Invited talks
Giulia Pisegna (MPI-DS) - Emergent polar order in nonpolar mixtures with nonreciprocal interactions
Abstract: Self-organization in living and active systems emerges from microscopic interactions, which are governed by symmetries and intrinsic properties of individual constituents. It is possible, however, that spontaneously formed composite units lead to the emergence of large-scale behavior that is completely different from what is expected for the single particles. An example of such occurrences is non-reciprocal active matter, where asymmetric interactions can induce polarity in nonpolar mixtures.
To study this phenomenon, I will present a generic class of active matter models with two scalar fields that represent the concentration of molecular species interacting non-reciprocally. We study the stability of the emergent ordered state, showing the existence of true long-range polar order in two dimensions and above, both at the linear level and by including all relevant nonlinearities in the Renormalization Group sense. We achieve this by uncovering a mapping to the Kardar-Parisi-Zhang universality class for the dynamics of fluctuations. This classification allows us to prove a conclusive violation of the Mermin-Wagner theorem and to predict the large-scale behavior of systems with non-reciprocal interactions at any dimension.
Moreover, natural systems are often three dimensional, leading to momentum conservation in the bulk. To address this scenario, I will extend this dry system to a wet case, incorporating hydrodynamic interactions in a momentum-conserving fluid. The dynamics of the polar order parameter reveal a fluid-mediated linear instability of the ordered state, which is ultimately stabilized by nonlinear effects in the regime of strong non-reciprocity. This result confirms that the non-equilibrium polar pattern is very robust also to hydrodynamic couplings.
Eric Clément (Sorbonne Université) - Scale-free bacteria turbulence
Abstract: Fluids loaded with swimming micro-organisms have become a rich domain of applications and a conceptual playground for the statistical physics of “active matter” [1]. Such active bacterial fluids display original emergent phases as well as unconventional macroscopic constitutive properties [2,3], hence leading to revisit many standard concepts in the physics and the hydrodynamics of colloidal suspensions. Here we show that above a critical concentration - scaling as the inverse of the vertical confinement- a suspension of active E.coli undergoes a hydrodynamic instability leading to a “turbulent-like” state characterized by a complex dynamics of vortices and jets [4]. The emergent structures and the temporal dynamics, scale over more than two decades, with the vertical confinement. Phenomenologically, the transition to active turbulence bears many similarities with a second order phase transitions limited by a spatial cut-off. We propose a finite size rescaling for both the correlations length divergence and the critical slowing-down dynamics that bring all our data onto a universal curve. We also observe in the critical turbulent domain, the stochastic emergence of very large transient vortices which diameters can reach the lateral circular confinement. We bring evidences for a spatio-temporal dynamics resulting in a complex interplay between coarsening and fragmentation that remains up to now, unraveled.
[1] Alert R. et al. Ann. Rev. Cond. Mat. Phys, 13, 143-170 (2022).
[2] Lopez M. et al. , Phys. Rev.Lett. 115, 028301 (2015).
[3] Martinez et al. , PNAS, 17:2326–233 (2020).
[4]Perez-Estay , B. et al., preprint (2025) arXiv:2509.15918.
Contributed talks
Callum Britton (Imperial College) - Looking back: field theory of transiently chiral active particles
Abstract: We derive a Doi-Peliti Field Theory for transiently chiral active particles in two dimensions, that is, active Brownian particles that undergo tumbles via a diffusing reorientation angle. Using this framework, we compute the mean-squared displacement for both uniformly distributed and fixed initial reorientations. We also calculate an array of orientation-based observables, to quantify the transiently chiral behaviour observed.
Rahil Valani (University of Oxford) - Intermittent migration of an active cell cluster in a confluent tissue
Abstract: During early mouse embryogenesis a group of cells, the distal visceral endoderm (DVE), are observed to move through intermittent bursts of motion. Motivated by this we model the dynamics of a self-propelled (active) cell cluster within a background of passive cells in a vertex model of confluent tissue. For small strengths of self-propulsion, the cluster comes to a halt, whereas for large strengths of self-propulsion, the cluster migrates steadily within the tissue. Interestingly, for intermediate strengths of self-propulsion, we find that the yield stress rheology of the surrounding passive cells results in a regime of intermittent motion of the migrating cluster, providing a potential mechanistic explanation of the experimental observations.
Tina Jia (Imperial College) - Conformational Trapping and Activity-Modulated Transport of Active Polymers in Compliant Environments
Abstract: We study the transport of a single active polymer moving through a 2D crowded environment composed of elastically tethered obstacles. Using Brownian dynamics simulations, we show that long-time transport cannot be characterized by mean-squared displacement alone. We identify a two-step mechanism governing transport: escape from local obstacle-induced cages and the unspiraling of compact polymer conformations. Flexible polymers readily form long-lived self-wrapped spiral states that strongly suppress transport, even when the center of mass partially decorrelates. Conversely, translational escape without conformational unwrapping is insufficient to restore sustained diffusion. To disentangle these effects, we introduce dynamical observables that separately quantify spatial persistence and conformational survival. Increasing activity primarily promotes cage escape, while increasing stiffness destabilizes spiral conformations by penalizing curvature. Based on these observables, we construct a phase diagram in the activity–stiffness plane that delineates localized and diffusive regimes.
Emir Sezik (Imperial College) - Non-Reciprocal yet Thermal: Emergent Equilibrium Dynamics in a Non-Reciprocal System
Abstract: Non-reciprocal interactions are the norm rather than the exception in non-equilibrium systems. They can be implemented in many ways, though the paradigmatic approach involves asymmetric couplings between two entities. These couplings generically induce spatio-temporal patterns and time-dependent steady-states that break time-translational invariance, representing a clear deviation from equilibrium physics. Though the physics at the ground state is far from equilibrium, the transition into these states in non-reciprocally coupled Ising spins have been found to be effectively equilibrium with the ordering dynamics falling under the XY universality class. In the present work, by performing the field-theoretic Renormalization Group (RG) procedure on the dynamics of two non-reciprocally coupled vector order parameters, we assess the effects of the non-reciprocity on the ordering dynamics. To lowest order in ε = 4-d, we find that the non-reciprocity is irrelevant and the ordering dynamics is subsumed under the Halperin and Hohenberg's Model A universality class. Finally, we analyse the dynamics of the resulting Goldstone modes due to the breaking of the symmetry and comment on the possibility of order in d=2. Our results highlight the stability of Halperin and Hohenberg's classification to the violations of detailed balance and the uniqueness of genuine non-equilibrium universality classes.
Eloise Lardet (Imperial College) - Kinetic Theory of Pattern Formation in a Generalized Multi-Species Vicsek Model
Abstract: A hallmark in natural systems, self-organization often stems from very simple interaction rules between individual agents. While single-species self-propelled particle (SPP) systems are well understood, the behavior of mixtures of self-propelled particles with general alignment interactions remains largely unexplored with a few scattered results hinting at the existence of a rich emergent phase behavior. Here, we first present a generalization of the two-species Vicsek model with reciprocal intra- and interspecies (anti-)alignment couplings, uncovering a rich phenomenology of emergent states. Notably, we show that rather than destroying polar order, anti-aligning interactions can promote phase separation and the emergence of global polar order. Secondly, we derive a kinetic theory for the system, finding good agreement between theoretical predictions and particle simulations. This includes a novel mechanism for microphase separation, as predicted by a Turing-Hopf instability. We finally show that these coexistence patterns can be generalized to multi-species systems with cyclic alignment interactions.
Gianmarco Spera (University of Oxford) - Low-Pass Filtering of Active Turbulent Flows to Liquid Substrates
Abstract: We investigate how an active nematic fluid affects a passive liquid substrate that is coupled to it through friction. As the active layer becomes turbulent, it drives disordered motion in the passive substrate, which shows a new type of turbulence generated by random forcing from the active material. The coupling between the layers smooths out small-scale fluctuations, effectively acting as a low-pass filter. We derive the relationship that links the dynamics of the two layers, predict how the substrate’s energy spectrum decays at large wave numbers, and examine how stresses and strain rates are transmitted across their interface. Our findings align with experiments on mixed active–passive microtubule systems and may help improve the interpretation of traction force microscopy of cell layers.
Jane Peltier (King's College) - Spontaneous chiral symmetry breaking in non-reciprocal chemical mixtures
Abstract: Pattern formation is ubiquitous in biological systems. Oftentimes, patterns that emerge from the nonequilibrium dynamics of the underlying physical system are critical for the organism to perform certain functions. One example is membraneless organelles which rely on liquid-liquid phase separation (LLPS) to exist in cells. Mathematically, LLPS is described by the Cahn-Hilliard equation. Recently, this equilibrium model was extended to couple liquids with interaction terms that drive it out of thermodynamic equilibrium, such as chemical reactions or non-reciprocal interactions. The latter drives the emergence of travelling waves, leading to the non-reciprocal Cahn-Hilliard model. We study, analytically and computationally, a minimal model involving two coupled fields governed by Cahn-Hilliard-like physics supplemented by both non-reciprocal couplings and transmutation. It was recently discovered that the superposition of non-reciprocity and switching drives interface instabilities. We confirm the existence of such an instability in our model, leading to travelling waves along the fields’ interfaces. Because of their non-linear two-dimensional shape, we coin them chainsaws. A numerical phase diagram allows us to locate them in the phase space of non-reciprocity and chemical reaction strengths, while a linear stability allows us to separate the regions of phase space where phase separation arises, and where oscillatory instability arises. By means of numerics and sharp interface limit analytics, we also show that in the oscillatory instability region, the only non-equilibrium pattern that emerges is chainsaws. We further show numerically the onset of chainsaws, that proves to be a two-step non-linear instability mechanism, which differs from previously observed interface instabilities in the context of non-reciprocal chemical mixtures.
