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Strong Casimir-like Forces in Flocking Active Matter.

Giuseppe Fava1,2, Andrea Gambassi3, Francesco Ginelli1,2

  • 1Dipartimento di Scienza e Alta Tecnologia and Center for Nonlinear and Complex Systems, <a href="https://ror.org/00s409261">Università degli Studi dell'Insubria</a>, Como, Italy.

Physical Review Letters
|October 18, 2024
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Summary
This summary is machine-generated.

Casimir-like forces emerge in flocking active matter due to boundary confinement. These forces, arising from nonequilibrium fluctuations, show unique boundary layer behavior and pressure effects in confined active vectorial fluids.

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Area of Science:

  • Statistical Mechanics
  • Active Matter Physics
  • Soft Condensed Matter

Background:

  • Equilibrium systems with long-range correlations exhibit effective boundary forces due to spatial confinement.
  • Active matter, such as flocking particles, represents a nonequilibrium system with unique emergent behaviors.

Purpose of the Study:

  • To investigate the presence and characteristics of Casimir-like forces in a nonequilibrium system of flocking active matter.
  • To analyze the impact of confinement on the boundary layers and pressure exerted by active vectorial fluids.

Main Methods:

  • Simulating a two-dimensional system of aligning self-propelled particles.
  • Confining the active matter using reflecting or partially reflecting walls.
  • Employing a hydrodynamic description of density and velocity fields for analysis.

Main Results:

  • Demonstrated Casimir-like forces in the nonequilibrium context of flocking active matter.
  • Observed extensive boundary layers in the ordered flocking phase, unlike finite layers in scalar active matter.
  • Identified a slow, algebraic decay of a finite-size, fluctuation-induced pressure contribution with increasing wall separation.

Conclusions:

  • Confinement in flocking active matter generates unique Casimir-like forces and boundary phenomena.
  • The observed effects, including extensive boundary layers and pressure contributions, display a degree of universality.
  • Hydrodynamic theory effectively describes these nonequilibrium phenomena in confined active vectorial fluids.