A supra new kind of froth
Posted on Thursday, June 05, 2008 @ 22:34:49 UTC by vlad
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To see the latest science of type-I superconductors, look no further
than the froth on a morning cup of cappuccino. A team of U.S.
Department of Energy's Ames Laboratory physicists and collaborating
students have found that the bubble-like arrangement of magnetic
domains in superconducting lead exhibits patterns that are very similar
to everyday froths like soap foam or frothed milk on a fancy coffee.
The similarities between the polygonal-shaped
patterns in conventional foams and "suprafroths," the patterns created
by a magnetic field in a superconductor, establish suprafroths as a
model system for the study of froths.
"There are certain statistical laws that govern
the behavior of froths, and we found that suprafroths satisfy these
laws," said Ruslan Prozorov, Ames Laboratory physicist and primary
investigator. "We can now apply what we know of suprafroths to all
other froths and complex froth-like systems."
Prozorov discovered the suprafroth pattern last year, seeing an
unexpected foam-like design when he applied a magnetic field to a lead
sample in a magneto-optics system. Since the term "superfroth" was
already in use for an unrelated product, Prozorov coined "suprafroths"
in a nod to history: in the 1930s, superconductors were called
"supraconductors."
To help characterize suprafroths, Prozorov pulled together a team
including Ames Lab senior physicist Paul Canfield, summer laboratory
assistant Andrew Fidler and graduate student Jacob Hoberg.
Canfield, who has an interest in pattern formation in nature,
supplied the original idea to compare suprafroths' patterns to
conventional froths.
"Last year, we were standing by Ruslan's poster on equilibrium
patterns in Pb (lead), and I was discussing one of his figures during a
break," said Canfield. "I recognized that the patterns he was showing
for his Pb sample were exceptionally similar to that of a classical
picture of bubbles.
"At first Ruslan was skeptical, but over the next few weeks we both
realized just how profound the similarity between suprafroths and
conventional froths was." Canfield continued.
The team's analysis revealed that suprafroths behave similarly to
other commonplace froths, despite their very different microscopic
origins: traditional froths' cell walls consist of material like
detergent, water or plastic, while suprafroths' cell boundaries consist
of superconducting phase lead.
One similarity between suprafroths and conventional froths is the
process of coarsening, or when froth cells grow or shrink and
eventually disappear. In everyday froths, this process is evident in a
sink full of dish soap bubbles that pop and disappear over time. The
process is similar in suprafroths when magnetic field is increased,
illustrating that suprafroths adhere to John von Neumann's law, the
widely accepted concept in froth physics that specifies the rate at
which froth cells grow or shrink.
"Seeing von Neumann's law at work in suprafroths shows that the
froth state is really an intrinsic property of this superconductor,"
said Prozorov.
"Suprafroths, like regular foams, adhere to the concept of area
tiling that says that if you want to cover a plane with polygons with
each having three vertices, the most probable polygon is a hexagon," he
continued.
Physicists have long believed in a connection
between the two statistical rules of froths. Common understanding has
been that the most probable polygon—the hexagon—was related to the
number of sides—six—that determines whether a froth cell shrinks or
grows during coarsening. But the Ames Lab team's analysis has decoupled
these two concepts in suprafroths.
"In our suprafroths, we found that the association between these
two ideas is a coincidence, said Prozorov. "There is no strict
correspondence between the most stable type of froth cell and the most
common number of sides in a froth cell."
In suprafroths, cells of all observed numbers of sides grow with an
increase in magnetic field, a discovery marking an important
contribution to the general study of froths.
But the most significant contribution suprafroths make to the
general physics of froth is as a model system that can be used to study
all froths. Suprafroths offer reversibility, a significant benefit over
conventional froths.
"In everyday froths, like soap foam, the agent of change is time,"
said Prozorov. "You have to wait for bubbles to simply dry out, and
that takes days. And it's not reversible. You cannot reverse time."
"Once the bubbles pop, the problem is that the physical and
chemical properties of the cells get modified, so that doesn't make for
a clean experiment," Prozorov continued. "In an ideal situation, you
want to only study the properties of the froth patterns and their
complexity. You want to easily be able to change some parameter and
change the structure of the froth."
Achieving an ideal froth experiment is possible in suprafroths
because the agents that create the superconducting phase cells are
magnetic field and temperature, both reversible parameters.
"Magnetic field and temperature can be tuned in the lab," said
Prozorov. "They can be increased or decreased, and therefore we are
able to study the pure statistical properties of froth without problems
associated with the irreversibility of time or with chemical property
changes."
Prozorov's comparison of suprafroths is also an important contribution is the study of superconductors.
"The statistical analysis shows suprafroths behave just like normal
froth, which is also new for superconductivity," said Prozorov. "Just
last year we found this new pattern in superconductors, and now we've
proven that the froth state is really an intrinsic property of
superconducting lead. It's a big deal for both the general physics of
froth and the growing physics of superconductors.
"In physics, if you can find model systems, like suprafroths, that
have similar patterns, then by studying these model systems you can
actually get additional information about the behavior of very complex
systems like galaxies, geophysics or biophysics" said Prozorov. "So,
the bottom line is that studying physics of everyday soap froth, or,
more reliably, suprafroths, can help us understand very complex,
difficult questions about the world around us."
Canfield said that the suprafroth project is a case study for how collaboration should work at research laboratories.
"Fruitful collaboration like this happens frequently at Ames Lab,"
he said. "As part of our extensive collaboration and interaction,
Ruslan and I discuss ideas, materials and results all the time."
Citation: "Suprafroth in Type-I Superconductors" by Ruslan Prozorov appears in Nature Physics' April issue: http://www.nature.com/nphys/journal/v4/n4/full/nphys888.html
Source: Ames Laboratory
Via: http://www.physorg.com/news131893688.html
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