Molecular machines might temporarily escape entropy

from the Who-ordered-this? dept.
A group of Australian scientists has published experiments demonstrating that microscopic systems (such as a nanomachine) followed for short time periods (as long as a second) could sometimes violate the Second Law of Thermodynamics, that is, they extract useable energy from the temperature of their surroundings. In so doing, these systems become spontaneously more ordered and entropy decreases, in violation of the second law. It has long been known that the second law is subject to statistical fluctuations in very small systems (a few molecules), but it is surprising that such fluctuations occur in systems microns in length followed for a second or more, systems containing many billions of atoms. It would appear that these results have implications for micron-sized molecular machine systems: how microbes and other cells function, and how nanomachines should be designed to take into account that they could run backwards for short periods. For a concise summary, see the AIP Bulletin of Physics News, Pushing the Second Law to the Limit From Pushing the Second Law to the Limit:

Australian researchers have experimentally shown that microscopic systems (a nano-machine) may spontaneously become more orderly for short periods of time–a development that would be tantamount to violating the second law of thermodynamics, if it happened in a larger system. Don't worry, nature still rigorously enforces the venerable second law in macroscopic systems, but engineers will want to keep limits to the second law in mind when designing nanoscale machines. The new experiment also potentially has important ramifications for an understanding of the mechanics of life on the scale of microbes and cells.


The researchers used optical tweezers to grab hold of a micron-sized bead and drag it through water. By measuring the motion of the bead and calculating the minuscule forces on it, the researchers were able to show that the bead was sometimes kicked by the water molecules in such a way that energy was transferred from the water to the bead. In effect, heat energy was extracted from the reservoir and used to do work (helping to move the bead) in apparent violation of the second law.

As it turns out, when the bead was briefly moved over short distances, it was almost as likely to extract energy from the water as it was to add energy to the water. But when the bead was moved for more than about 2 seconds at a time, the second law took over again and no useful energy could be extracted from the motion of the water molecules, eliminating the possibility of micron-sized perpetual motion machines that run for more than a few seconds.

The research was published in Physical Review Letters, print issue of 29 July 2002: Experimental Demonstration of Violations of the Second Law of Thermodynamics for Small Systems and Short Time Scales

We experimentally demonstrate the fluctuation theorem, which predicts appreciable and measurable violations of the second law of thermodynamics for small systems over short time scales, by following the trajectory of a colloidal particle captured in an optical trap that is translated relative to surrounding water molecules. From each particle trajectory, we calculate the entropy production/consumption over the duration of the trajectory and determine the fraction of second law-defying trajectories. Our results show entropy consumption can occur over colloidal length and time scales.

Nanodot users noted the following web coverage of this unexpected result:

Gina Miller writes "BBC news reports Beads of Doubt by Dr David Whitehouse, July 18, 2002. The report says that Australian National University (ANU) scientists find that building molecular machines may not work in the way we thought. This theory is derived from laser and microscopic bead experiments that do not seem to follow the Second Law of Thermodynamics. Considering that this law was thought unavoidable it implies that we do not fully understand the dynamics of molecular interactions. The group states that this work is instrumental to the development of nanotechnology which could one day provide society with drug delivery systems to destroy disease."

Anonymous Coward writes "A New Scientist article Second law of thermodynamics 'broken' reads 'One of the most fundamental rules of physics, the second law of thermodynamics, has for the first time been shown not to hold for microscopic systems. The demonstration, by chemical physicists in Australia, could place a fundamental limit on miniaturisation, because it suggests that the micro-scale devices envisaged by nanotechnologists will not behave like simple scaled-down versions of their larger counterparts – they could sometimes run backwards.' I appeal to the better minds out there; prove it isn't so …"

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