Brownian ratchet
In the philosophy of thermal and statistical physics, the Brownian ratchet or Feynman–Smoluchowski ratchet is an apparent perpetual motion machine of the second kind (converting thermal energy into mechanical work), first analysed in 1912 as a thought experiment by Polish physicist Marian Smoluchowski.
The simple machine, consisting of a tiny paddle wheel and a ratchet, appears to be an example of a Maxwell's demon, able to extract mechanical work from random fluctuations (heat) in a system at thermal equilibrium, in violation of the second law of thermodynamics.
The molecules constitute a heat bath in that they undergo random Brownian motion with a mean kinetic energy that is determined by the temperature.
[2] A simple but rigorous proof that no net motion occurs no matter what shape the teeth are was given by Magnasco.
The Feynman ratchet model led to the similar concept of Brownian motors, nanomachines which can extract useful work not from thermal noise but from chemical potentials and other microscopic nonequilibrium sources, in compliance with the laws of thermodynamics.
[3][4] Diodes are an electrical analog of the ratchet and pawl, and for the same reason cannot produce useful work by rectifying Johnson noise in a circuit at uniform temperature.
[7] In 1912, Polish physicist Marian Smoluchowski[1] gave the first correct qualitative explanation of why the device fails; thermal motion of the pawl allows the ratchet's teeth to slip backwards.
In 1996, Juan Parrondo and Pep Español used a variation of the above device in which no ratchet is present, only two paddles, to show that the axle connecting the paddles and ratchet conducts heat between reservoirs; they argued that although Feynman's conclusion was correct, his analysis was flawed because of his erroneous use of the quasistatic approximation, resulting in incorrect equations for efficiency.
[12] Researchers from the University of Twente, the University of Patras in Greece, and the Foundation for Fundamental Research on Matter have constructed a Feynman–Smoluchowski engine which, when not in thermal equilibrium, converts pseudo-Brownian motion into work by means of a granular gas,[13] which is a conglomeration of solid particles vibrated with such vigour that the system assumes a gas-like state.
The constructed engine consisted of four vanes which were allowed to rotate freely in a vibrofluidized granular gas.
However, this system is not in perfect thermal equilibrium: energy is constantly being supplied to maintain the fluid motion of the beads.
Unlike an ideal gas, though, in which tiny particles move constantly, stopping the shaking would simply cause the beads to drop.
For very strong shaking, the vanes of the paddle wheel interacted with the gas, forming a convection roll, sustaining their rotation.
