Controlling vertical transport driven by active carpets at viscosity interfaces

Microorganisms accumulate at or near fluid interfaces, forming floating biofilms that may impact their environment. Here, we discuss the case of a quasi-2D floating biofilm, known as "active carpet", confined between a free surface and a "viscocline"—an interface between fluids of different viscosities. The active carpet, located near the viscocline, is formed by microorganisms swimming in the horizontal plane, each exerting hydrodynamic distortions to the surrounding 3D fluid system and collectively generating anisotropic hydrodynamic fluctuations that energize biogenic transport. We focus on quantifying how the system's vertical geometry and the viscosity ratio affect the degree of confinement. Our results show confinement shifts the dominant transport component across the water column, being the vertical component stronger near the viscocline and the horizontal components near the free surface. This work sheds light on the non-equilibrium properties of microbial life near fluid interfaces and how active carpets might shape their fluid environment.