pjk pjk at design.eng.yale.edu
Wed Jul 17 19:37:08 EDT 2002

Subject:   NanoThermodynamics

PHYSICS NEWS UPDATE                         
The American Institute of Physics Bulletin of Physics News
Number 598  July 17, 2002   by Phillip F. Schewe, Ben Stein, and
James Riordon

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
   There are numerous ways to summarize the second law of
thermodynamics. One of the simplest is to note that it's impossible
simply to extract the heat energy from some reservoir and use it to do
work. Otherwise, machines could run on the energy in a glass of water,
for example, by extracting heat and leaving behind a lump of ice. If
this were possible, refrigerators and freezers could create electrical
power rather that consuming it. The second law typically concerns
collections of many trillions of particles - such as the molecules in
an iron rod, or a cup of tea, or a helium balloon - and it works well
because it is essentially a statistical statement about the collective
behavior of countless particles we could never hope to track
individually. In systems of only a few particles, the statistics are
grainier, and circumstances may arise that would be highly improbable
in large systems. Therefore, the second law of thermodynamics is not
generally applied to small collections of particles.
   The experiment at the Australian National University in Canberra
and Griffith University in Brisbane (Edith Sevick,
sevick at rsc.anu.edu.au, 011+61-2-6125-0508) looks at aspects of
thermodynamics in the hazy middle ground between very small and very
large systems. 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.  Nevertheless,
many physicists will be surprised to learn that the second law is not
entirely valid for systems as large as the bead- and-water experiment,
and for periods on the order of seconds. After all, even a cubic
micron of water contains about thirty billion molecules. While it's
still not possible to do useful work by turning water into ice, the
experiment suggests that nanoscale machines may have to deal with
phenomena that are more bizarre than most engineers realize. Such tiny
devices may even end up running backwards for brief periods due to the
counterintuitive energy flow. The research may also be important to
biologists because many of the cells and microbes they study comprise
systems comparable in size to the bead-and-water experiment.(G.M.Wang
et al., Physical Review Letters, 29 July 2002)

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