Thermodynamic Limits to Nanomachines Evans, DenisView Profile. Australasian Science24.10 (Nov/Dec 2003): 23-24. Turn on hit highlighting for speaking browsers Abstract (summary) Translate Abstract Evans reveals why nanomachines relates energy will run backwards part of the time, undoing some of the work they do during their normal operations. Fluctuatuation Theorem places absolute limits on what one can do with nanotechnology in a very precise mathematical way, and has even shown that the laws of thermodynamics can be violated in nanosystems. Full text Translate Full text Headnote Denis Evans reveals why nanomachines will run backwards part of the time, undoing some of the work they do during their normal operations. View Image - Denis Evans has determined that nanomachines will sometimes run backwards. Thermodynamics relates energy to heat and temperature, and until 10 years ago I thought that it could not say much about extremely small systems. As an undergraduate I had been taught that one of the important things in thermodynamics is to look at large systems in the so-called thermodynamic limit, where properties like temperature and pressure no longer depend on the number of particles or the mass of the total system that you are studying. Thermodynamics is a very general subject that is independent of the existence of atoms and molecules. If there is a subject that shouldn't apply to nanotechnology it would be thermodynamics. However, the Fluctuation Theorem adapts the second law of thermodynamics to finite systems that are observed for finite times. It places absolute limits on what you can do with nanotechnology in a very precise mathematical way, and has even shown that the laws of thermodynamics can be violated in nanosystems. PERPETUAL MOTION The laws of thermodynamics say that you cannot build a perpetual motion machine that violates energy conservation - you cannot construct a perpetual motion machine of the first kind. A much more subtle statement that thermodynamics makes is that you cannot construct a machine that converts ambient heat into useful work. If you could construct such a thing it would be called a perpetual motion machine of the second kind. Imagine a motorboat cruising around in the ocean. Sea water is not at a temperature of absolute zero, so there is some energy content in the water. This hypothetical boat extracts energy from the warm sea water to turn the propeller and propel the boat. But if the energy extracted from the water comes from heat, the temperature of the water must decrease. In order to conserve energy - the first law of thermodynamics - the boat must chill the water that it takes in and then throw ice cubes out the back. This is an example of a perpetual motion machine of the second kind. View Image - These things can't be constructed, and the US Patent Office routinely uses these two formulations of the laws of thermodynamics to throw out patent applications. Now, however, the Fluctuation Theorem has shown that the second law can be violated in small systems for brief periods of time. To explain how this occurs we first need to understand a crucial variable in the Fluctuation Theorem - the rate of entropy production. The laws of thermodynamics dictate that entropy - the amount of energy in a system that is not available to do work - increases as a system gets bigger. But the founders of statistical mechanics and thermodynamics knew that this was not valid for small systems. They knew that entropy would not always increase if you looked at just a small collection of particles. At a macroscopic level, behaviour is generally irreversible. Car engines do not run backwards, producing petrol and oxygen and absorbing heat. For more than 100 years mathematicians have not been able to resolve why macroscopic behaviour is irreversible even though that behaviour is governed by reversible equations of motion. Now this has been resolved by the Fluctuation Theorem, which calculates that you should be able to violate the second law of thermodynamics in small systems observed for short periods of time. LIGHT FORCE Einstein was the first to point out that light can exert a force on matter. Edith Sevick and Genmiao Wang of the Australian National University's Research School of Chemistry have used this principle to pull small colloid particles around an aqueous medium. According to the laws of electrodynamics, if the refractive index of the colloid particle is greater than the surrounding water, then the particle will be attracted to a region where the intensity of light is greatest. We are dealing with very weak forces here in the realm of 3x10^sup -15^N, and these move the particle at a velocity of about 1 μm/s. In this way we can test the second law of thermodynamics for these little suspended particles. When we moved the laser beam back and forth for 10 seconds in each direction and then computed the rate of entropy production, we found that entropy was negative for periods as long as 2 seconds. In these instances, moving the laser beam to the left caused the particle to move to the right. Where does the particle get the energy to move in the wrong direction? Like the perpetual motion motorboat, it gets it by extracting the heat from the solvent. This would be in violation of the second law of thermodynamics if you assumed that it applied to such small systems. LIMITS TO NANOTECHNOLOGY The Fluctuation Theorem generalises the laws of thermodynamics so that they apply to systems that are very small and observed for very small periods of time. Just like thermodynamics itself, it is a fundamental physical limitation that you cannot beat. It is just as fundamental as saying that you cannot beat the speed of light. The media were interested in the Fluctuation Theorem because it places absolute limits on what you can do with nanotechnology in a very precise mathematical way. It doesn't say you cannot create nanomachines. It does say, though, that a motorboat shrunk down to the nanometre scale will make one rev backward for every two revs forward. And with each rev backwards the engine will be sucking in heat, consuming CO2, and generating petrol and oxygen for a short while. In fits and starts it will be a perpetual motion machine of the second kind. The smaller the boat, the more of its time will be spent running backwards. If it got down to the scale of a couple of atoms it would run backwards as often as it ran forwards, and could do not any useful net work at all. The Fluctuation Theorem doesn't say that nanomachines can't work. They clearly can, as nature has harnessed them ever since life existed on this planet. But small engines and organelles must obey this fundamental theorem of nature if they are to work. Sidebar The Laws of Thermodynamics Zeroth law: If two systems are each in thermal equilibrium with a third system then they are in thermal equilibrium with each other. First law: The total energy of a thermodynamic system remains constant although it may be transformed from one form to another (conservation of energy). Second law: Heat can never pass spontaneously from a body at a lower temperature to one at a higher temperature, or equivalently no process is possible whose only result is the removal of heat from a single heat reservoir and the performance of an equivalent amount of work. Third law: The entropy of a substance approaches zero as its temperature approaches absolute zero. Sidebar "... a motorboat shrunk down to the nanometre scale will make one rev backward for every two revs forward". AuthorAffiliation Denis Evans is Dean of the Australian National University's Research School of Chemistry. This is an edited version of a presentation he gave to the Australian Academy of Science. Copyright Control Publications Pty Ltd Nov/Dec 2003