Molecular Machine - Historical Insight and Studies

Historical Insight and Studies

There are two thought experiments that form the historical basis for molecular machines: Maxwell's demon and Feynman's Ratchet (or Brownian ratchet). Maxwell's Demon is well described elsewhere, and a slightly different interpretation of Richard Feynman's ratchet is given here.

Imagine a very small system (seen below) of two paddles or gears connected by a rigid axle and that it is possible to keep these two paddles at two different temperatures. One of the gears (at T2) has a pawl that is rectifying the system motion, and therefore, the axle can only move in a clockwise rotation, and in doing so, it could lift a weight (m) upward upon ratcheting. Now imagine if the paddle in box T1 was in a much hotter environment than the gear in box T2; it would be expected that the kinetic energy of the gas molecules (red circles) hitting the paddle in T1 would be much higher than the gas molecules hitting the gear at T2. Therefore, with lower kinetic energy of the gases in T2, there would be very little resistance from the molecules on colliding with the gear in the statistically opposite direction. Further, the ratcheting would allow for directionality, and slowly over time, the axle would rotate and ratchet, lifting the weight (m).

As described, this system may seem like a perpetual motion machine; however, the key ingredient is the heat gradient within the system. This ratchet does not threaten the second law of thermodynamics, because this temperature gradient must be maintained by some external means. Brownian motion of the gas particles provides the power to the machine, and the temperature gradient allows the machine to drive the system cyclically away from equilibrium. In Feynman's ratchet, random Brownian motion is not fought against, but instead, harnessed and rectified. Unfortunately, temperature gradients cannot be maintained over molecular scale distances because of molecular vibration redistributing the energy to other parts of the molecule. Furthermore, despite Feynman's machine doing useful work in lifting the mass, using Brownian motion to power a molecular level machine does not provide any insight on how that power (or potential energy of the lifted weight, m) can be used to perform nanoscale tasks.