Full Article. Making the Paper. Nature Nanotech N&V's. How it Works.
‘Exercising demons’. Click on image to download high resolution version (jpeg).
[This illustration by Regina Fernandes – Illugraphics.]
Chemists at the University of Edinburgh have created a molecular machine that operates via a mechanism inspired by a 140 year-old thought experiment.
The ‘molecular information ratchet’ uses light energy to fuel information transfer, a fundamentally new type of motor-mechanism for artificial nanomachines.
Nature uses molecular-sized motors and machines in virtually every important biological process and their extraordinary success is inspiring scientists to
try to create synthetic devices that mimic the function of these amazing natural systems. However, it is far from obvious to see how to design such machines because mechanical
behaviour at the molecular level, where everything is constantly moving (under kinetic energy supplied by the heat of the surroundings) and being buffeted by other atoms and
molecules (Brownian motion), is very different to that which we observe in our everyday world. One cannot just scale down the design of a motor car to the nanoscale, for example,
it simply could not operate because friction, heat dissipation and many other factors are so different. The problem of controlling motion on the molecular level is not a recent one,
however, it has occupied the minds of scientists since as far back as the middle of the 19th Century.
James Clerk Maxwell
James Clerk Maxwell (1831–1879),[1]
born and raised in Edinburgh, was arguably one of the three (along with Isaac
Newton and Albert Einstein) most important and influential scientists of all
time ["From a long view of the
history of mankind - seen from, say, ten thousand years from now - there can be
little doubt that the most significant event of the 19th century will be judged
as Maxwell's discovery of the laws of electrodynamics." — Richard
Feynman; "The work of James Clerk
Maxwell changed the world forever" — Albert Einstein; "Maxwell's equations have had a greater
impact on human history than any ten presidents." — Carl Sagan].
During a lifetime of contributions to science that began at age 14, two pieces
of Maxwell’s work are generally the most celebrated: His electromagnetic theory
of light; and his contributions to the kinetic theory of gases, which for the
first time explained real-world properties in terms of the statistical
behaviour of atoms and molecules. These two theories have become cornerstones
of modern physical science but ‘Maxwell’s Demon’,[2] an offshoot of his
work on the kinetic theory of gases, has had its own extraordinary impact.[3]
Time and again it has captured the imagination and interest of scientists in
different fields, profoundly influencing the development of statistical and
quantum physics, information theory, computer science and cybernetics. Now, 140
years after its conception, it is the inspiration for a new motor mechanism for
nanomachine systems reported by Serreli et
al in Nature.[4]
‘The molecular information ratchet team in the James Clerk Maxwell
Building, University of Edinburgh, 8th December 2006’ Left-to-right: Dr Euan Kay, Dr Chin-Fa Lee, Dott.
Viviana Serreli and Prof Dave Leigh. [Click photo to enlarge]
Maxwell’s Demon
In 1867,[2] Maxwell proposed
the thought experiment which has come to be known as ‘Maxwell’s Demon’ (Figure
1). In the original version of this imaginary system, a tiny intelligent being –
a ‘demon’ – is able to open and close a gate connecting two boxes filled with gas
so as to allow only fast (‘hot’) gas molecules to flow into one box and only slow
(‘cold’) gas molecules into the other – creating a temperature difference
between the two compartments (Figure 1a). If the demon can perform such a task
without expending any energy (using a frictionless gate which he opens and
closes very slowly), then such a result would be in violation of the Second Law
of Thermodynamics (‘heat cannot spontaneously pass from a colder to a hotter
body’ or, more generally, ‘the entropy of an isolated system not at equilibrium
will tend to increase over time, approaching a maximum value’) which is one of
the most fundamental principles of physics. Maxwell appreciated that other
types of ‘sorting demon’ could be imagined that would also violate the Second
Law, for example a system that allowed particles to pass between compartments
in one direction but not the other without an energy input (Figure 1b).
Figure 1. The Maxwell Demon thought experiments. a Maxwell’s ‘temperature demon’[2a,b]
in which a gas at uniform temperature is sorted into ‘hot’ (red) and ‘cold’ (blue)
molecules. The demon opens the gate between the compartments when it detects a cold particle approaching the
gate from the left or a hot particle coming from the right, thus separating the particles according to their
thermal energy and creating a temperature differential between the compartments. b A Maxwellian ‘pressure
demon’[2c] in which a concentration gradient is established by the gate being opened only when a particle
approaches it from the left. In both versions of the thought experiment the idea is that the demon’s actions
involve no work being done, but as the end-result is a reduction in the entropy of the gas this is in conflict with
the Second Law of Thermodynamics. Maxwell appreciated that the successful operation of the demon in the thought experiment
somehow relied on its intelligence as an animate being. Subsequent analysis by several generations of scientists revealed a
fundamental link between entropy and information, significantly influencing the development of statistical and quantum physics
and chemistry, information theory and computer science.
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