We investigate the nonequilibrium energetics near a critical point of a non-linear driven oscillator immersed in an active bacterial bath. At the critical point, we reveal a scaling exponent of the average power $\langle\dot{W}\rangle\sim (D_{\rm a}/τ)^{1/4}$ where $D_{\rm a}$ is the effective diffusivity and $τ$ the correlation time of the bacterial bath described by a Gaussian colored noise. Other features that we investigate are the average stationary power and the variance of the work both below and above the saddle-node bifurcation. Above the bifurcation, the average power attains an optimal, minimum value for finite $τ$ that is below its zero-temperature limit. Furthermore, we reveal a finite-time uncertainty relation for active matter which leads to values of the Fano factor of the work that can be below $2k_{\rm B}T_{\rm eff}$, with $T_{\rm eff}$ the effective temperature of the oscillator in the bacterial bath. We analyze different Markovian approximations to describe the nonequilibrium stationary state of the system. Finally, we illustrate our results in the experimental context by considering the example of driven colloidal particles in periodic optical potentials within an E. Coli bacterial bath.
The motion of a pair of micro-wedges ("carriers") in a turbulent bacterial bath is explored using computer simulations with explicit modeling of the bacteria and experiments. The orientation of the two micro-wedges is parallel and fixed by an applied magnetic field but the translational coordinates can move freely as induced by the bacterial bath. As a result, two carriers of same orientation move such that their mutual distance decreases. Eventually the two carriers stack on each other with no intervening bacteria exhibiting a stable dynamical mode where the two micro-wedges follow each other…
The equilibrium structure and dynamics of a single polymer chain in a thermal solvent is by now well-understood in terms of scaling laws. Here we consider a polymer in a bacterial bath, i.e. in a solvent consisting of active particles which bring in nonequilibrium fluctuations. Using computer simulations of a self-avoiding polymer chain in two dimensions which is exposed to a dilute bath of active particles, we show that the Flory-scaling exponent is unaffected by the bath activity provided the chain is very long. Conversely, for shorter chains, there is a nontrivial coupling between the bacte…
Micromotors pushed by biological entities, like motile bacteria, constitute a fascinating way to convert chemical energy into mechanical work at the micrometer scale. Here we show, by using numerical simulations, that a properly designed asymmetric object can be spontaneously set into the desired motion when immersed in a chaotic bacterial bath. Our findings open the way to conceive new hybrid microdevices exploiting the mechanical power production of bacterial organisms. Moreover, the system provides an example of how, in contrast with equilibrium thermal baths, the irreversible chaotic motio…
Recent experiments on particle diffusion in bacterial baths indicate the formation of correlated structures in the form of bacterial swirls. Here we predict that such a structural ordering would give rise to the new effect of propagating shear waves in a bacterial bath at length scales of the order of a swirl, which corresponds to time scales of the order of the lifetime of a swirl. Our prediction can be tested against future experiments in bacterial baths.
Transport and Energetics of Bacterial Rectification
Courant Institute of Mathematical Sciences, New York University（纽约大学柯朗数学科学研究所）；Center for Soft Matter Research, Department of Physics, New York University（纽约大学物理系软物质研究中心）；Department of Chemical Engineering and Materials Science, University of Minnesota（明尼苏达大学化学工程与材料科学系）；Simons Center for Computational Physical Chemistry, Department of Chemistry, New York University（纽约大学化学系西蒙斯计算物理化学中心）
We demonstrate that collective turbulent-like motion in a bacterial bath can power and steer directed transport of mesoscopic carriers through the suspension. In our experiments and simulations, a microwedge-like "bulldozer" draws energy from a bacterial bath of varied density. We obtain that a maximal transport speed is achieved in the turbulent state of the bacterial suspension. This apparent rectification of random motion of bacteria is caused by polar ordered bacteria inside the cusp region of the carrier, which is shielded from the outside turbulent fluctuations.
We present a joint experimental and computational study of the effect of bacterial motion on micron-scale colloids contained in a two-dimensional suspension of Bacillus subtilis. With respect to previous work using E. coli, here we introduce a novel experimental set-up that allows us to realise a two-dimensional bacterial suspension insensitive to either evaporation or fluid flow. By analysing the mean square displacements of both bacteria and colloids, we confirm the existence of a crossover from super-diffusive behaviour at short time scales to normal diffusion at longer times. We also study…
Self-propelling bacteria are a dream of nano-technology. These unicellular organisms are not just capable of living and reproducing, but they can swim very efficiently, sense the environment and look for food, all packaged in a body measuring a few microns. Before such perfect machines could be artificially assembled, researchers are beginning to explore new ways to harness bacteria as propelling units for micro-devices. Proposed strategies require the careful task of aligning and binding bacterial cells on synthetic surfaces in order to have them work cooperatively. Here we show that asymmetr…
We probe non-equilibrium properties of an active bacterial bath through measurements of correlations of passive tracer particles and the response function of a driven, optically trapped tracer. These measurements demonstrate violation of the fluctuation-dissipation theorem and enable us to extract the power spectrum of the active stress fluctuations. In some cases, we observe $1/\sqrtω$ scaling in the noise spectrum which we show can be derived from a theoretical model incorporating coupled stress, orientation, and concentration fluctuations of the bacteria.
Active matter such as swarming bacteria and motile colloids exhibits exotic properties different from conventional equilibrium materials. Among these properties, the enhanced diffusion of tracer particles is generally deemed as a hallmark of active matter. Here, rather than spherical tracers, we investigate the diffusion of isolated ellipsoids in quasi-two-dimensional bacterial bath. Our study reveals a nonlinear enhancement of both translational and rotational diffusions. More importantly, we uncover an anomalous coupling between translation and rotation that is strictly prohibited in the cla…
We consider a colloidal particle immersed in an active bath and derive a Smoluchowski equation that governs the dynamics of colloidal particle. We address this as active Smoluchowski equation. Our analysis based on this active Smoluchowski equation shows a short time superdiffusive behavior that strongly depends on the activity. Our model also predicts a non-monotonic dependence of mean energy dissipation against time, a signature of activity-induced dynamics. By introducing a frequency-dependent effective temperature, we show that the mean rate of entropy production is time dependent unlike a…
From synthetic active devices such as self-propelling Janus colloids to micro-organisms like bacteria, micro-algae, living cells in tissues, active fluctuations are ubiquitous. Thermodynamics of small systems involving thermal as well as active fluctuations are of immense importance. They can be employed to extract thermodynamic work. Here we propose a simple model system that can produce thermodynamic work exploiting active fluctuations. We consider a Brownian particle, trapped by an externally controlled harmonic confinement which time-periodically contracts and expands by modulating its spr…
