Strong Casimir-like Forces in Flocking Active Matter

Flocking – the collective motion exhibited by certain active matter (AM) systems – is a ubiquitous phenomenon, observed in a wide array of different living systems and on an even wider range of scales.

Our knowledge of the bulk behavior, well described by the Toner & Tu (TT) theory, of free collective motion is now fairly complete, at least in the so-called “dry approximation”, while much less is known regarding the collective behavior of confined flocking AM. This is a problem of great relevance for many experimental realizations with active colloids, where confinement by hard boundaries is practically unavoidable.

Confinement effects have been so far mostly investigated in scalar active matter (where the density field is the unique hydrodynamic mode), either in ”dry” systems or active suspensions.

In this work I describe genuinely Casimir-like long-ranged forces that arise when a flocking AM system is confined between flat reflecting boundaries – either elastic or inelastic – in the direction transversal to collective motion. Direct numerical simulations and analytical results show that non-equilibrium fluctuations induce an unusually strong Casimir-like force, characterized by a rather slow algebraic decay. I also discuss the nature of the hydrodynamic layer at the active fluid-boundary interface and present an approximate analytical argument that connects our findings with the bulk exponents of TT theory.