Directed transport from thermal noise in a broken-symmetry potential
An asymmetric periodic potential (ratchet) can rectify Brownian motion into directed transport — without violating thermodynamics — when the potential is flashed on/off.
When ON: particles equilibrate near potential minima. When OFF: they diffuse freely. Re-activating the potential "captures" more particles on the right side of each tooth, causing net drift.
This models molecular motors (kinesin, myosin) which use ATP hydrolysis to drive a ratchet cycle.