Number 730, May 5, 2005
by Phil Schewe and Ben Stein
Room Temperature Liquid Sodium
Room temperature liquid sodium can occur but only under pressures of
a million atmospheres. Melting is a mystery. It happens when the thermal
agitation among atoms in a solid overcomes the inter-atom bonds. Applying
pressure to a solid sample usually helps to negate the effect of thermal
agitation and so the melting temperature usually goes up with pressure.
In a few materials, such as water, above a certain pressure the melting
point begins to drop.
Now, the most dramatic case yet seen of such a “negative melting curve”
has been studied by scientists at the Carnegie Institution of Washington
looking at one of the simplest metals known, sodium. What happens is
this: With zero pressure applied, sodium melts at a temperature of 371
K. As pressure is added, the melting temperature goes up too, up to
1000 K at a pressure of 30 giga-pascals (30 GPa), or about 300,000 atm.
Then strange things happen. As the pressure is taken up further, the
melting point starts to drop, reaching a low of 300 K (below its ambient
melting point) at pressures of 118 GPa (see graph at www.aip.org/png).
All previous materials exhibiting negative melting curves have gone
negative very reluctantly, over pressure ranges of a few GPa or temperature
ranges of a few K. Sodium, by contrast, goes negative over a range of
700 K and 80 GPa.
According to Carnegie researcher Eugene Gregoryanz (e.gregoryanz@gl.ciw.edu),
at a pressure of a million atmospheres his sodium sample melts at room
temperature. The liquid is denser than the solid (water shares this
trait), and might have strange plastic or mechanical properties. It
might even be superconducting under some circumstances, he says. (Gregoryanz
et al., Physical Review Letters, upcoming article.
An Optical Conveyor Belt
An optical conveyor belt for moving sub-micron objects has been achieved
by collaborating physicists at the Institute of Scientific Instruments
in Brno, Czech Republic and at the University of St. Andrews in Scotland.
Their set-up used a special type of non-diffracting laser light that
forms a very narrow beam existing over long distance without changing
its width.
Two such counter-propagating laser beams establish up a lace-like standing
wave pattern which can suspend and hold tiny polystyrene spheres of
just the right size. The balls, which range in size from 400 nm to one
micron, have a density comparable to water. Previously, scientists have
used such non-diffracting "optical lace" beams to move particles with
the force of radiation pressure, but without the ability to stop them
using only a single beam.
The Czech and Scottish researchers, by contrast, set up a light lace
pattern with numerous knots, corresponding to intensity maxima (antinodes)
of the standing wave. Furthermore a particle can be confined near a
knot and all the knots can then be moved simultaneously over large distances
by changing the relative phases of the counter-propagating laser beams.
Moreover thanks to the self-healing property of the non-diffracting
beams, many particles can be confined simultaneously in the standing
wave structure (near the knots) without significantly spoiling the beam
properties. The positioning accuracy, related to the precision of the
phase shift and the optical trap depth (the size of the knots), is at
the micron level and will get better.
Pavel Zemanek (zemanek@isibrno.cz) says that possible applications for
his device include the delivery of biological or colloidal microparticles
or even ultracold atoms. (Cizmaret al., Applied Physics Letters, 25 April 2005; lab site at http://www.isibrno.cz/omitec/index.php?swt.html
) (A few years we wrote about a different kind of photon conveyor belt:
http://www.aip.org/pnu/1997/split/pnu321-1.htm
)
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