Confined floating active carpets generate coherent vortical flows that enhance transport

Slicks are thin viscous films found at the air-water interface in lakes, rivers, and oceans. These microlayers, enriched in surfactants, organic matter and microorganisms, exhibit steep physical and chemical gradients across tens to hundreds of micrometers. In such geometrically confined environments, the hydrodynamics and transport of nutrients, pollutants and microorganisms are strongly constrained, yet they sustain key biogenic processes. However, it remains largely unexplored how biogenic transport depends on the vertical extent of slicks relative to the size of microbial colonies. Here we address this question by combining analytical and numerical approaches to model a microbial colony as an active carpet-a two-dimensional distribution of microswimmers exerting dipolar forces. We show that an optimal ratio of the carpet size to the confinement height maximizes particle transport toward the colony edges via advective flows that organize into three-dimensional, vortex-ring-like structures whose characteristic length is set by the confinement height. Our results demonstrate that finite, coherent vortex-ring-like flows can emerge solely from the ratio between slick thickness and colony size. These findings highlight the role of geometric confinement in shaping collective activity and nonequilibrium transport, and provide insight into how microbial communities form, spread, and persist in surface slicks.