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Journal of Statistical Physics

, Volume 156, Issue 6, pp 1093–1110 | Cite as

Fluctuation–Dissipation Relation for Systems with Spatially Varying Friction

  • Oded FaragoEmail author
  • Niels Grønbech-Jensen
Article

Abstract

When a particle diffuses in a medium with spatially dependent friction coefficient \(\alpha (r)\) at constant temperature \(T\), it drifts toward the low friction end of the system even in the absence of any real physical force \(f\). This phenomenon, which has been previously studied in the context of non-inertial Brownian dynamics, is termed “spurious drift”, although the drift is real and stems from an inertial effect taking place at the short temporal scales. Here, we study the diffusion of particles in inhomogeneous media within the framework of the inertial Langevin equation. We demonstrate that the quantity which characterizes the dynamics with non-uniform \(\alpha (r)\) is not the displacement of the particle \(\Delta r=r-r^0\) (where \(r^0\) is the initial position), but rather \(\Delta A(r)=A(r)-A(r^0)\), where \(A(r)\) is the primitive function of \(\alpha (r)\). We derive expressions relating the mean and variance of \(\Delta A\) to \(f\), \(T\), and the duration of the dynamics \(\Delta t\). For a constant friction coefficient \(\alpha (r)=\alpha \), these expressions reduce to the well known forms of the force-drift and fluctuation–dissipation relations. We introduce a very accurate method for Langevin dynamics simulations in systems with spatially varying \(\alpha (r)\), and use the method to validate the newly derived expressions.

Keywords

Fluctuation–dissipation theorem Langevin dynamics Itô–Stratonovich Dilemma Fick’s law Diffusion Computer simulations 

Notes

Acknowledgments

This work was supported by the US Department of Energy Project DE-NE0000536 000.

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.Department of Biomedical Engineering and Ilse Katz Institute for Nanoscale Science and TechnologyBen Gurion University of the NegevBe’er ShevaIsrael
  2. 2.Department of Mechanical and Aerospace EngineeringUniversity of CaliforniaDavidUSA
  3. 3.Department of MathematicsUniversity of CaliforniaDavidUSA

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