Physicists Create World's Smallest Droplets in Lab

Physicists have performed some amazing feats in the lab. Now, they may have created some of the smallest drops of liquid ever made. The droplets could lead to further understanding of collective behavior.

The short-lived droplets are miniscule--about the size of three to five protons. That's about one-100,000^th the size of a hydrogen atom, or one-100,000,000^th the size of a virus. They actually "flow" in a manner similar to the behavior of the quark-gluon plasma, a state of matter that is a mixture of the sub-atomic particles that make up protons and neutrons and only exists at extreme temperatures and densities. In fact, cosmologists propose that the entire universe once consisted of this strongly interacting elixir for a fraction of a second after the Big Bang when conditions were dramatically hotter and denser than they are today.

Actually "seeing" these droplets, though, was a challenge. In fact, researchers were only able to detect them through another experiment. Scientists saw evidence of the droplets from the results of colliding protons with lead ions at velocities approaching the speed of light.  

"With this new discovery, we seem to be seeing the very origin of collective behavior," said Julia Velkovska, one of the researchers, in a news release. "Regardless of the material that we are using, collisions have to be violent enough to produce about 50 sub-atomic particles before we begin to see collective, flow-like behavior."

Scientists have actually been trying to recreate the quark-gluon plasma associated with the Big Bang since the early 2000s by colliding gold nuclei using the Relativistic Heavy Ion Collider (RHIC). Although the RHIC scientists expected the plasma to behave like a gas, they found that it instead acted like a liquid. Eventually, the scientists duplicated the results with the LHC by colliding lead nuclei--and saw evidence of the plasma.

"The proton-lead collisions are something like shooting a bullet through an apple while lead-lead collisions are more like smashing two apples together: A lot more energy is released in the latter," said Velkovska in a news release.

The researchers created two models in order to explain their observations. Of the two, the plasma droplet model seemed to fit the observations best. In fact, the new data is forcing the authors of a competing model--color glass condensate, which attributes the particle correlations to the internal gluon structure of the protons themselves--to incorporate hydrodynamic effects. Essentially, both models need to describe the phenomenon as liquid droplets.

The findings could help advance physicists understanding of these collisions. In addition, it could allow them to further understand exactly how our universe works and perhaps may lead them to clues as to how it was formed.

The findings are published in the journal Physical Review.