There's now a new understanding of the word "wet" thanks to physicists who created the thinnest film of liquid ever. A team from the University of Vermont engineered a new class of coatings and lubricants that are only a few atoms in thickness.
"We've learned what controls the thickness of ultra-thin films grown on graphene," said Sanghita Sengupta, a doctoral student at UVM and the lead author on the new study. "And we have a good sense now of what conditions -- like knobs you can turn -- will change how many layers of atoms will form in different liquids."
The team published its results in a recent edition of the journal Physical Review Letters. While the study itself gained popularity, the researchers explained a simpler way to understand what they accomplished. When rain falls on someone's iPhone, beads form on the screen of the phone. Those beads can quickly be shaken off and won't cause the phone damage. Alternatively, if you take your iPhone into the bathroom with you to listen to music while you shower, your phone will also be covered with a thin layer of water due to steam. The researchers explained these two concepts both contribute to the same larger idea.
"These are two extreme examples of the physics of wetting," noted UVM physicist Adrian Del Maestro, a co-author on the new study. "If interactions inside the liquid are stronger than those between the liquid and surface, the liquid atoms stick together, forming separate droplets. In the opposite case, the strong pull of the surface causes the liquid to spread, forming a thin film."
However, over a half a century ago, physicists theorized there could be a third possibility of wetting. They developed the idea of "critical wetting" -- a time when atoms of liquid would form on a film's surface but stop after getting just a few atoms thick. The famous Soviet scientist Evgeny Lifshitz understood the concept, but like many of his fellow physicists at the time, he doubted if they were real. If critical wetting was real, it could not easily be tested in a laboratory setting.
Ironically, the 2010 Nobel Prize in physics went to two Russian scientists for the creation of graphene -- the material the Vermont research team just used to prove "critical wetting" was real.
Within the last few years, graphene has proven itself to be the super material of all super materials. It conducts electricity well, interacts with water in unique ways, and can be tailored to do just about anything scientists across a variety of fields could need.
For Del Maestro and his team, graphene was the "ideal surface to test for critical wetting." From there, the physicists compared various amounts of hydrogen, helium and nitrogen behaved when exposed to graphene. Using a vacuum as well as in other conditions, the team discovered a liquid layer of the various gases would form on the ultra-thin graphene. However, as team member Valeri Kotov pointed out, the film stops growing when "it is ten or 20 atoms thick."
The film sticks to the graphene thanks to Van Der Waals forces. The weak forces of the liquid to the graphene might not seem impressive when compared to the strength with which water clings to objects. But getting such weak forces to still stick and stick in a laboratory setting was a challenge in its own right, the researchers noted.
"What's important is that we can tune this thickness," Sengupta noted.
While this discovery won't completely reshape water-proof materials or water-proof electronics, the team noted it would give other engineers the ability to understand and control critical wetting. It also provided significant steps forward in what researchers know about graphene.