A new derivation of the relationship between diffusion coefficient and entropy in classical Brownian motion by the ensemble method

TL;DR

This paper derives an analytic relationship between diffusion coefficient and entropy in classical Brownian motion using the ensemble method, revealing a nonlinear correlation in thermal equilibrium.

Contribution

It introduces a new derivation of the D-entropy relationship in Brownian motion, employing the canonical ensemble and a novel entropy definition.

Findings

Derived an explicit formula: D = (ħ / eM) * exp[S / (k_B d)]

Established a nonlinear D-entropy correlation in thermal equilibrium

Connected the relation to known scaling laws like Rosenfeld's expansion

Abstract

The diffusion coefficient--a measure of dissipation, and the entropy--a measure of fluctuation are found to be intimately correlated in many physical systems. Unlike the fluctuation dissipation theorem in linear response theory, the correlation is often strongly non-linear. To understand this complex dependence, we consider the classical Brownian diffusion in this work. Under certain rational assumption, i.e. in the bi-component fluid mixture, the mass of the Brownian particle $M$ is far greater than that of the bath molecule $m$, we can adopt the weakly couple limit. Only considering the first-order approximation of the mass ratio $m/M$, we obtain a linear motion equation in the reference frame of the observer as a Brownian particle. Based on this equivalent equation, we get the Hamiltonian at equilibrium. Finally, using canonical ensemble method, we define a new entropy that is similar to the Kolmogorov-Sinai entropy. Further, we present an analytic expression of the relationship between the diffusion coefficient $D$ and the entropy $S$ in the thermal equilibrium, that is to say, $D =\frac{\hbar}{eM} \exp{[S/(k_Bd)]}$, where $d$ is the dimension of the space, $k_B$ the Boltzmann constant, $\hbar $ the reduced Planck constant and $e$ the Euler number. This kind of scaling relation has been well-known and well-tested since the similar one for single component is firstly derived by Rosenfeld with the expansion of volume ratio.