Tiny details may affect nanomachine operation

There is a lot of talk on how nanotechnology will soon be allowing us to build a wonderful array of nanomachines, from micron-sized motors to nanoscale train sets made from Teflon and cow's brains.

But theoretical work led by Professor Denis Evans from the Australian national university Research School of Chemistry, shows that the very scale at which nanomachines operate will place limits on the work they're capable of performing.

Professor Evans demonstrated that things work quite differently at very small scales of time and space. For example, as machines are made ever smaller, the probability that they will run in the reverse direction becomes ever greater until at a certain point, a machine is just as likely to run forwards as backwards.

This arises from the work of Professor Evans to resolve the paradox between the Second Law of Thermodynamics, which says that the disorder of the Universe can only increase in time, and the equations of classical and quantum mechanics, which are time reversible.

The solution was arrived at several years ago in the form of the Fluctuation Theorem. The Theorem states that the probability of the Second Law of Thermodynamics being violated decreases exponentially as the size of the system and/or the duration of the violation increases.

Therefore at human scales, the Second Law always applies and machines only ever run in one direction. However, when working at the nanoscale over extremely short periods of time, things can work in either direction. As a result, nano-engines can either consume fuel to produce work, heat and waste or they can consume work, heat and waste to produce fuel. As the size of the engine gets smaller, the probabilities that the engine will run forwards or backwards become more nearly equal.

The full consequences of this are only just starting to be understood, but one practical outcome is that the Theorem sets a fundamental limit to the work available from nanomachines.

Item provided courtesy of the Australian National University.