle current, a vortex, centered at the base of the meniscus, whose size and strength increase with the ABP activity. The origin of these vortices can be traced back to the confinement of the ABPs in a box: already the stationary state of ideal (non-interacting) ABPs without gravitation displays circular currents that arrange in a highly symmetric way in the eight octants of the box. Gravitation distorts this vortex configuration downward, leaving two major vortices at the two side walls, with a strong downward flow along the walls. Repulsive interactions between the ABPs change this situation only as soon as motility induced phase separation (MIPS) sets in and forms a dense, sedimented liquid region at the bottom, which pushes the center of the vortex upwards towards the liquid-gas interface. Self-propelled particles therefore represent an impressive realization of scalar active matter that forms stationary particle currents being able to perform visible work against gravity or any other external field, which we predict to be observable experimentally in active colloids under gravitation.

[17] M. Karmakar, S. Chatterjee, M. Mangeat, H. Rieger, and R. Paul, Jamming and flocking in the restricted active Potts model, Phys. Rev. E 108, 014604 (2023). [links][doi:10.1103/PhysRevE.108.014604] [arXiv:2212.10251] [gitHub] [pdf] [abstract]Abstract: We study the active Potts model with either site occupancy restriction or on-site repulsion to explore jamming and kinetic arrest in a flocking model. The incorporation of such volume exclusion features leads to a surprisingly rich variety of self-organized spatial patterns. While bands and lanes of moving particles commonly occur without or under weak volume exclusion, strong volume exclusion along with low temperature, high activity, and large particle density facilitates traffic jams. Through several phase diagrams, we identify the phase boundaries separating the jammed and free-flowing phases and study the transition between these phases which provide us with both qualitative and quantitative predictions of how jamming might be delayed or dissolved. We further formulate and analyze a hydrodynamic theory for the restricted APM with that predicts various features of the microscopic model.

[16] S. Chatterjee, M. Mangeat, C.-U. Woo, H. Rieger, and J. D. Noh, Flocking of two unfriendly species: The two-species Vicsek model, Phys. Rev. E 107, 024607 (2023). [links][doi:10.1103/PhysRevE.107.024607] [arXiv:2211.10494] [gitHub] [pdf] [abstract]Abstract: We consider the two-species Vicsek model (TSVM) consisting of two kinds of self-propelled particles, A and B, that tend to align with particles from the same species and to antialign with the other. The model shows a flocking transition that is reminiscent of the original Vicsek model: it has a liquid-gas phase transition and displays micro-phase-separation in the coexistence region where multiple dense liquid bands propagate in a gaseous background. The interesting features of the TSVM are the existence of two kinds of bands, one composed of mainly A particles and one mainly of B particles, the appearance of two dynamical states in the coexistence region: the PF (parallel flocking) state in which all bands of the two species propagate in the same direction, and the APF (antiparallel flocking) state in which the bands of species A and species B move in opposite directions. When PF and APF states exist in the low-density part of the coexistence region they perform stochastic transitions from one to the other. The system size dependence of the transition frequency and dwell times show a pronounced crossover that is determined by the ratio of the band width and the longitudinal system size. Our work paves the way for studying multispecies flocking models with heterogeneous alignment interactions.

[15] S. Chatterjee, M. Mangeat, and H. Rieger, Polar flocks with discretized directions: the active clock model approaching the Vicsek model, EPL 138, 41001 (2022). [links][doi:10.1209/0295-5075/ac6e4b] [arXiv:2203.01181] [gitHub] [pdf] [abstract]Abstract: We consider the off-lattice two-dimensional q-state active clock model (ACM) as a natural discretization of the Vicsek model (VM) describing flocking. The ACM consists of particles able to move in the plane in a discrete set of q equidistant angular directions, as in the active Potts model (APM), with an alignment interaction inspired by the ferromagnetic equilibrium clock model. We find that for a small number of directions, the flocking transition of the ACM has the same phenomenology as the APM, including macrophase separation and reorientation transition. For a larger number of directions, the flocking transition in the ACM becomes equivalent to the one of the VM and displays microphase separation and only transverse bands, i.e. no re-orientation transition. Concomitantly also the transition of the q→∞ limit of the ACM, the active XY model (AXYM), is in the same universality class as the VM. We also construct a coarse-grained hydrodynamic description for the ACM and AXYM akin to the VM.

[14] A. Alexandre, M. Mangeat, T. Guérin, and D. S. Dean, How Stickiness Can Speed Up Diffusion in Confined Systems, Phys. Rev. Lett. 128, 210601 (2022). [links][doi:10.1103/PhysRevLett.128.210601] [arXiv:2112.05532] [pdf] [abstract]Abstract: The paradigmatic model for heterogeneous media used in diffusion studies is built from reflecting obstacles and surfaces. It is well known that the crowding effect produced by these reflecting surfaces slows the dispersion of Brownian tracers. Here, using a general adsorption desorption model with surface diffusion, we show analytically that making surfaces or obstacles attractive can accelerate dispersion. In particular, we show that this enhancement of diffusion can exist even when the surface diffusion constant is smaller than that in the bulk. Even more remarkably, this enhancement effect occurs when the effective diffusion constant, when restricted to surfaces only, is lower than the effective diffusivity with purely reflecting boundaries. We give analytical formulas for this intriguing effect in periodic arrays of spheres as well as undulating microchannels. Our results are confirmed by numerical calculations and Monte Carlo simulations.

[13] M. Mangeat, T. Guérin, and D. S. Dean, Steady state of overdamped particles in the non-conservative force field of a simple non-linear model of optical trap, J. Stat. Mech. 2021, 113205 (2021). [links][doi:10.1088/1742-5468/ac3907] [arXiv:2110.04362] [pdf] [abstract]Abstract: Optically trapped particles are often subject to a non-conservative scattering force arising from radiation pressure. In this paper we present an exact solution for the steady state statistics of an overdamped Brownian particle subjected to a commonly used force field model for an optical trap. The model is the simplest of its kind that takes into account non-conservative forces. In particular, we present exact results for certain marginals of the full three dimensional steady state probability distribution as well as results for the toroidal probability currents which are present in the steady state, as well as for the circulation of theses currents. Our analytical results are confirmed by numerical solution of the steady state Fokker-Planck equation.

[12] M. Mangeat and H. Rieger, Narrow escape problem in two-shell spherical domains, Phys. Rev. E 104, 044124 (2021). [links][doi:10.1103/PhysRevE.104.044124] [arXiv:2104.13125] [gitHub] [pdf] [abstract]Abstract: Intracellular transport in living cells is often spatially inhomogeneous with an accelerated effective diffusion close to the cell membrane and a ballistic motion away from the centrosome due to active transport along actin filaments and microtubules, respectively. Recently it was reported that the mean first passage time (MFPT) for transport to a specific area on the cell membrane is minimal for an optimal actin cortex width. In this paper we ask whether this optimization in a two-compartment domain can also be achieved by passive Brownian particles. We consider a Brownian motion with different diffusion constants in the two shells and a potential barrier between the two and investigate the narrow escape problem by calculating the MFPT for Brownian particles to reach a small window on the external boundary. In two and three dimensions, we derive asymptotic expressions for the MFPT in the thin cortex and small escape region limits confirmed by numerical calculations of the MFPT using the finite element method and stochastic simulations. From this analytical and numeric analysis we finally extract the dependence of the MFPT on the ratio of diffusion constants, the potential barrier height and the width of the outer shell. The first two are monotonous whereas the last one may have a minimum for a sufficiently attractive cortex, for which we propose an analytical expression of the potential barrier height matching very well the numerical predictions.

[11] M. Mangeat, S. Chatterjee, R. Paul, and H. Rieger, Flocking with a q-fold discrete symmetry: band-to-lane transition in the active Potts model, Phys. Rev. E 102, 042601 (2020). [links][doi:10.1103/PhysRevE.102.042601] [arXiv:2007.14875] [gitHub] [pdf] [abstract]Abstract: We study the q-state active Potts model (APM) on a two-dimensional lattice in which self-propelled particles have q internal states corresponding to the q directions of motion. A local alignment rule inspired by the ferromagnetic q-state Potts model and self-propulsion via biased diffusion according to the internal particle states elicits collective motion at high densities and low noise. We formulate a coarse-grained hydrodynamic theory with which we compute the phase diagrams of the APM for q=4 and q=6 and analyze the flocking dynamics in the coexistence region, where the high-density (polar liquid) phase forms a fluctuating stripe of coherently moving particles on the background of the low-density (gas) phase. A reorientation transition of the phase-separated profiles from transversal band motion to longitudinal lane formation is found, which is absent in the Vicsek model and the active Ising model. The origin of this reorientation transition is revealed by a stability analysis: for large velocities the transverse diffusivity approaches zero and stabilizes lanes. Computer simulations corroborate the analytical predictions of the flocking and reorientation transitions and validate the phase diagrams of the APM.

[10] S. Chatterjee, M. Mangeat, R. Paul, and H. Rieger, Flocking and re-orientation transition in the 4-state active Potts model, EPL 130, 66001 (2020). [links][doi:10.1209/0295-5075/130/66001] [arXiv:1911.13067] [gitHub] [pdf] [abstract]Abstract: We study the active 4-state Potts model (APM) on the square lattice in which active particles have four internal states corresponding to the four directions of motion. A local alignment rule inspired by the ferromagnetic 4-state Potts model and self-propulsion via biased diffusion according to the internal particle states leads to flocking at high densities and low noise. We compute the phase diagram of the APM and explore the flocking dynamics in the region, in which the high-density (liquid) phase coexists with the low-density (gas) phase and forms a fluctuating band of coherently moving particles. As a function of the particle self-propulsion velocity, a novel reorientation transition of the phase-separated profiles from transversal to longitudinal band motion is revealed, which is absent in the Vicsek model and the active Ising model. We further construct a coarse-grained hydrodynamic description of the model which validates the results for the microscopic model.

[09] M. Mangeat, T. Guérin, and D. S. Dean, Effective diffusivity of Brownian particles in a two dimensional square lattice of hard disks, J. Chem. Phys. 152, 234109 (2020). [links][doi:10.1063/5.0009095] [arXiv:2111.04354] [pdf] [abstract]Abstract: We revisit the classic problem of the effective diffusion constant of a Brownian particle in a square lattice of reflecting impenetrable hard disks. This diffusion constant is also related to the effective conductivity of non-conducting and infinitely conductive disks in the same geometry. We show how a recently derived Green’s function for the periodic lattice can be exploited to derive a series expansion of the diffusion constant in terms of the disk’s volume fraction φ. Second, we propose a variant of the Fick–Jacobs approximation to study the large volume fraction limit. This combination of analytical results is shown to describe the behavior of the diffusion constant for all volume fractions.

[08] M. Mangeat and H. Rieger, The narrow escape problem in a circular domain with radial piecewise constant diffusivity, J. Phys. A: Math. Theor. 52, 424002 (2019). [links][doi:10.1088/1751-8121/ab4348] [arXiv:1906.06975] [gitHub] [pdf] [abstract]Abstract: The stochastic motion of particles in living cells is often spatially inhomogeneous with a higher effective diffusivity in a region close to the cell boundary due to active transport along actin filaments. As a first step to understand the consequence of the existence of two compartments with different diffusion constant for stochastic search problems we consider here a Brownian particle in a circular domain with different diffusion constants in the inner and the outer shell. We focus on the narrow escape problem and compute the mean first passage time (MFPT) for Brownian particles starting at some pre-defined position to find a small region on the outer reflecting boundary. For the annulus geometry we find that the MFPT can be minimized for a specific value of the width of the outer shell. In contrast for the two-shell geometry we show that the MFPT depends monotonously on all model parameters, in particular on the outer shell width. Moreover we find that the distance between the starting point and the narrow escape region which maximizes the MFPT depends discontinuously on the ratio between inner and outer diffusivity.

[07] M. Mangeat, Y. Amarouchene, Y. Louyer, T. Guérin, and D. S. Dean, Role of nonconservative scattering forces and damping on Brownian particles in optical traps, Phys. Rev. E 99, 052107 (2019). [links][doi:10.1103/PhysRevE.99.052107] [arXiv:1812.09188] [pdf] [abstract]Abstract: We consider a model of a particle trapped in a harmonic optical trap but with the addition of a nonconservative radiation induced force. This model is known to correctly describe experimentally observed trapped particle statistics for a wide range of physical parameters, such as temperature and pressure. We theoretically analyze the effect of nonconservative force on the underlying steady state distribution as well as the power spectrum for the particle position. We compute perturbatively the probability distribution of the resulting nonequilibrium steady states for all dynamical regimes underdamped through to overdamped and give expressions for the associated currents in phase space (position and velocity). We also give the spectral density of the trapped particle's position in all dynamical regimes and for any value of the nonconservative force. Signatures of the presence of nonconservative forces are shown to be particularly strong for the underdamped regime at low frequencies.

[06] Y. Amarouchene, M. Mangeat, B. Vidal Montes, L. Ondic, T. Guérin, D. S. Dean, and Y. Louyer, Nonequilibrium Dynamics Induced by Scattering Forces for Optically Trapped Nanoparticles in Strongly Inertial Regimes, Phys. Rev. Lett. 122, 183901 (2019). [links][doi:10.1103/PhysRevLett.122.183901] [arXiv:1812.06804] [pdf] [abstract]Abstract: The forces acting on optically trapped particles are commonly assumed to be conservative. Nonconservative scattering forces induce toroidal currents in overdamped liquid environments, with negligible effects on position fluctuations. However, their impact in the underdamped regime remains unexplored. Here, we study the effect of nonconservative scattering forces on the underdamped nonlinear dynamics of trapped nanoparticles at various air pressures. These forces induce significant low-frequency position fluctuations along the optical axis and the emergence of toroidal currents in both position and velocity variables. Our experimental and theoretical results provide fundamental insights into the functioning of optical tweezers and a means for investigating nonequilibrium steady states induced by nonconservative forces.

[PhD] M. Mangeat, De la dispersion aux vortex browniens dans des systèmes hors-équilibres confinés, Thèse de doctorat, Université de Bordeaux (2018). [links][tel] [pdf] [abstract]Abstract: Cette thèse vise à caractériser la dynamique stochastique hors-équilibre de particules browniennes sous l’effet de confinement. Ce confinement est appliqué ici par des potentiels attractifs ou des frontières imperméables créant des barrières entropiques. Dans un premier temps, nous regardons la dispersion de particules sans interactions dans les milieux hétérogènes. Un nuage de particules browniennes s’étale au cours du temps sans atteindre la distribution d’équilibre de Boltzmann, et son étalement est alors caractérisé par une diffusivité effective inférieure à la diffusivité microscopique. Dans un premier chapitre, nous nous intéressons au lien entre la géométrie de confinement et la dispersion dans le cas particulier des microcanaux périodiques. Pour cela, nous calculons la diffusivité effective sans hypothèse de réduction de dimensionnalité, contrairement à l’approche standard dite de Fick-Jacobs. Une classification des différents régimes de dispersion est alors réalisée, pour toute géométrie autant pour les canaux continus que discontinus. Dans un second chapitre, nous étendons cette analyse à la dispersion dans les réseaux périodiques d’obstacles sphériques attractifs à courte portée. La présence d’un potentiel attractif peut, de manière surprenante, augmenter la dispersion. Nous quantifions cet effet dans le régime dilué, et montrons alors son optimisation pour plusieurs potentiels ainsi que pour une diffusion médiée par la surface des sphères. Ensuite, nous étudions la dynamique stochastique de particules browniennes dans un piège optique en présence d’une force non conservative créée par la pression de radiation du laser. L’expression perturbative des courants stationnaires, décrivant les vortex browniens, est dérivée pour les basses pressions en conservant le terme inertiel dans l’équation de Langevin sous-amortie. L’expression de la densité spectrale est également calculée permettant d’observer les anisotropies du piège et les effets de la force non conservative. La plupart des expressions analytiques obtenues durant cette thèse sont asymptotiquement exactes et vérifiées par des analyses numériques basées sur l’intégration de l’équation de Langevin ou la résolution d’équation aux dérivées partielles.

[05] M. Mangeat, T. Guérin, and D. S. Dean, Dispersion in two-dimensional periodic channels with discontinuous profiles, J. Chem. Phys. 149, 124105 (2018). [links][doi:10.1063/1.5045183] [arXiv:1807.05366] [pdf] [abstract]Abstract: The effective diffusivity of Brownian tracer particles confined in periodic micro-channels is smaller than the microscopic diffusivity due to entropic trapping. Here, we study diffusion in two-dimensional periodic channels whose cross section presents singular points, such as abrupt changes of radius or the presence of thin walls, with openings, delimiting periodic compartments composing the channel. Dispersion in such systems is analyzed using the Fick-Jacobs (FJ) approximation. This approximation assumes a much faster equilibration in the lateral than in the axial direction, along which the dispersion is measured. If the characteristic width a of the channel is much smaller than the period L of the channel, i.e., ε=a/L is small, this assumption is clearly valid for Brownian particles. For discontinuous channels, the FJ approximation is only valid at the lowest order in ε and provides a rough, though on occasions rather accurate, estimate of the effective diffusivity. Here we provide formulas for the effective diffusivity in discontinuous channels that are asymptotically exact at the next-to-leading order in ε. Each discontinuity leads to a reduction of the effective diffusivity. We show that our theory is consistent with the picture of effective trapping rates associated with each discontinuity, for which our theory provides explicit and asymptotically exact formulas. Our analytical predictions are confirmed by numerical analysis. Our results provide a precise quantification of the kinetic entropic barriers associated with profile singularities.

[04] M. Mangeat, T. Guérin, and D. S. Dean, Dispersion in two dimensional channels — the Fick-Jacobs approximation revisited, J. Stat. Mech. 2017, 123205 (2017). [links][doi:10.1088/1742-5468/aa9bb5] [arXiv:1710.02699] [pdf] [abstract]Abstract: We examine the dispersion of Brownian particles in a symmetric two dimensional channel, this classical problem has been widely studied in the literature using the so called Fick–Jacobs’ approximation and its various improvements. Most studies rely on the reduction to an effective one dimensional diffusion equation, here we derive an explicit formula for the diffusion constant which avoids this reduction. Using this formula the effective diffusion constant can be evaluated numerically without resorting to Brownian simulations. In addition, a perturbation theory can be developed in ε=h₀/L where h₀ is the characteristic channel height and L the period. This perturbation theory confirms the results of Kalinay and Percus [PRE 74, 041203 (2006)], based on the reduction, to one dimensional diffusion are exact at least to O(ε⁶) . Furthermore, we show how the Kalinay and Percus pseudo-linear approximation can be straightforwardly recovered. The approach proposed here can also be exploited to yield exact results in the limit ε→∞, we show that here the diffusion constant remains finite and show how the result can be obtained with a simple physical argument. Moreover, we show that the correction to the effective diffusion constant is of order 1/ε and remarkably has some universal characteristics. Numerically we compare the analytic results obtained with exact numerical calculations for a number of interesting channel geometries.

[03] M. Mangeat, T. Guérin, and D. S. Dean, Geometry controlled dispersion in periodic corrugated channels, EPL 118, 40004 (2017). [links][doi:10.1209/0295-5075/118/40004] [arXiv:1709.03722] [pdf] [abstract]Abstract: The effective diffusivity D_e of tracer particles diffusing in periodically corrugated axisymmetric two- and three-dimensional channels is studied. The majority of the previous studies of this class of problems are based on perturbative analyses about narrow channels, where the problem can be reduced to an effectively one-dimensional one. Here we show how to analyze this class of problems using a much more general approach which even includes the limit of infinitely wide channels. Using the narrow- and wide-channel asymptotics, we provide a Padé approximant scheme that is able to describe the dispersion properties of a wide class of channels. Furthermore, we systematically identify all the exact asymptotic scaling regimes of D_e and the accompanying physical mechanisms that control dispersion, clarifying the distinction between smooth channels and compartmentalized ones, and identifying the regimes in which D_e can be linked to first passage problems.

[02] X. Zhou, R. Zhao, K. Schwarz, M. Mangeat, E. C. Schwarz, M. Hamed, I. Bogeski, V. Helms, H. Rieger, and B. Qu, Bystander cells enhance NK cytotoxic efficiency by reducing search time, Scientific Reports 7, 44357 (2017). [links][doi:10.1038/srep44357] [pdf] [abstract]Abstract: Natural killer (NK) cells play a central role during innate immune responses by eliminating pathogen-infected or tumorigenic cells. In the microenvironment, NK cells encounter not only target cells but also other cell types including non-target bystander cells. The impact of bystander cells on NK killing efficiency is, however, still elusive. In this study we show that the presence of bystander cells, such as P815, monocytes or HUVEC, enhances NK killing efficiency. With bystander cells present, the velocity and persistence of NK cells were increased, whereas the degranulation of lytic granules remained unchanged. Bystander cell-derived H₂O₂ was found to mediate the acceleration of NK cell migration. Using mathematical diffusion models, we confirm that local acceleration of NK cells in the vicinity of bystander cells reduces their search time to locate target cells. In addition, we found that integrin β chains (β₁, β₂ and β₇) on NK cells are required for bystander-enhanced NK migration persistence. In conclusion, we show that acceleration of NK cell migration in the vicinity of H₂O₂-producing bystander cells reduces target cell search time and enhances NK killing efficiency.

[01] M. Mangeat and F. Zamponi, Quantitative approximation schemes for glasses, Phys. Rev. E 93, 012609 (2016). [links][doi:10.1103/PhysRevE.93.012609] [arXiv:1510.03808] [pdf] [abstract]Abstract: By means of a systematic expansion around the infinite-dimensional solution, we obtain an approximation scheme to compute properties of glasses in low dimensions. The resulting equations take as input the thermodynamic and structural properties of the equilibrium liquid, and from this they allow one to compute properties of the glass. They are therefore similar in spirit to the Mode Coupling approximation scheme. Our scheme becomes exact, by construction, in dimension d→∞, and it can be improved systematically by adding more terms in the expansion.

[21] The 29th international conference on Statistical Physics (StatPhys29), Florence, July 2025, Emergent phases in a discrete flocking model with non-reciprocal interaction (poster). [link] [pdf] [abstract]Abstract: Non-reciprocal interactions arise in systems that seemingly violate Newton’s third law “actio=reactio”. They are ubiquitous in active and living systems that break detailed balance at the microscale, from social forces to antagonistic inter-species interactions in bacteria. Non-reciprocity affects non-equilibrium phase transitions and pattern formation in active matter and represents a rapidly growing research focus in the field. In this work, we have undertaken a comprehensive study of the non-reciprocal two-species active Ising model (NRTSAIM), a non-reciprocal discrete-symmetry counterpart of the continuous-symmetry two-species Vicsek model [PRE 107, 024607 (2023)]. Our study uncovers a distinctive run-and-chase dynamical state that emerges under significant non-reciprocal frustration. In this state, A-particles chase B-particles to align with them, while B-particles avoid A-particles, resulting in B-particle accumulation at the opposite end of the advancing A-band. This run-and-chase state represents a non-reciprocal discrete-symmetry analog of the chiral phase seen in the non-reciprocal Vicsek model [Nature 592, 363 (2021)]. Additionally, we find that self-propulsion destroys the oscillatory state obtained for the non-motile case, and all the NRTSAIM steady-states are metastable due to spontaneous droplet excitation and exhibit motility-induced interface pinning. A hydrodynamic theory supports our simulations and confirms the reported phase diagrams.

[20] DPG Meeting of the Condensed Matter Section, Regensburg 2025, University of Regensburg (Germany), March 2025, Emergent phases in a discrete flocking model with reciprocal interaction (talk). [link] [pdf] [abstract]Abstract: We have undertaken a comprehensive study of the two-species active Ising model (TSAIM), a discrete-symmetry counterpart of the continuous-symmetry two-species Vicsek model, motivated by recent interest in the impact of complex and heterogeneous interactions on active matter systems. In the TSAIM, two species of self-propelled particles undergo biased diffusion in two dimensions, interacting via local intraspecies alignment and reciprocal interspecies anti-alignment, along with the possibility of species interconversion. We observe a liquid-gas phase transition, exhibiting macrophase-separated bands, and the emergence of a high-density parallel flocking state, a feature not seen in previous flocking models. With species interconversion (species-flip dynamics), the TSAIM corresponds to an active extension of the Ashkin-Teller model and exhibits a broader range of steady-state phases, including microphase-separated bands that further enrich the coexistence region. We also find that the system is metastable due to droplet excitation and exhibits spontaneous motility-induced interface pinning, preventing the system from reaching long-range order at sufficiently low noise. A hydrodynamic theory complements our computer simulations of the microscopic model and confirms the reported phase diagrams.

[19] DPG Meeting of the Condensed Matter Section, Berlin 2024, TU Berlin (Germany), March 2024, Stationary particle currents in sedimenting active matter wetting a wall (talk). [link] [pdf] [abstract]Abstract: Recently it was predicted, on the basis of a lattice gas model, that scalar active matter would rise against gravity up a confining wall in spite of repulsive particle-wall interactions [PRL 124, 048001 (2020)]. We confirm this prediction with sedimenting active Brownian particles (ABPs) in a box and elucidate the mechanism leading to the formation of a meniscus rising above the bulk of the sedimentation region. The height of the meniscus increases algebraically with the activity, and the formation of the meniscus is determined by a stationary circular particle current centered at the base of the meniscus. The origin of these vortices can be traced back to the confinement of the ABPs in a box: already the stationary state of non-interacting ABPs without gravitation displays highly symmetric circular currents. Gravitation distorts this vortex configuration downward, leaving two major vortices at the two side walls, with a strong downward flow along the walls. Repulsive interactions between the ABPs change this situation only as soon as motility induced phase separation (MIPS) sets in and forms a dense, sedimented liquid region at the bottom, which pushes the center of the vortex upwards towards the liquid-gas interface. Self-propelled particles therefore represent an impressive realization of scalar active matter that forms stationary particle currents being able to perform visible work against gravity, which we predict to be observable experimentally.

[18] DPG Meeting of the Condensed Matter Section, Berlin 2024, TU Berlin (Germany), March 2024, Flocking of two unfriendly species (poster). [link] [pdf] [abstract]Abstract: Complex systems are typically heterogeneous as individuals vary in their properties, their response to the external environment and to each other. In particular, many biological systems that show flocking involve self-propelled particles with heterogeneous interactions, which motivates the study of populations with multiple species. In this work, we consider the two-species variant of the Vicsek model (TSVM) and the active Ising model (TSAIM), consisting of two kinds of self-propelled particles that tend to align with particles from the same species and to antialign with the other. These two-species models show a flocking transition that is reminiscent of the original one-species model, as a liquid-gas phase transition, and display phase-separation in the coexistence region where dense liquid bands of each species propagate in a gaseous background. The interesting feature of these models is the appearance of two dynamical states in the coexistence region: the PF (parallel flocking) state in which all bands of the two species propagate in the same direction, and the APF (antiparallel flocking) state in which the bands of two different species move in opposite directions. PF and APF states perform stochastic transitions from one to the other only in TSVM, and the APF liquid phase of the TSVM is replaced by a high density PF state in the TSAIM. We also study the impact of particle switching from one species to another.

[17] Cell Physics 2023, Saarland University (Saarbrücken, Germany), October 2023 (attendee). [link]

[16] DPG Meeting of the Condensed Matter Section, Dresden 2023, TU Dresden (Germany), March 2023, Wetting of reflecting plates by an active Brownian fluid (poster). [link] [pdf] [abstract]Abstract: We study, using interacting active Brownian particles (ABP), the wall-wetting mechanism of active sedimenting fluid. We consider a minimal model of active particles under gravitational field, inside a two-dimensional rectangular box. An accumulation of particles near the bottom wall is observed, as well as the wetting of vertical plates by the rise of active particles against the gravity, even without any attractive force within the system. We characterize this wall-wetting by the meniscus height, calculated from stationary density profile and depending on the inter-particle repulsion. The maximum wetting height depends super-linearly on active sedimentation length for interacting ABP, and linearly for non-interacting ABP. We also observe two large vortices concentrated close to the meniscus, due to the persistence motion of ABP against the gravity. Moreover, with non-interacting ABP, a current flow is present near the boundaries for which we propose a coarse-grained description.

[15] DPG Meeting of the Condensed Matter Section, Dresden 2023, TU Dresden (Germany), March 2023, Polar flocks with discretized directions: the active clock model approaching the Vicsek model (talk). [link] [pdf] [abstract]Abstract: We study the off-lattice two-dimensional q-state active clock model (ACM) [EPL 138, 41001 (2022)] as a natural discretization of the Vicsek model (VM) [PRL 75, 1226 (1995)] describing flocking. The ACM consists of particles able to move in the plane in a discrete set of q equidistant angular directions, as in the active Potts model (APM) [EPL 130, 66001 (2020); PRE 102, 042601 (2020)], with a local alignment interaction inspired by the ferromagnetic equilibrium clock model. A collective motion emerges at high densities and low noise. We compute phase diagrams of the ACM and explore the flocking dynamics in the region, in which the high-density (polar liquid) phase coexists with the low-density (gas) phase. We find that for a small number of directions, the flocking transition of the ACM has the same phenomenology as the APM, including macrophase separation and reorientation transition from transversal to longitudinal band motion as a function of the particle self-propulsion velocity. For a larger number of directions, the flocking transition in the ACM becomes equivalent to the one of the VM and displays microphase separation and only transverse bands, i.e. no reorientation transition. Concomitantly also the transition of the q→∞ limit of the ACM, the active XY model, is in the same universality class as the VM. We also construct a coarse-grained hydrodynamic description akin to the VM.

[14] DPG Meeting of the Condensed Matter Section, Regensburg 2022, University of Regensburg (Germany), September 2022, Polar flocks with discretized directions: the active clock model approaching the Vicsek model (poster). [link] [pdf] [abstract]Abstract: We study the off-lattice two-dimensional q-state active clock model (ACM) as a natural discretization of the Vicsek model (VM) [PRL 75, 1226 (1995)] describing flocking. The ACM consists of particles able to move in the plane in a discrete set of q equidistant angular directions, as in the active Potts model (APM) [EPL 130, 66001 (2020); PRE 102, 042601 (2020)], with a local alignment interaction inspired by the ferromagnetic equilibrium clock model. A collective motion emerges at high densities and low noise. We compute phase diagrams of the ACM and explore the flocking dynamics in the region, in which the high-density (polar liquid) phase coexists with the low-density (gas) phase. We find that for a small number of directions, the flocking transition of the ACM has the same phenomenology as the APM, including macrophase separation and reorientation transition from transversal to longitudinal band motion as a function of the particle self-propulsion velocity. For a larger number of directions, the flocking transition in the ACM becomes equivalent to the one of the VM and displays microphase separation and only transverse bands, i.e. no reorientation transition. Concomitantly also the transition of the q→∞ limit of the ACM, the active XY model (AXYM), is in the same universality class as the VM. We also construct a coarse-grained hydrodynamic description for the ACM and AXYM akin to the VM.

[13] Cell Physics 2021, Saarland University (Saarbrücken, Germany), September 2021 (attendee). [link]

[12] Virtual DPG Spring Meeting 2021, online, March 2021, The narrow escape problem in two-shell circular domains (poster). [link] [pdf] [abstract]Abstract: The stochastic motion of particles in living cells is often spatially inhomogeneous with a higher effective diffusivity in a region close to the cell boundary due to active transport along actin filaments [PRL 117, 068101 (2016); Phys. Biol. 13, 066003 (2016); Biophys. J 114, 1420-1432 (2018)]. As a first step to understand the consequence of the existence of two compartments for stochastic search problems we consider here a Brownian particle in a circular domain with different diffusivities and potentials in the inner and the outer shell. We focus on the narrow escape problem and compute the mean first passage time (MFPT) for Brownian particles starting at some pre-defined position to find a small region on the outer reflecting boundary (cell membrane). We find that the MFPT can be minimized for a specific value of the width of the outer shell only if the particle is sufficiently attracted in the outer shell whereas the MFPT depends monotonously on all model parameters without attraction. A criterion on the difference of potential between the two shells can be calculated analytically with respect to the escape region size and the ratio of diffusivities. Moreover we show that the limit of small width of the outer shell is equivalent to the surface-mediated diffusion problem [PRE 86, 041135 (2012)].

[11] Virtual DPG Spring Meeting 2021, online, March 2021, Flocking and reorientation transition in the q-state active Potts model (poster). [link] [pdf] [abstract]Abstract: We study the q-state active Potts model (APM) on a two-dimensional lattice in which active particles have q internal states corresponding to the q directions of motion. A local alignment rule inspired by the ferromagnetic q-state Potts model and self-propulsion via biased diffusion according to the internal particle states leads to a collective motion at high densities and low noise. We formulate a coarse-grained hydrodynamic theory with which we compute the phase diagram of the APM and explore the flocking dynamics in the region, in which the high-density (polar liquid) phase coexists with the low-density (gas) phase and forms a fluctuating band of coherently moving particles. As a function of the particle self-propulsion velocity, a novel reorientation transition of the phase-separated profiles from transversal to longitudinal band motion is found, which is absent in the Vicsek model [PRL 75, 1226 (1995)] and the active Ising model [PRL 111, 078101 (2013)]. The origin of this reorientation transition is revealed by a stability analysis: for large velocities the transverse diffusion constant approaches zero and then stabilizes longitudinal band motion. Computer simulations corroborate the analytical predictions of the flocking and reorientation transitions and validate the phase diagrams of the APM.

[10] Seminar, Laboratoire de Physique Théorique et Modèles Statistiques (Orsay, France, online), January 2021, Flocking and reorientation transition in the q-state active Potts model (talk). [link] [pdf] [abstract]Abstract: We study the q-state active Potts model (APM) on a two-dimensional lattice in which active particles have q internal states corresponding to the q directions of motion. A local alignment rule inspired by the ferromagnetic q-state Potts model and self-propulsion via biased diffusion according to the internal particle states leads to a collective motion at high densities and low noise. We formulate a coarse-grained hydrodynamic theory with which we compute the phase diagram of the APM and explore the flocking dynamics in the region, in which the high-density (polar liquid) phase coexists with the low-density (gas) phase and forms a fluctuating band of coherently moving particles. As a function of the particle self-propulsion velocity, a novel reorientation transition of the phase-separated profiles from transversal to longitudinal band motion is found, which is absent in the Vicsek model [PRL 75, 1226 (1995)] and the active Ising model [PRL 111, 078101 (2013); PRE 92, 042119 (2015)]. The origin of this reorientation transition is revealed by a stability analysis: for large velocities the transverse diffusion constant approaches zero and then stabilizes longitudinal band motion. Computer simulations corroborate the analytical predictions of the flocking and reorientation transitions and validate the phase diagrams of the APM [EPL 130, 66001 (2020); PRE 102, 042601 (2020)].

[09] Seminar, Laboratoire de Physique Théorique (Toulouse, France, online), December 2020, Flocking and reorientation transition in the q-state active Potts model (talk). [link] [pdf] [abstract]Abstract: We study the q-state active Potts model (APM) on a two-dimensional lattice in which active particles have q internal states corresponding to the q directions of motion. A local alignment rule inspired by the ferromagnetic q-state Potts model and self-propulsion via biased diffusion according to the internal particle states leads to a collective motion at high densities and low noise. We formulate a coarse-grained hydrodynamic theory with which we compute the phase diagram of the APM and explore the flocking dynamics in the region, in which the high-density (polar liquid) phase coexists with the low-density (gas) phase and forms a fluctuating band of coherently moving particles. As a function of the particle self-propulsion velocity, a novel reorientation transition of the phase-separated profiles from transversal to longitudinal band motion is found, which is absent in the Vicsek model [PRL 75, 1226 (1995)] and the active Ising model [PRL 111, 078101 (2013); PRE 92, 042119 (2015)]. The origin of this reorientation transition is revealed by a stability analysis : for large velocities the transverse diffusion constant approaches zero and then stabilizes longitudinal band motion. Computer simulations corroborate the analytical predictions of the flocking and reorientation transitions and validate the phase diagrams of the APM [EPL 130, 66001 (2020); PRE 102, 042601 (2020)].

[08] Microswimmers International Conference 2020: Motile Active Matter, Forschungszentrum caesar (Bonn, Germany, online), October 2020, Flocking and reorientation transition in the q-state active Potts model (poster). [link] [pdf] [abstract]Abstract: We study the q-state active Potts model (APM) on a two-dimensional lattice in which active particles have q internal states corresponding to the q directions of motion. A local alignment rule inspired by the ferromagnetic q-state Potts model and self-propulsion via biased diffusion according to the internal particle states leads to a collection motion at high densities and low noise. We compute the phase diagram of the APM and explore the flocking dynamics in the region, in which the high-density (polar liquid) phase coexists with the low-density (gas) phase and forms a fluctuating band of coherently moving particles. As a function of the particle self-propulsion velocity, a novel reorientation transition of the phase-separated profiles from transversal to longitudinal band motion is revealed, which is absent in the Vicsek model [PRL 75, 1226 (1995)] and the active Ising model [PRL 078101 (2013)]. We further formulate a coarse-grained hydrodynamic theory of the model which predicts the phase diagram of the microscopic model and reveals the origin of the flocking transition in a stability analysis: for large velocities the transverse diffusion constant approaches zero and then stabilizes longitudinal band motion.

[07] Frontiers in Computational Methods for Active Matter, Centre Européen de Calcul Atomique et Moléculaire (Lausanne, Switzerland), February 2020, Flocking and reorientation transition in the 4-state active Potts model (poster). [link] [pdf] [abstract]Abstract: We study the 4-state active Potts model in two dimensions, which is an active lattice gas model on the square lattice with particles performing nearest-neighbor hopping biased according to one of four possible motility states and ferromagnetic on-site alignment. A collective motion emerges at low temperatures from a spontaneous breaking of the discrete symmetry in the form of bands aligned with one of the four directions. This flocking transition is a first-order liquid-gas phase transition with an infinite critical density, as found in previously studied models. We compute the velocity-density and the temperature-density phase diagrams and we find a novel reorientation transition of the phase-separated profiles from transversal to longitudinal band motion, which is absent in the Vicsek model [PRL 75, 1226 (1995)] and the active Ising model [PRL 111, 078101 (2013)]. The longitudinal motion occurs at high velocities due to a strongly biased diffusion. We present a hydrodynamic theory which reproduces very well the results of computer simulations of the microscopic model.

[06] Cell Physics 2019, Saarland University (Saarbrücken, Germany), October 2019, The narrow escape problem in a circular domain with radial piecewise constant diffusivity (talk). [link] [pdf] [abstract]Abstract: The stochastic motion of particles in living cells is often spatially inhomogeneous with a higher effective diffusivity in a region close to the cell boundary due to active transport along actin filaments [PRL 117, 068101 (2016)]. As a first step to understand the consequence of the existence of two compartments with different diffusion constant for stochastic search problems we consider a Brownian particle in a circular domain with different diffusion constants in the inner and the outer shell. We focus on the narrow escape problem and compute the mean first passage time (MFPT) for Brownian particles starting at some predefined position to find a small region on the outer reflecting boundary. The asymptotic expression of the MFPT are obtained following [Multiscale Model. Simul. 8, 803 (2010)]. For the annulus geometry we find that the MFPT can be minimized for a specific value of the width of the outer shell. In contrast for the two-shell geometry we show that the MFPT depends monotonously on the outer shell width. Then, the MFPT can be optimized only when a mechanism enforce the particle to stay close to the surface [PRL 105, 150606 (2010)]. Moreover we find that the distance between the starting point and the narrow escape region which maximizes the MFPT depends discontinuously on the ratio between inner and outer diffusivity.

[05] DPG Spring Meeting of the Condensed Matter Section, Regensburg 2019, University of Regensburg (Germany), April 2019, Controlled dispersion in periodic microchannels and regular obstacle parks (talk). [link] [pdf] [abstract]Abstract: The dispersion of Brownian particles in heterogeneous media is a widely studied problem which appears in many contexts (chemical reactions, biological systems, zeolites, porous media, pollutant spreading, ...). A cloud of particles disperses over time without reaching the Boltzmann equilibrium distribution and its spreading is then characterized by an effective long-time diffusivity D_e lower than the microscopic diffusivity. The analytical expression of D_e is given by an exact Kubo-type formula [PRE 92, 062103 (2015)] for periodic systems. The dispersion in periodic microchannels is controlled by the confinement geometry via an entropic trapping. Three different dispersion regimes are then identified for continuous and discontinuous channels [EPL 118, 40004 (2017)]. The expression of D_e is thus well-described by the Fick-Jacobs’ approximation, narrow escape problems or the diffusion problem in comb-like geometries in each regime. This analysis can be extended to the dispersion in regular obstacle parks. The presence of short-range attractive potential on the surface of obstacles enhance the dispersion of Brownian particles. The optimal value of D_e is then analytically characterized in the dilute limit of obstacles.

[04] LOMA Theory Day 2018, Bordeaux University (Talence, France), May 2018, Geometry controlled dispersion in periodic channels (talk). [link] [pdf]

[03] Seminar, Saarland University (Saarbrücken, Germany), April 2018, Geometry controlled dispersion in periodic channels (talk). [pdf] [abstract]Abstract: We study the dispersion of Brownian particles in periodic two- and three-dimensional channels through the long time effective diffusivity D_e. The majority of the previous studies of this class of problems are based on perturbative analyses about narrow channels, where the problem can be reduced to an effectively one-dimensional one, using the Fick-Jacobs’ approximation. We show how to analyse the dispersion using a more general approach which even includes the wide channel limit, where D_e remains finite. We finally identify all the exact asymptotic scaling regimes of D_e and the accompanying physical mechanism that control the dispersion.

[02] Journées de Physique Statistique 2018, ESPCI (Paris, France), January 2018, Dispersion in periodic channels (talk). [link] [pdf]

[01] International Summer School "Fundamental Problems in Statistical Physics XIV", Bruneck (Italy), July 2017, Geometry controlled dispersion in periodic corrugated channels (poster). [link] [pdf]

Multispecies flocking and non-reciprocal interactions.

Flocking transition of active spins.