Laboratoire de Physique Théorique

de la Matière Condensée

Fernando Peruani (LPTM, Cergy Paris Université)

Cluster formation, order, and aggregation in Active Matter

Active matter is strongly based on idealized models and processes. The underlying assumption is that, as in equilibrium, “universality classes” depend only on the symmetry and range of the interactions, as well as on the presence or absence of conserved quantities. For example, the emergence of polar order is explained assuming the existence of a short-range velocity alignment mechanism, which is believed to lead generically to active polar fluids that fall into the Toner-Tu class. Similarly, phase separation in active systems is usually rationalized in the context of short-range repulsive active Brownian particles that fall into the mobility-induced phase separation (MIPS) class. However, it can be argued that in general the emergence of order and phase separation are closely interconnected in several active systems. For instance, in self-propelled rods the emergence of (local) polar order precludes MIPS and leads to a phase separation process that exhibits statistical features that are different from those reported in MIPS. Similarly, active particles with non-reciprocal attractive interactions phase separate via a distinct process, inconsistent with MIPS, into high-density structures displaying either polar, neutral, or nematic order depending on noise value and the non-reciprocity parameter. We will also see that it is possible to conceive a model that exhibits short-range polar velocity alignment and short-range attraction and repulsion, where particle aggregation and emergence of order are intrinsically interconnected, that displays a novel type of phase transition to polar order fundamentally different to the one observed in the Vicsek model and inconsistent with Toner-Tu ordered phase.