Antidiffusive instabilities in active matter: granular experiments, theory and outlook

This article presents a perspective on the widespread occurrence of antidiffusive instabilities in systems in which free energy is supplied in a sustained manner directly to the constituent particles. Defining this distinct category allows a discussion of living matter as a special kind of non-equilibrium material. After an introduction surveying diverse manifestations of antidiffusion, including some in which momentum transfer through an ambient fluid plays a central role, we describe experiments and theory on a striking and extreme form that arises in a vibrated monolayer of macroscopic particles. This article has two aims: to bring active matter as a novel hydrodynamic arena to the attention of researchers in large-scale fluid and plasma flows, and to highlight open issues arising in our work with wider implications for a statistical thermodynamics of active matter. This article is part of the theme issue 'Anti-diffusive transport and layering: from sub-cellular to astrophysical scales'.