

Toward Advanced Nanosystems: Mechanical engineering meets thermal fluctuations - ph0rque
http://metamodern.com/2009/02/08/toward-advanced-nanosystems-materials-3/

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kurtosis
This is one of my favorite topics! - as they say "small is different" - This
was known for quantum systems for about ~100 years but the laws of
thermodynamics also defy macroscopic intuition when applied to nanoscale
objects.

This article gives a great example.

For another example, the second law of thermodynamics which states the the
entropy of an isolated system must increase is only true on _average_. It is
possible for systems of very small size to display temporary fluctuations
where the entropy of a system and it's heat bath both _decrease_.

This is like your car inhaling CO2 from the exhaust, running backwards, and
combining the CO2 to give oxygen and gasoline which is pumped into the fuel
tank.

<http://en.wikipedia.org/wiki/Fluctuation_theorem>

This process has real implications for how the cell works. Biological
organisms are full of microscopic motors that do all kinds of mechanical work
- like transporting molecules against concentration gradients, viruses packing
RNA into their protein capsids etc.. These motors are all very strange in that
they only work forward on average.

Another phenomena which is very different at the nanoscale is melting and
phase transitions. Everyone is familiar with the fact that sodium metal melts
at a sharply defined temperature. But it turns out that this "melting"
temperature is only sharp for macroscopic sized samples of sodium metal. For
sodium clusters (sodium crystals with only a few hundred atoms) the melting
temperature is blurred over a large range of temperatures.

See for instance..

[http://www.lassp.cornell.edu/sethna/CrystalShapes/CopperClus...](http://www.lassp.cornell.edu/sethna/CrystalShapes/CopperCluster.html)

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davi
Very nice link on the melting copper clusters.

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kurtosis
Yeah I like Sethna a lot but he is a theorist - the experiment was done for Na
clusters by a guy named Haberland. It was published in Science a few years
ago.

[edit]

Sorry I mean nature: M. Schmidt, R. Kusche, B. v. Issendorff und H. Haberland,
Irregular variations in the melting point of size-selected atomic clusters,
Nature, 393, 238-240 (1998).

[http://cluster.physik.uni-
freiburg.de/lit/preprints/Natmelt....](http://cluster.physik.uni-
freiburg.de/lit/preprints/Natmelt.pdf)

There experiment is pretty hard to understand though.

------
davi
"the most important mechanical tolerances in atomically precise fabrication
are (with a caveat or two) proportional to the lattice constant (unit cell
size) of the material in question"

