Active bacterial baths in droplets.

Area of Science:

• Colloidal science
• Soft matter physics
• Active matter systems

Background:

• Active baths of self-propelled objects challenge traditional thermodynamics.
• Key descriptors for active baths, unlike thermal baths, are not well-defined.
• Confined active environments, like bacterial droplets, are biologically and technologically relevant.

Purpose of the Study:

• To characterize the properties of an active bath formed by motile bacteria within a confined droplet.
• To utilize buoyant passive tracers as probes to extract information about the active bath.
• To introduce and validate a new metric, 'bath diffusivity,' for out-of-equilibrium systems.

Main Methods:

• Employing buoyant passive tracers to probe bacterial suspensions in droplet confinement.
• Modeling the bacterial suspension as colored noise acting on the tracer.
• Extracting temporal memory and characteristic intensity of the noise.

Main Results:

• Identified temporal memory and intensity of the colored noise characterizing the active bath.
• Introduced 'bath diffusivity' as a predictor of momentum transfer properties.
• Demonstrated that bath diffusivity scales linearly with bacterial concentration, modulated by confinement.

Conclusions:

• Bath diffusivity offers a unifying concept for understanding momentum transfer in confined active baths.
• Findings provide insights into transport and mixing in active emulsions.
• This work lays groundwork for further theoretical and experimental exploration of active matter systems.