Applications
Real-life versions of Maxwellian demons occur, but all such "real demons" have their entropy-lowering effects duly balanced by increase of entropy elsewhere.
Single-atom traps used by particle physicists allow an experimenter to control the state of individual quanta in a way similar to Maxwell's demon.
Molecular-sized mechanisms are no longer found only in biology; they are also the subject of the emerging field of nanotechnology.
A large-scale, commercially available pneumatic device, called a Ranque-Hilsch vortex tube separates hot and cold air. It sorts molecules by exploiting the conservation of angular momentum: hotter molecules are spun to the outside of the tube while cooler molecules spin in a tighter whirl within the tube. Gas from the two different temperature whirls may be vented on opposite ends of the tube. Although this creates a temperature difference, the energy to do so is supplied by the pressure driving the gas through the tube.
If hypothetical mirror matter exists, Zurab Silagadze proposes that demons can be envisaged, "which can act like perpetuum mobiles of the second kind: extract heat energy from only one reservoir, use it to do work and be isolated from the rest of ordinary world. Yet the Second Law is not violated because the demons pay their entropy cost in the hidden (mirror) sector of the world by emitting mirror photons."
In 1962 lectures, to illustrate thermodynamics, physicist Richard Feynman analyzed a putative Maxwell's demon device, a tiny paddlewheel attached to a ratchet, showing why it cannot extract energy from molecular motion of a fluid at equilibrium. This Brownian ratchet is a popular teaching tool.
Read more about this topic: Maxwell's Demon