Brownian ratchet

A mechanism that converts random thermal motion into directed movement using an asymmetric potential and a source of nonequilibrium energy. It does not extract work from equilibrium fluctuations alone.

Flashing ratchet: A Brownian ratchet whose asymmetric potential switches between states, allowing particles to diffuse and become preferentially trapped. Its alternating potential is a concrete way to create directed motion from thermal fluctuations.
Brownian motion: The irregular motion of particles caused by collisions with surrounding molecules. It supplies the random fluctuations that the ratchet rectifies.
Kinesin: A motor protein that moves along microtubules while hydrolyzing ATP. Kinesin's stepping can be modeled as directed motion powered by chemical energy and thermal fluctuations.
Feynman–Smoluchowski ratchet: A thought experiment involving a pawl-and-wheel device intended to extract work from thermal fluctuations. Its equilibrium failure demonstrates why a ratchet needs nonequilibrium energy input.
Rocking ratchet: A Brownian ratchet driven by a time-varying force, often one with zero average over a cycle. It uses an oscillating drive rather than switching the potential itself.
Thermal noise: Random fluctuations in a system caused by its interaction with a thermal environment. The mechanism acts on these fluctuations, but cannot extract net work from equilibrium noise alone.
Dynein: A motor protein that transports cellular cargo along microtubules, usually toward their minus ends. Its ATP-driven movement illustrates how molecular systems can rectify stochastic motion.
Thermal ratchet paradox: The apparent possibility of extracting work from heat using an asymmetric ratchet, resolved by accounting for all components' thermal motion. It marks the error of treating the pawl as immune to thermal fluctuations.
Fokker–Planck equation: A differential equation describing how probability density evolves under drift and diffusion. It models how the ratchet reshapes a particle distribution over time.
Nonequilibrium thermodynamics: The study of systems that exchange energy or matter while remaining outside thermodynamic equilibrium. A sustained energy flow is what permits net ratchet transport.

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