Draft:Pressure in active matter
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Comment: This page is currently an essay which is has a short paragraph on a range of pressure terms used in papers on "active matter". Very little is explained, it is not an encyclopedic article. It fails as being WP:SYNTH, collecting a number of concepts together to make something. I do not see how this can be repaired, it should be rewritten completely. Ldm1954 (talk) 20:59, 26 August 2026 (UTC)
- Comment: In accordance with Wikipedia's Conflict of interest guideline, I disclose that I have a conflict of interest regarding the subject of this article. Chandranshu phy (talk) 11:58, 28 June 2026 (UTC)
Pressure in active matter
[edit]The pressure in active matter is a mechanical stress built up in systems of self-propelled particles. Unlike passive fluids, pressure in active matter contains additional contributions from persistent self-propulsion, and its properties depend on particle interactions, confinement geometry, and inertial effects.
Background
[edit]In equilibrium statistical mechanics, pressure is a state function determined by bulk properties. In active matter systems, however, particles continuously convert energy into directed motion, driving the system out of equilibrium. This self-propulsion introduces new contributions to the mechanical stress that have no counterpart in passive fluids. For active Brownian particles (ABPs), the total pressure can be separated into contributions from thermal fluctuations, interparticle interactions, and active self-propulsion.[1]
Swim pressure
[edit]Swim pressure, or active pressure, results from the constant propulsion of active particles.[2] It is a distinctive feature of active systems and has no equivalent in thermal equilibrium fluids. The swim pressure is commonly obtained using virial approaches applied to the active propulsion forces acting on each particle.
Equation of state
[edit]Unlike equilibrium fluids, the pressure of generic active systems can depend on the nature of particle–wall interactions and confinement geometry; therefore, a universal equation of state does not generally exist for active matter.[3] However, for specific systems such as spherical ABPs without aligning interactions, a well-defined bulk pressure and equation of state can be obtained.[4]
Global and local pressure
[edit]The global pressure of an active system represents the spatially averaged mechanical stress and is commonly obtained using virial approaches or momentum transport arguments. In contrast, local pressure describes the spatial variation of stress inside the system and provides information about inhomogeneous regions such as interfaces, boundaries, and density gradients.
Studies of local stress in active Brownian systems have shown that, for spherical ABPs, the local bulk stress can correspond to the mechanical pressure measured at confining walls, supporting the existence of an equation of state.[4] However, the active contribution to the global pressure, particularly the swim pressure, does not always directly correspond to the local stress tensor. This difference becomes important in confined, inhomogeneous, or anisotropic active systems where particle orientation and boundary-induced polarization influence stress transmission.[3]
Inertial active matter
[edit]In active systems with translational and rotational inertia, the virial stress has contributions from mass, moment of inertia, interparticle forces and active propulsion forces. The translational inertial contribution is related to particle momentum, while rotational inertia results in a further stress contribution associated with angular motion. These terms together describe the mechanical stress carried by inertial active particles and generalize the stress description of active Brownian systems.[5]
Studies of active Brownian particles with translational and rotational inertia have shown that the swim stress obtained from the global virial expression does not necessarily represent the local stress tensor, demonstrating that global pressure and local stress can have different physical interpretations in inertial active matter systems.[5]
Overdamped limit
[edit]In the overdamped limit, where translational and rotational inertia are neglected, the inertial contributions disappear and the stress reduces to the commonly used active Brownian particle pressure formulations.[1] This limit corresponds to the regime most commonly studied in the literature on active matter, where viscous forces dominate over inertial effects.
References
[edit]- 1 2 Winkler, Roland G.; Wysocki, Adam; Gompper, Gerhard (2015). "Virial pressure in systems of spherical active Brownian particles". Soft Matter. 11: 6680–6691. doi:10.1039/C5SM01412C.
- ↑ Takatori, Sho C.; Brady, John F. (2015). "Swim stress, motion, and deformation of active matter: effect of hydrodynamic interactions". Physical Review E. 91 (3) 032117. doi:10.1103/PhysRevE.91.032117.
- 1 2 Solon, Alexandre P.; Cates, Michael E.; Tailleur, Julien (2015). "Pressure is not a state function for generic active fluids". Nature Physics. 11: 673–678. doi:10.1038/nphys3377.
- 1 2 Das, Shibananda; Gompper, Gerhard; Winkler, Roland G. (2019). "Local stress and pressure in an inhomogeneous system of spherical active Brownian particles". Scientific Reports. 9: 6608. doi:10.1038/s41598-019-43077-x.
- 1 2 Tiwari, Chandranshu; Singh, Sunil P.; Winkler, Roland G. (2026). "Virial stress in systems of active Brownian particles in the presence of translational and rotational inertia". The Journal of Chemical Physics. 164: 224901.
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