Identify the fundamental descriptors of confined active baths

Determine the fundamental quantitative descriptors that characterize active baths formed by motile bacterial suspensions in confined environments, specifying the minimal set of parameters required to describe the stochastic driving experienced by passive tracers in spherical droplets.

Background

In contrast to thermal baths, which are characterized by temperature, the paper underscores that the appropriate descriptors for active baths—particularly in confinement—are not firmly established. The authors experimentally infer two parameters (memory time and diffusivity) from tracer dynamics, but explicitly note that, in general, what constitutes the fundamental descriptors remains unresolved.

Clarifying the foundational descriptors is necessary to systematize modeling frameworks and to compare active baths across geometries and concentrations, especially when boundaries strongly influence non-equilibrium dynamics.