Mob Rules

*\"It would indeed be remarkable if Nature fortified herself against
further advances in knowledge behind the analytical difficulties of the
many-body problem.\" ([Max Born](http://en.wikipedia.org/wiki/Max_Born),
1960)*

One water molecule all by itself isn\'t very interesting. One oxygen.
Two hydrogens. What more can you say? But throw a lot of water molecules
together.... You get snowflakes, rainbows, rock-hard ice, a soft cooling
mist, infinitely flexible water.

Atoms and molecules, en masse, can do almost anything, and physicists
know it. In fact, they dream about it. By combining the right kinds of
molecules under the right conditions, it should be possible to craft,
*e*.*g*., uncrackable metal alloys, room-temperature superconductors,
self-healing spaceship skins impervious to meteoroids and cosmic rays.
You name it.

The promise of these \"designer materials\" is great, but there\'s a
problem: Just as crowds of people are hard to predict, so are crowds of
molecules. Only in idealized cases do physicists have simple rules, like
the ideal gas law, to help them calculate what will happen. Nature,
though, is rarely ideal.

Consider the numbers. Physics can deal with one or two particles.
Newton\'s laws describe the motion of a single planet around the sun so
simply and beautifully that school children can solve the equations. Now
add another planet; all three bodies (sun, planet and planet) tug on
each other as they move. The \"three body problem\" is famously
complicated, and computers are required to solve it.

There\'s a three-body problem in quantum mechanics, too. Schrodinger\'s
equation is easy to solve for two particles, *e.g.*, an electron and a
proton in a hydrogen atom. Add one more electron and, once again, a
computer is required for an exact solution.

Now imagine, not three, but 10^23^ particles. That\'s the number of
atoms or molecules in a typical lump of matter you can hold in your
hand. They tug on each other, bond with each other, crash into each
other. The number of interactions going on at any instant is
mind-boggling.

\"It\'s impossible to solve the \[equations\] exactly for such a
system,\" says physicist Peter Lu of Harvard University. \"With 10^23^
particles, you have to write down a term for each interaction. So, of
course, you make approximations because the exact calculation is far,
far beyond anyone\'s (or any computer\'s) abilities.\"

What do you do when even supercomputers can\'t handle the math? Enter
the International Space Station (ISS).

Lu and his PhD thesis adviser Prof. Dave Weitz are experimenting with a
device onboard the ISS that might succeed, in some respects, where
supercomputers fail.

It\'s simple: Take a jar of \"organic goop,\" says Lu, and mix in
millions of tiny Plexiglas spheres. Add some molecular coils, billions
of them, and float the mixture in space.

This \"device\" is a colloidal mixture, says Lu, and it\'s a good model
for many-particle interactions.

Colloids are systems of particles suspended in fluid. (Pulpy orange
juice is an everyday example.) Physicists have known for a long time
that carefully crafted colloids could be used to simulate swarms of
atoms or molecules. Colloidal particles arrange themselves as crystals;
flow like fluids; expand and contract like a gas. They exhibit all the
mob behaviors of molecules, with one big advantage:

\"We can see colloids,\" says Lu. While it\'s impossible to watch
individual atoms or molecules, colloidal particles are big enough to see
through an ordinary microscope. Their interactions, and the structures
they form, can be observed directly. No supercomputer required!

On Earth colloidal simulations are limited. The particles, weighted by
gravity, tend to settle to the bottom of the jar. Floating onboard the
ISS, however, they remain suspended, interacting for as long as a
physicist cares to watch.

What kinds of atoms and molecules do colloids mimic?

It depends on the make-up of the colloid. Some colloid-spheres can carry
a charge, \"so we can make them attract or repel\" like ions, says Lu.
Picture this: a pair of colloid-spheres, grafted together, one
positively charged, the other negatively charged. A bipolar molecule?
The possibilities are endless.

But the research is just beginning.

In recent months, Lu and Weitz have been experimenting with a simple
colloid onboard the ISS. It\'s made of 400-nm Plexiglas spheres, polymer
coils (long molecules coiled up like a Sinky), and an organic
fluid\--all inside a transparent jar about the size of a Pringles can.

The spheres are stand-ins for atoms or molecules. The polymer coils
force the spheres to interact. Lu explains: \"Polymer coils act like an
ideal gas. They swarm through the liquid, applying pressure to the
bigger spheres. When two spheres approach one another, the gap between
them becomes too small for the coils. Inside the gap, pressure drops and
the spheres are pulled together. By controlling the size and
concentration of the polymer coils, we control the strength of
interactions between the Plexiglas spheres.\"

This colloid mixture, called BCAT, short for Binary Colloid Alloy Test,
turns out to be a promising model for supercritical fluids.

A supercritical fluid is a high-pressure, high-temperature state of
matter best described as a liquid-like gas, and a marvelous solvent.
Water becomes supercritical in some steam turbines\--and it tends to
dissolve the tips of the turbine blades. Supercritical carbon dioxide is
used to remove caffeine from coffee beans, and sometimes to dry-clean
clothes. Liquid-fueled rocket propellant is also supercritical when it
emerges from the tail of spaceship.

Of special interest is the \"critical point\"\--the pressure and
temperature where a substance becomes supercritical. Near the critical
point, matter fluctuates. Bubbles and droplets, some as small as a few
atoms, some as wide as the jar itself, appear and disappear, merge and
split. Lu and Weitz have been able to observe some of these structures
in BCAT.

It\'s only a start, says Lu. If you want to design a supercritical fluid
... good luck! Maybe one day, though, all you\'ll need is a good
colloid.