Starts 15 Mar 2022 11:00
Ends 15 Mar 2022 12:00
Central European Time
via Zoom
Lorenzo Caprini 
(Düsseldorf U)


Systems of active matter, such as bacterial colonies and cell monolayers, show intriguing patterns in their velocity field, displaying spatial velocity correlations. In this talk, I show that spherical Active Brownian Particles (ABP) reproduce these phenomena because of the interplay between persistent active forces and pure repulsive interactions, without invoking explicit alignment forces. Both in phase-separated and homogeneous configurations, ABPs display coherent domains where the particle velocities are spontaneously aligned and spatial velocity correlations are observed. These spontaneous collective effects have a non-thermal origin, being independent of active and solvent temperatures, while their dynamical nature is corroborated by a strong dependence on the active force persistence and inertial time. We build a phase diagram as a function of packing fraction and persistence time to compare the structural properties of the system (i.e. liquid, hexatic and solid phases) with the emergent velocity order (i.e. the correlation length of the spatial velocity correlations). For active solids, our findings are corroborated by a microscopic theory while, for active liquid, we developed a hydrodynamic theory, derived by the microscopic model under suitable approximations. In the liquid case, the spatial structure and the slowdown of the velocity field are shown through the analytical expressions for the dynamical correlation functions.