Constraint dependence of average potential energy of a passive particle in an active bath

We quantify the mean potential energy of a passive colloidal particle harmonically confined in a bacterial solution using optical traps.We find that the average potential energy of the passive particle depends on the trap stiffness,in contrast to the equilibrium case where energy partition is independent of the external constraints.The constraint dependence of the mean potential energy originates from the fact that the persistent collisions between the passive particle and the active bacteria are influenced by the particle relaxation dynamics.Our experimental results are consistent with the Brownian dynamics simulations,and confirm the recent theoretical prediction.

Effective diffusion of a tracer in active bath:A path-integral approach

We investigate the effective diffusion of a tracer immersed in an active particle bath consisting of self-propelled particles.Utilising the Dean's method developed for the equilibrium bath and extending it to the nonequilibrium situation,we derive a generalized Langevin equation(GLE)for the tracer particle.The complex interactions between the tracer and bath particles are shown as a memory kernel term and two colored noise terms.To obtain the effective diffusivity of the tracer,we use path integral technique to calculate all necessary correlation functions.Calculations show the effective diffusion decreases with the persistent time of active force,and has rich behavior with the number density of bath particles,depending on different activities.All theoretical results regarding the dependence of such diffusivity on bath parameters have been confirmed by direct computer simulation.

Symmetry properties of fluctuations in an actively driven rotor

We investigate rotational dynamics of an actively driven rotor through experiments and numerical simulations. While probability density distributions of rotor angular velocity are strongly non-Gaussian, relative probabilities of observing rotation in opposite directions are shown to be linearly related to the angular velocity magnitude. We construct a stochastic model to describe transitions between different states from rotor angular velocity data and use the stochastic model to show that symmetry properties in probability density distributions are related to the detailed fluctuation relation(FR) of entropy productions.

Study of neutron spectra in a water bath from a Pb target irradiated by 250 MeV protons

Spallation neutrons were produced by the irradiation of Pb with 250 MeV protons. The Pb target was surrounded by water which was used to slow down the emitted neutrons. The moderated neutrons in the water bath were measured by using the resonance detectors of Au, Mn and In with a cadmium （Cd） cover. According to the measured activities of the foils, the neutron flux at different resonance energies were deduced and the epithermal neutron spectra were proposed. Corresponding results calculated with the Monte Carlo code MCNPX were compared with the experimental data to check the validity of the code. The comparison showed that the simulation could give a good prediction for the neutron spectra above 50 eV, while the finite thickness of the foils greatly effected the experimental data in low energy. It was also found that the resonance detectors themselves had great impact on the simulated energy spectra.