Anti-diffusive transport and layering: from sub-cellular to astrophysical scales

Abstract Anti-diffusion refers to a remarkable class of processes, in which particles, fluid elements, etc., tend to migrate up-gradient, contrary to expected diffusive relaxation. A particularly dramatic and fascinating anti-diffusive phenomenon is layering or staircase formation, in which turbulent stirring leads to the formation of mixed regions, interspersed by layers of steepened gradients. Anti-diffusive dynamics is of interest across a wide range of physical systems—e.g. in geophysical and astrophysical fluid dynamics, in magnetically confined plasmas and in active matter. To bring together researchers from a variety of different backgrounds, the Isaac Newton Institute hosted a six-month programme in 2024 entitled Anti-Diffusive Dynamics: from Sub-Cellular to Astrophysical Scales. This special issue is a product of that programme, containing papers aimed at a broad readership covering observations, experiments, and theoretical and computational developments in a range of topics in fluid dynamics, plasma physics and active matter. This article is part of the theme issue ‘Anti-diffusive transport and layering: from sub-cellular to astrophysical scales’.