Active bacterial baths in droplets (2501.14088v1)

Abstract: Suspensions of self-propelled objects represent a novel paradigm in colloidal science. In such active baths traditional concepts, such as Brownian motion, fluctuation-dissipation relations, and work extraction from heat reservoirs, must be extended beyond the conventional framework of thermal baths. Unlike thermal baths, which are characterized by a single parameter, the temperature, the fundamental descriptors of an active bath remain elusive, especially in confined environments. In this study, buoyant, passive tracers are employed as generalized probes to investigate an active bath comprising motile bacteria confined within a droplet. We demonstrate that momentum transfer from the bath to the tracer can be effectively described as colored noise, characterized by temporal memory and an enhanced effective diffusivity significantly larger compared to thermal Brownian motion values. Using a stochastic analytical framework, we extract the temporal memory and diffusivity parameters that define such an active bath. Notably, the diffusivity scales linearly with bacterial concentration, modulated by a factor representing the role of confinement, expressed as the ratio of the confining radius to the probe radius. This finding, while still awaiting a complete theoretical explanation, offers new insights into the transport properties of confined active baths and paves the way for a deeper understanding of active emulsions driven by confined active matter.

• The paper establishes that confinement in droplets alters bacterial activity, leading to distinct velocity fluctuations and temporal memory in particle motion.
• It utilizes 2D and 3D tracking of passive tracers to quantify effective diffusivity, which increases linearly with bacterial concentration and confinement degree.
• The experimental results are supported by a stochastic model that captures non-thermal behavior, bridging empirical data with theoretical predictions for active matter.

Overview of "Active bacterial baths in droplets"

The paper "Active bacterial baths in droplets" explores the dynamics of suspensions of motile bacteria confined within micrometer-scale droplets, advancing the paper of active matter systems. The authors establish a connection between bacterial activity and the observed velocity fluctuations inside restricted environments. With a focus on droplet encapsulation, this research employs buoyant passive tracers as probes to characterize the stochastic properties of bacterial baths.

Investigation of Bacterial Baths in Confined Spaces

The paper builds on previous research efforts that have highlighted the unique behaviors that arise in bacterial suspensions, challenging the conventional understanding of thermal baths. The authors specifically address the limitations imposed by droplet boundaries on the transport properties of active matter systems. Unlike classical thermal baths characterized by a temperature parameter, these active bacterial baths involve complex parameters such as bacterial concentration and available space. This research utilizes both 2D and 3D tracking techniques to monitor the kinematics of particles within active baths.

Methodology and Experimental Setup

The researchers employ an experimental system composed of bacterial suspensions within droplets of hexadecane oil, studying the dynamics of encapsulated tracer particles ranging from solid beads to oil spheres with varying sizes. This setup allows the exploration of different bacterial concentration regimes and confinement ratios. The motility of Escherichia coli is harnessed to evaluate how bacterial interaction with passive tracers leads to enhanced diffusion and non-trivial memory effects confined by geometry.

Key Findings and Parameters

1. Velocity Fluctuations and Memory: The authors demonstrate that the interaction between bacteria and passive particles is characterized by colored noise with significant temporal memory. This deviates from the memoryless behavior typical of thermal baths.
2. Effective Diffusivity: The research shows an increase in effective diffusivity with bacterial concentration, scaling linearly with density modulated by the confinement degree, highlighting the influence of geometric constraints on the dynamics.
3. Stochastic Framework: A stochastic analytical model captures the particle dynamics and extracts parameters such as temporal memory and diffusivity. These parameters reveal a novel understanding of confined active baths in motile bacterial suspensions.
4. Experimental Validation: By comparing experimental data against a comprehensive set of simulations within a wide parameter space, the paper confirms that the active bath's diffusivity and dynamics can be quantitatively captured, linking theory and empirical observation.
5. Implications of Confinement: The paper finds that the spatial boundaries imposed by the droplet walls significantly influence the transport properties of the active baths, a result that opens new avenues for understanding similar active systems in biological contexts.

Implications and Future Directions

This research provides valuable insights into the mechanics of active systems, emphasizing the critical role of boundaries and spatial constraints. The findings have potential applications in biotechnologies such as droplet microfluidics, which leverage bacterial suspensions for devices like bioreactors, and possibly in atmospheric science related to bacterial behavior in cloud droplets. Future exploration could focus on complex boundary conditions in non-dilute suspensions and potential nonlinear scaling behaviors beyond the regimes tested in this work. The paper fosters deeper theoretical developments in the physics of active matter, proposing an interdisciplinary path for advancements in colloidal science and fluid dynamics associated with living systems.