Friday, November 02, 2007

Granular Liquids vs. RHIC

People often use the word "glass" in relation to RHIC physics (via the Color Glass Condensate), but this is a bit more literal. From the upcoming Physics News Update #845, now RHIC is being used as a "benchmark" for fluid (or even liquid) behavior, even when the fluid is made of glass beads:
GRANULAR LIQUIDS WITH ZERO SURFACE TENSION. New experiments with spherical glass beads show that liquid behavior can arise simply from rapid collisions among a sufficiently dense stream of particles. The experiment was undertaken by Xiang Cheng, Heinrich Jaeger and Sidney Nagel and their colleagues at the University of Chicago, experts on discovering novel effects with granular materials (see http://www.aip.org/pnu/2005/split/725-3.html and http://www.aip.org/pnu/2005/split/759-2.html). If one or two beads are dropped from above on a horizontal surface, they will bounce back in the direction from which they came. If, however, many beads are dropped all at once---constituting a dense granular stream hitting a target---then something else happens: the grains deflect out laterally in the form of a very thin, symmetrical sheet or cone as if they were a liquid. Indeed, the experiments using granular matter quantitatively reproduce results obtained with streams of water. However, with beads, the *liquid* is one in the limit of vanishing surface tension. (To ensure there was no cohesiveness between the beads, which range in size between 50 microns and 2 millimeter, they were baked in a vacuum oven beforehand, evaporating any lurking moisture.) During the short interval the beads inside the stream collide with each other in front of the target, liquid-like conditions are established whose observable consequence are the thin sheets. This novel, zero-surface-tension liquid state, the experimenters believe, might be of interest to physicists at the Relativistic Heavy Ion Collider (RHIC), where heavy nuclei colliding at high energies (see http://www.aip.org/pnu/2005/split/728-1.html) form a plasma of quarks and gluons that also resembles a liquid. Intriguingly, the collision pattern produced by the completely classical, macroscopic granular liquid can match that produced by the quark-gluon plasma. (Cheng et al., Physical Review Letters, 2 November 2007)

4 comments:

Blake Stacey said...

So, does this mean we're going to see the AdS/CFT correspondence applied to granular materials now?

Corey said...

Maybe I don't really understand the word "plasma".

I don't blame the reporter for using it -- many RHIC physicists refer to a "sQGP". But doesn't the word "plasma" imply that the (color) charges in the matter are free?

But the data seems to support an extremely small mean free path. So why do we keep calling it a plasma? Nostalgia?

Peter said...

no-one really calls it a plasma anymore, except as code for "perfect liquid". and now that liquids can have vanishing surface tension, i can accept lots of new things: black is white, night is day, etc.

Anonymous said...

A code for "perfect liquid"? Peter, maybe you should encourage people to adjust their decoder rings. I mean, we still believe in asymptotic freedom in such a tightly packed system of quarks, right? If the quarks aren't bound, why wouldn't "plasma" mean plasma?

Corey, I don't think that the mean free path alone is enough to say whether something should be a plasma or not. If your charges are freely moving and they're not recombining, then what does it matter if they bump into each other?

If you're like me, you think of the ionized gas in a fluorescent light when you think of a plasma. But this is a weakly coupled plasma, while the QGP is strongly coupled. The following link helped me understand the distinction between the two, but it's not obvious to me a priori why the QGP would couple strongly.

http://farside.ph.utexas.edu/teaching/plasma/lectures/node8.html
(and thank you to Professor Fitzpatrick for publishing your class notes online!)