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We relate the breakdown of equations of states for the mechanical pressure of generic dry active systems to the lack of momentum conservation in such systems. We show how sources and sinks of momentum arise generically close to confining…

Statistical Mechanics · Physics 2017-11-21 Yaouen Fily , Yariv Kafri , Alexandre P. Solon , Julien Tailleur , Ari Turner

We derive a general closed expression for the local pressure exerted onto the corrugated walls of a channel confining a fluid medium. When the fluid medium is at equilibrium the local pressure is a functional of the shape of the walls. It…

Soft Condensed Matter · Physics 2018-02-07 Paolo Malgaretti , Markus Bier

We study, from first principles, the pressure exerted by an active fluid of spherical particles on general boundaries in two dimensions. We show that, despite the non-uniform pressure along curved walls, an equation of state is recovered…

Statistical Mechanics · Physics 2016-08-31 Nikolai Nikola , Alexandre P. Solon , Yariv Kafri , Mehran Kardar , Julien Tailleur , Raphaël Voituriez

We derive from first principles the mechanical pressure $P$, defined as the force per unit area on a bounding wall, in a system of spherical, overdamped, active Brownian particles at density $\rho$. Our exact result relates $P$, in closed…

Soft Condensed Matter · Physics 2015-07-10 A. P. Solon , J. Stenhammar , R. Wittkowski , M. Kardar , Y. Kafri , M. E. Cates , J. Tailleur

We discuss different definitions of pressure for a system of active spherical particles driven by a non-thermal coloured noise. We show that mechanical, kinetic and free-energy based approaches lead to the same result up to first order in…

Statistical Mechanics · Physics 2016-03-18 Umberto Marini Bettolo Marconi , Claudio Maggi , Simone Melchionna

We have considered a model of a small finite system with internal particles and surface degrees of freedom. All the main statistical distributions were explicitly obtained, on a pre thermodynamic limit basis. The concept of temperature or…

Statistical Mechanics · Physics 2025-08-11 D. M. Naplekov , V. V. Yanovsky

The well-known Wall theorem states a simple and precise relation among temperature, pressure and density of a fluid at contact with a confining hard wall in thermodynamic equilibrium. In this Letter we develop an extension of the Wall…

Statistical Mechanics · Physics 2019-10-23 Ignacio Urrutia , Ivan Paganini , Claudio Pastorino

We analyze the surface tension exerted at the interface between an active fluid and a solid boundary in terms of tangential forces. Focusing on active systems known to possess an equation of state for the pressure, we show that interfacial…

We study fluctuations of pressure in equilibrium for classical particle systems. In equilibrium statistical mechanics, pressure for a microscopic state is defined by the derivative of a thermodynamic function or, more mechanically, through…

Statistical Mechanics · Physics 2018-10-05 Ken Hiura , Shin-ichi Sasa

We experimentally determine the force exerted by a bath of active particles onto a passive probe as a function of its distance to a wall and compare it to the measured averaged density distribution of active particles around the probe.…

Soft Condensed Matter · Physics 2022-08-10 Shuvojit Paul , Ashreya Jayaram , N Narinder , Thomas Speck , Clemens Bechinger

The pressure exerted on a wall by a gas at equilibrium does not depend on the shape of the confining potential defining the wall. In contrast, it has been shown recently [A.P. Solon et al., Nat. Phys. 11, 673 (2015)] that a gas of…

Soft Condensed Matter · Physics 2016-03-11 Marc Joyeux , Eric Bertin

In the last decade, the study of pressure in active matter has attracted growing attention due to its fundamental relevance to nonequilibrium statistical physics. Active matter systems are composed of particles that consume energy to…

Soft Condensed Matter · Physics 2025-08-14 Guo Yu , Ruiyao Li , Fukang Li , Jiayu Zhang , Xiyue Li , Zequ Chen , Joscha Mecke , Yongxiang Gao

The stress of a fluid on a confining wall is given by the mechanical wall forces, independent of the nature of the fluid being passive or active. At thermal equilibrium, an equation of state exists and stress is likewise obtained from…

Soft Condensed Matter · Physics 2024-06-03 Shibananda Das , Gerhard Gompper , Roland G. Winkler

Cells and other soft particles are often forced to flow in confined geometries in both laboratory and natural environments, where the elastic deformation induces an additional drag and pressure drop across the particle. In contrast with…

Fluid Dynamics · Physics 2025-07-09 Charles Paul Moore , Hiba Belkadi , Brouna Safi , Gabriel Amselem , Charles N. Baroud

The pressure of suspensions of self-propelled objects is studied theoretically and by simulation of spherical active Brownian particles (ABP). We show that for certain geometries, the mechanical pressure as force/area of a confined systems…

Statistical Mechanics · Physics 2024-06-03 Roland G. Winkler , Adam Wysocki , Gerhard Gompper

The extent to which active matter might be described by effective equilibrium concepts like temperature and pressure is currently being discussed intensely. Here we study the simplest model, an ideal gas of non-interacting active Brownian…

Statistical Mechanics · Physics 2016-06-15 Thomas Speck , Robert L. Jack

In solid phase the pressure correlates to the elastic related volume change while the temperature to the thermal related volume change. These volume changes are not compatible with the exception of constant volume condition when the…

General Physics · Physics 2007-05-23 Jozsef Garai , Alexandre Laugier

The unique pressure exerted by active particles -- the "swim" pressure -- has proven to be a useful quantity in explaining many of the seemingly confounding behaviors of active particles. However, its use has also resulted in some puzzling…

Soft Condensed Matter · Physics 2020-01-22 Ahmad K. Omar , Zhen-Gang Wang , John F. Brady

By the mechanical pressure we mean that the pressure in the fundamental thermodynamic equation with the naive form of the electromagnetic work used, while the thermodynamic one we mean that in the equation with proper thermodynamic form of…

Classical Physics · Physics 2025-01-08 Q. H. Liu

We explore the pressure of active particles on curved surfaces and its relation to other interfacial properties. We use both direct simulations of the active systems as well as simulations of an equilibrium system with effective (pair)…

Soft Condensed Matter · Physics 2019-05-14 René Wittmann , Frank Smallenburg , Joseph M. Brader
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