Giuseppe Fava — Università dell'Insubria #
Strong Casimir-like Forces in Flocking Active Matter #
 Confining in space the equilibrium fluctuations of statistical systems with long-range correlations is known to result into effective forces on the boundaries.  Here we demonstrate the occurrence of Casimir-like forces in the non-equilibrium context provided by flocking active matter. Free flocks, as described by the Toner-Tu theory, are characterized by a strongly fluctuating ordered phase displaying truly long-ranged order even in two spatial dimensions. Here we consider a system of aligning self-propelled particles in two spatial dimensions, confined in the direction transversal to that of flocking.  We first discuss how bulk correlations are affected by such boundaries, then show that in the ordered flocking phase this confined active vectorial fluid is characterized by extensive boundary layers, as opposed to the finite ones usually observed in confined scalar active matter.  Moreover we show that non-equilibrium fluctuations induce unusually strong Casimir-like forces, characterized by a remarkably slow algebraic decay. We explain our findings – which display a certain degree of universality – within a hydrodynamic description of the density and velocity fields.