We discover a new contribution to the pressure (or stress) exerted by a suspension of self-propelled bodies. Through their self-motion, all active matter systems generate a unique swim pressure that is entirely athermal in origin. The origin of the swim pressure is based upon the notion that an active body would swim away in space unless confined by boundaries-this confinement pressure is precisely the swim pressure. Here we give the micromechanical basis for the swim stress and use this new perspective to study self-assembly and phase separation in active soft matter. The swim pressure gives rise to a nonequilibrium equation of state for active matter with pressure-volume phase diagrams that resemble a van der Waals loop from equilibrium gas-liquid coexistence. Theoretical predictions are corroborated by Brownian dynamics simulations. Our new swim stress perspective can help analyze and exploit a wide class of active soft matter, from swimming bacteria to catalytic nanobots to molecular motors that activate the cellular cytoskeleton. DOI: 10.1103/PhysRevLett.113.028103 PACS numbers: 47.63.mf, 05.65.+b, 47.63.Gd, 64.75.Xc From flocks of animals and insects to colonies of living bacteria, so-called active matter exhibits intriguing phenomena owing to its constituents' ability to convert chemical fuel into mechanical motion. Active matter systems generate their own internal stress, which drives them far from equilibrium and thus frees them from conventional thermodynamic constraints, and by so doing they can control and direct their own behavior and that of their surrounding environment. This gives rise to fascinating behavior such as spontaneous self-assembly and pattern formation [1-3] but also makes the theoretical understanding of their complex dynamical behaviors a challenging problem in the statistical physics of soft matter.In this Letter we have identified a new principle that all active matter systems display-namely, through their selfmotion they generate an intrinsic swim stress that impacts their dynamic and collective behavior. In contrast to thermodynamic quantities such as the chemical potential and free energy, the mechanical pressure (or stress) is valid out of equilibrium because it comes directly from the micromechanical equations of motion. To motivate this new perspective, we focus on the simplest model of active matter-a suspension of self-propelled spheres of radii a immersed in a continuous Newtonian solvent with viscosity η. The active particles translate with a constant, intrinsic swim velocity U 0 and tumble with a reorientation time τ R . We do not include the effects of hydrodynamic interactions among the particles.The origin of the swim pressure is based upon a simple notion-a self-propelled body would swim away in space unless confined by boundaries. The pressure exerted by the surrounding walls to contain the particle is precisely the swim pressure. This is similar to the kinetic theory of gases, where molecular collisions with the container walls exert a pressure. Although it is...

We present a general theory for determining the force (and torque) exerted on a boundary (or body) in active matter. The theory extends the description of passive Brownian colloids to self-propelled active particles and applies for all ratios of the thermal energy k B T to the swimmer's activity k s T s = ζ U 2 0 τ R /6, where ζ is the Stokes drag coefficient, U 0 is the swim speed and τ R is the reorientation time of the active particles. The theory, which is valid on all length and time scales, has a natural microscopic length scale over which concentration and orientation distributions are confined near boundaries, but the microscopic length does not appear in the force. The swim pressure emerges naturally and dominates the behaviour when the boundary size is large compared to the swimmer's run length = U 0 τ R . The theory is used to predict the motion of bodies of all sizes immersed in active matter.

Net (as opposed to random) motion of active matter results from an average swim (or propulsive) force. It is shown that the average swim force acts like a body force - an internal body force. As a result, the particle-pressure exerted on a container wall is the sum of the swim pressure [Takatori et al., Phys. Rev. Lett., 2014, 113, 028103] and the 'weight' of the active particles. A continuum description is possible when variations occur on scales larger than the run length of the active particles and gives a Boltzmann-like distribution from a balance of the swim force and the swim pressure. Active particles may also display 'action at a distance' and accumulate adjacent to (or be depleted from) a boundary without any external forces. In the momentum balance for the suspension - the mixture of active particles plus fluid - only external body forces appear.

Abstract:In this study, we report the antibacterial activities of six polyphenols (i.e., luteolin, quercetin, scutellarin, apigenin, chlorogenic acid, and resveratrol) against 29 clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA), and in vitro antibacterial activities of two-drug combinations. All of the MRSA strains evaluated were clinical isolates from patients with MRSA bacteremia. The antibacterial activities were determined by agar dilution method, and the two-drug antibacterial activities were determined by the checkerboard agar dilution method. It was found that luteolin, quercetin and resveratrol show obvious antibacterial activities against MRSA, and the results of two-drug antibacterial activity show either synergy or additivity, without evidences of antagonistic effects.