arXiv subjects

Fernando Peruani

Publications and source records attributed to Fernando Peruani.

At least 19 recordsLinked to original sources

arXiv2026

Minority-Triggered Reorientations Yield Macroscopic Cascades and Enhanced Responsiveness in Swarms

Collective motion in animals and cells often exhibits rapid reorientations and scale-free velocity correlations. This allows information to spread rapidly through the group, allowing an adequate collective response to environmental changes and threats such as predators. To explain this phenomenon, we introduce a simple, biologically plausible mechanism: a minority-triggered reorientation rule. When local order is high, agents sometimes follow a strongly deviating neighbor instead of the majority. This rule qualitatively changes the macroscopic system behavior compared to traditional flocking models, as it generates heavy-tailed cascades of reorientations over broad parameter ranges. Our mechanism preserves cohesion while markedly enhancing collective responsiveness because localized directional cues elicit amplified group-level reorientation. Our results provide a parsimonious, biologically interpretable route to critical-like fluctuations and high responsiveness during flocking.

q-bio.QM↗

arXiv2026

Collective behavior based on agent-environment interactions

We present a model of active particles interacting through a dynamic, heterogeneous environment, leading to emergent collective behaviors without direct agent-to-agent communication. Expanding the resource-dependent framework introduced in Briozzo et al., 2025, arXiv:2512.08762, agents perform a persistent random walk combined with chemotaxis, directing toward nutrient-rich patches, whose resources are generated by logistic regrowth. We identify distinct phases of collective organization, ranging from disordered gas-like states to polar traveling waves and nematic independent clusters, depending on the interplay between chemotactic sensitivity and angular noise. The system exhibits spontaneous symmetry breaking and density waves driven purely by the coupling between population dynamics (birth-death processes) and environmental feedback. Our results bridge active matter physics and movement ecology, demonstrating that complex spatiotemporal patterns can arise without direct interaction between agents, but solely from the maximization of resource intake in a reactive environment.

physics.bio-ph↗

arXiv2025

Spreading processes on heterogeneous active systems: spreading threshold, immunization strategies, and vaccination noise

We study spreading processes in two-dimensional systems of heterogeneous active agents that exhibit different individual active speeds. We obtain, combining kinetic and complex network theory, an analytical expression for the spreading threshold that depends not only on the first but also second moment of the speed distribution. Moreover, we prove that spreading can even occur for vanishing average active speed. Furthermore, we find that random vaccination strategies are ineffective in heterogeneous active systems, whereas targeted ones are effective. We also show that vaccination acts as (quenched) noise: it can decrease or increase the outbreak size. Our results offer insights into how information propagates in heterogeneous populations of active agents.

physics.bio-ph↗

arXiv2025

Zoology of collective patterns modulated by non-reciprocal, long-range interactions

We investigate active particles that exhibit long-range interactions only restricted by a field of view, which is characterized by an angle $\beta$. We show that constraining attractive interactions to a field of view leads to the emergence of a complex pattern that exhibits - depending on the value of $\beta$ and initial conditions - significantly different topologies and transport properties. We find, in two dimensions, a nematic closed filament in the form of a ring that moves as a chiral active particle, a closed polar filament with one singular topological point that exhibits net polar order and moves ballistically, a structure with two singular topological points that rotates, or an open polar filament that behaves as a persistent random walk. Furthermore, we investigate the process that transforms one structure into another by slowly varying $\beta$ and observe that the process is non-reversible and presents strong hysteresis. Finally, we find that in three dimensions similar patterns also emerge. The analysis sheds light on the physics of single-species active particles with long-range, non-reciprocal interactions in two and three dimensions, characterized by the absence of gas phases, and provides evidence that in these systems, topological and transport properties are closely related.

physics.bio-ph↗

arXiv2025

Confinement controls bacterial spreading at all scales

Navigation of microorganisms is controlled by internal processes ultimately sensitive to mechanical or chemical signaling encountered along the path. In many natural environments, such as porous soils or physiological ducts, motile species alternate between bulk and surface motion displaying in each case, distinct kinematics. This inherent complexity is key to many practical biological and ecological issues involving spreading and contamination, essential for understanding the spatiotemporal structuring of populations in their environment. However grasping the interplay between geometrical confinement and kinematics driven by internal biological responses remains poorly understood from a physical and biological standpoint. Here, we address this question through experimental and theoretical analysis in the heuristic situation of two parallel confining surfaces. We track wild-type E. coli - a model peritrichous flagellated bacterium - in 3D over extended periods of time. We obtain the first experimental measurements of the emerging diffusivity and bulk/surface residence times as a function of confinement height and the specific chiral kinematics at surfaces. All experimental results are quantitatively reproduced, without parametric adjustment, by a non-Markovian stochastic (BV) model that incorporates the internal biochemical memory carried by a phosphorylated protein switching the motor rotation. By matching the results with a Markovian (memoryless) companion model, we derive an analytical expression for the diffusivity and demonstrate how confining walls influence microbial long-range dispersion. This approach also provides a general conceptual basis for understanding how microorganisms navigate complex environments, in which their movement alternates between bulk and surfaces.

physics.bio-ph↗

arXiv2024

Activity leads to topological phase transition in 2D populations of heterogeneous oscillators

Populations of heterogeneous, noisy oscillators on a two-dimensional lattice display short-range order. Here, we show that if the oscillators are allowed to actively move in space, the system undergoes instead a Berezenskii-Kosterlitz-Thouless transition and exhibits quasi-long-range order. This fundamental result connects two paradigmatic models -- XY and Kuramoto model -- and provides insight on the emergence of order in active systems.

cond-mat.stat-mech↗

arXiv2024

Self-trapping of active particles with non-reciprocal interactions in disordered media

We study systems of active particles, whose perception is constrained by a vision cone, that are attracted to other particles and repelled from static obstacles. We report a novel self-trapping mechanism: active particles with non-reciprocal attraction form particle chains, which eventually become closed loops that shrink around one or many obstacles. These closed loops act as effective aggregation centers. Long-lived, self-organized closed loops require to enclose obstacles to exist. Furthermore, we find that closed loops that initially exhibit local polar order, transition to a nematic state as they absorb more particles. The unveiled mechanism corresponds to a pinning behavior that strongly hinders particle diffusion. In short, closed loops dominate the large-scale properties of active systems with non-reciprocal attraction in disordered media.

cond-mat.stat-mech↗

arXiv2024

Vectorial active matter on the lattice: polar condensates and nematic filaments

We introduce a novel lattice-gas cellular automaton (LGCA) for compressible vectorial active matter with polar and nematic velocity alignment. Interactions are, by construction, zero-range. For polar alignment, we show the system undergoes a phase transition that promotes aggregation with strong resemblance to the classic zero-range process. We find that above a critical point, the states of a macroscopic fraction of the particles in the system coalesce into the same state, sharing the same position and momentum (polar condensate). For nematic alignment, the system also exhibits condensation, but there exist fundamental differences: a macroscopic fraction of the particles in the system collapses into a filament, where particles possess only two possible momenta. Furthermore, we derive hydrodynamic equations for the active LGCA model to understand the phase transitions and condensation that undergoes the system. We also show that generically the discrete lattice symmetries -- e.g. of a square or hexagonal lattice -- affect drastically the emergent large-scale properties of on-lattice active systems. The study puts in evidence that aligning active matter on the lattice displays new behavior, including phase transitions to states that share similarities to condensation models.

cond-mat.soft↗

arXiv2024

Structural dynamics and optimal transport of an active polymer

We study the spontaneous configuration transitions of an active semi-flexible polymer between {\it spiral} and {\it non-spiral} states, and show that the configuration dynamics is fully described by a {\it subcritical pitchfork} bifurcation. Exploiting the fact that active polymer barely moves in {\it spiral} states and exhibits net displacements in {\it non-spiral} states, we theoretically prove that the motion of the active polymer is consistent with a {\it run-and-tumble}-like dynamics. Moreover, we find that there exists an {\it optimal} self-propelling {\it force}, at which the probabilities of finding the polymer in the {\it spiral} and {\it non-spiral} state become equal, that maximizes the diffusion coefficient.

cond-mat.soft↗

arXiv2023

A generic coupling between internal states and activity leads to activation fronts and criticality in active systems

To understand the onset of collective motion, we investigate active systems where particles switch on and off their self-propulsion. We prove that even when the only possible transition is off$\to$on, an active 2-state system behaves as an effective 3-state system that exhibits a sharp phase transition in 1D, and critical behavior in 2D, with scale-invariant activity avalanches. The obtained results show how criticality can naturally emerge in active systems, providing insight into the way collectives distribute, process, and respond to local environmental cues.

physics.bio-ph↗

arXiv2023

Active stop and go motion: a strategy to improve spatial exploration and survival

We consider active Brownian particles that intermittently switch between active and inactive states. Such behavior is ubiquitous at all scales, from bacteria to animals and in artificial active systems. We derive exact expressions for key transport properties, including velocity autocorrelations and diffusion coefficients, and demonstrate that particle dispersion is highly sensitive to minute details, such as whether the memory of active orientation is retained across a stop event. Extending the model to include minimal metabolic dynamics, we show that the average survival time of the particle is maximized at (i)~an optimal stopping frequency and (ii)~a non-trivial optimal diffusivity, which itself depends on the stopping frequency.

cond-mat.stat-mech↗

arXiv2023

When does an active bath behave as an equilibrium one?

Active baths are characterized by a non-Gaussian velocity distribution and a quadratic dependence with active velocity $v_0$ of the kinetic temperature and diffusion coefficient. While these results hold in over-damped active systems, inertial effects lead to normal velocity distributions, with kinetic temperature and diffusion coefficient increasing as $\sim v_0^\alpha$ with $1<\alpha<2$. Remarkably, the late-time diffusivity and mobility decrease with mass. Moreover, we show that the equilibrium Einstein relation is asymptotically recovered with inertia. In summary, the inertial mass restores an equilibrium-like behavior.

cond-mat.stat-mech↗

arXiv2020

Topological flocking models in spatially heterogeneous environments

Flocking models with metric and topological interactions are supposed to exhibit distinct features, as for instance the presence and absence of moving polar bands. On the other hand, quenched disorder (spatial heterogeneities) has been shown to dramatically affect large-scale properties of active systems with metric interactions, while the impact of quenched disorder on active systems with metric-free interactions has remained, until now, unexplored. Here, we show that topological flocking models recover several features of metric ones in homogeneous media, when placed in a heterogeneous environment. In particular, we find that order is long-ranged even in the presence of spatial heterogeneities, and that the heterogeneous environment induces an effective density-order coupling facilitating emergence of traveling bands, which are observed in wide regions of parameter space. We argue that such a coupling results from a fluctuation-induced rewiring of the topological interaction network, strongly enhanced by the presence of spatial heterogeneities.

physics.bio-ph↗

arXiv2020

A novel approach to chemotaxis: active particles guided by internal clocks

Motivated by the observation of non-exponential run-time distributions of bacterial swimmers, we propose a minimal phenomenological model for taxis of active particles whose motion is controlled by an internal clock. The ticking of the clock depends on an external concentration field, e.g. a chemical substance. We demonstrate that these particles can detect concentration gradients and respond to them by moving up- or down-gradient depending on the clock design, albeit measurements of these fields are purely local in space and instantaneous in time. Altogether, our results open a new route in the study of directional navigation, by showing that the use of a clock to control motility actions represents a generic and versatile toolbox to engineer behavioral responses to external cues, such as light, chemical, or temperature gradients.

physics.bio-ph↗

arXiv2019

Bacteria display optimal transport near surfaces -- bacteria as intermittent active chiral particles: trapped by hydrodynamics, escaping by adhesion

The near-surface swimming patterns of bacteria are strongly determined by the hydrodynamic interactions between bacteria and the surface, which trap bacteria in smooth circular trajectories that lead to inefficient surface exploration. Here, we show by combining experiments and a data-driven mathematical model that surface exploration of enterohemorrhagic Escherichia coli (EHEC) -- a pathogenic strain of E. coli causing serious illnesses such as bloody diarrhea -- results from a complex interplay between motility and transient surface adhesion events. These events allow EHEC to break the smooth circular trajectories and regulate their transport properties by the use stop-adhesion events that lead to a characteristic intermittent motion on surfaces. We find that the experimentally measured frequency of stop-adhesion events in EHEC is located at the value predicted by the developed mathematical model that maximizes bacterial surface diffusivity. We indicate that these results and the developed model apply to other bacterial strains on different surfaces, which suggests that swimming bacteria use transient adhesion to regulate surface motion.

physics.bio-ph↗

arXiv2019

Reaction processes among self-propelled particles

We study a system of self-propelled disks that perform run-and-tumble motion, where particles can adopt more than one internal state. One of those internal states can be transmitted to another particle if the particle carrying this state maintains physical contact with another particle for a finite period of time. We refer to this process as a reaction process and to the different internal states as particle species making an analogy to chemical reactions. The studied system may fall into an absorbing phase, where due to the disappearance of one of the particle species no further reaction can occur or remain in an active phase where particles constantly react. Combining individual-based simulations and mean-field arguments, we study the dependence of the equilibrium densities of particle species with motility parameters, specifically the active speed $v_0$ and tumbling frequency $λ$. We find that the equilibrium densities of particle species exhibit two very distinct, non-trivial scaling regimes with $v_0$ and $λ$ depending on whether the system is in the so-called ballistic or diffusive regime. Our mean-field estimates lead to an effective renormalization of reaction rates that allow building the phase-diagram $v_0$--$λ$ that separates the absorbing and active phase. We find an excellent agreement between numerical simulations and estimates. This study is a necessary step to an understanding of phase transitions into an absorbing state in active systems and sheds light on the spreading of information/signaling among moving elements.

physics.bio-ph↗

arXiv2019

Large-scales patterns in a minimal cognitive flocking model: incidental leaders, nematic patterns, and aggregates

We study a minimal cognitive flocking model, which assumes that the moving entities navigate using exclusively the available instantaneous visual information. The model consists of active particles, with no memory, that interact by a short-ranged, position-based, attractive force that acts inside a vision cone (VC) and lack velocity-velocity alignment. We show that this active system can exhibit -- due to the VC that breaks Newton's third law -- various complex, large-scale, self-organized patterns. Depending on parameter values, we observe the emergence of aggregates or milling-like patterns, the formation of moving -- locally polar -- files with particles at the front of these structures acting as effective leaders, and the self-organization of particles into macroscopic nematic structures leading to long-ranged nematic order. Combining simulations and non-linear field equations, we show that position-based active models, as the one analyzed here, represent a new class of active systems fundamentally different from other active systems, including velocity-alignment-based flocking systems. The reported results are of prime importance in the study, interpretation, and modeling of collective motion patterns in living and non-living active systems.

physics.bio-ph↗

arXiv2019

Hydrodynamic Equations for Flocking Models without Velocity Alignment

The spontaneous emergence of collective motion patterns is usually associated with the presence of a velocity alignment mechanism that mediates the interactions among the moving individuals. Despite of this widespread view, it has been shown recently that several flocking behaviors can emerge in the absence of velocity alignment and as a result of short-range, position-based, attractive forces that act inside a vision cone. Here, we derive the corresponding hydrodynamic equations of a microscopic position-based flocking model, reviewing and extending previously reported results. In particular, we show that three distinct macroscopic collective behaviors can be observed: i) the coarsening of aggregates with no orientational order, ii) the emergence of static, elongated nematic bands, and iii) the formation of moving, locally polar structures, which we call worms. The derived hydrodynamic equations indicate that active particles interacting via position-based interactions belong to a distinct class of active systems fundamentally different from other active systems, including velocity-alignment-based flocking systems.

physics.bio-ph↗