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.