Two talks on active matter

Alexandre Solon

Sorbonne Université, Paris, France

Title: Hydrodynamic theory of polar active matter

Understanding how self-propelling entities manage to move collectively is a problem that has puzzled physicists for the last 30 years, since the original contributions of Vicsek and Toner and Tu. In this seminar, I will present a generic hydrodynamic theory for the ordered phase of collective motion, constrained only by the symmetries of the system. It differs from the Toner-Tu theory in selecting the appropriate hydrodynamic modes and allows, in two dimensions, to prove the existence of long-ranged order and derive exact scaling exponents.

H. Chaté and A. Solon, Phys. Rev. Lett. 132, 268302 (2024)

Hugues Chaté

CEA – Saclay, France, & Beijing CSRC, China

Title: Condensation and Synchronization in Aligning Chiral Active Matter

Spontaneous segregation of active matter into dense and sparse domains is ubiquitous.

In systems with local interactions, it is usually well described as phase separation, and occurs not only in scalar active matter ("motility-induced phase separation'') but also in vectorial, aligning systems. This is in particular the generic situation for the simple but important case of self-propelled particles locally aligning their velocities against some noise. In such dry aligning active matter, the order-disorder transition is not direct, and the homogeneous orientationally-ordered liquid is generically separated from disorder by a coexistence phase in which dense ordered regions evolve in a remaining vapor.

We show that in collections of aligning circle swimmers with this phase separation scenario is replaced by a condensation phenomenon. The condensates, which take the form of vortices or rotating polar packets, can absorb a finite fraction of the particles in the system, and keep a finite or slowly growing size as their mass increases. Our results are obtained both at particle and continuous levels. We consider both ferromagnetic and nematic alignment, and both identical and disordered chiralities. Condensation implies synchronization, even though our systems are in 2D and bear strictly local interactions. We propose a phenomenological theory based on observed mechanisms that accounts qualitatively for our results.