Brownian Ratchet in a Thermal Bath Driven by Coulomb Friction

Abstract

The rectification of unbiased fluctuations, also known as the ratchet effect, is normally obtained under statistical nonequilibrium conditions. Here we propose a new ratchet mechanism where a thermal bath solicits the random rotation of an asymmetric wheel, which is also subject to Coulomb friction due to solid-on-solid contacts. Numerical simulations and analytical calculations demonstrate a net drift induced by friction. If the thermal bath is replaced by a granular gas, the well-known granular ratchet effect also intervenes, becoming dominant at high collision rates. For our chosen wheel shape the granular effect acts in the opposite direction with respect to the friction-induced torque, resulting in the inversion of the ratchet direction as the collision rate increases. We have realized a new granular ratchet experiment where both these ratchet effects are observed, as well as the predicted inversion at their crossover. Our discovery paves the way to the realization of micro and submicrometer Brownian motors in an equilibrium fluid, based purely upon nanofriction.

Voltage Fluctuation to Current Converter with Coulomb-Coupled Quantum Dots

“…Extracting work from random fluctuations by energy conversion to a unidirectional particle flow is a key enabling technology and has consequently triggered substantial experimental and theoretical work [1][2][3][4]. The exploitation of temperature and fluctuation gradients for energy harvesting has led to new concepts such as Brownian and Büttiker-Landauer motors [5][6][7][8], phonon rectifiers [9,10], and piezoelectric nanogenerators [11][12][13]. Challenging factors in miniaturizing heat engines are an efficient energy conversion and the maintenance of welldefined hot and cold spots [14].…”

Coulomb Friction Driving Brownian Motors

“…In the case of elastic collisions, the rates W ǫ (v|v ′ ) satisfy detailed balance with respect to G T (v), which however does not imply that detailed balance holds for the model, because of the presence of F nl (v). In previous works it has been considered the more general case where the collisions between the tracer and the gas particles can also be inelastic: however this is an additional mechanism of dissipation, not strictly necessary to get a motor effect, introduced in order to describe granular experiments [3]. This mechanism is not discussed here.…”

“…Cases which have been demonstrated to break detailed balance with F nl = 0 are: 1) in the presence of collisional noise, W ǫ = 0 (even for elastic collisions) [3,13]; 2) in the presence of a spatial potential, U(x) = 0 [14]. We consider three possible examples of the first case, Section III, and one example of the second case, Section IV.…”

“…A finite drift could be achieved in the case of inelastic collisions, which has been considered elsewhere [3,26].…”

“…One of the main interesting behaviors, peculiar to nonequilibrium dynamics, is the possibility of rectifying unbiased fluctuations, when a spatial asymmetry is present in the system. Among the many ways to break the time-reversal symmetry in statistical models, the action of Coulomb friction has been recently put in evidence [1][2][3][4]. This is a form of energy dissipation that is observed in the relative motion of sliding surfaces of macroscopic objects, and its microscopic theory is still at the core of an intense debate [5].…”