Entropy is one of the most important quantities in the universe. It’s defined as the measure of the disorder of a system and it has connections to many physics fundamentals, possibly even explaining why time has a direction.
Physicists understand entropy really well when it comes to systems that are at equilibrium, but now researchers at Brown have studied entropy beyond the equilibrium states for the first time. Their results are reported in Physical Review Letters.
"It's not clear what entropy even means when you're moving away from equilibrium, so to have this interplay between a non-equilibrium phenomenon and an entropic state is surprising," co-author Derek Stein said in a statement. "It's the tension between these two fundamental things that is so interesting."
The researchers investigated a recently discovered non-equilibrium phenomenon known as a "giant acceleration of diffusion" or GAD. The set-up of this system is often likened to a miniature washboard with particles trapped within the bumps. If the washboard is tilted, some of the particles can escape more easily in certain directions and that moves the system out of equilibrium.
Although GAD was only proposed in 2001, scientists have been able to make accurate predictions based on it.
The Brown University researchers recreated a GAD system using DNA molecules organized in very narrow channels in which the molecules could move through. The channels were lined with nanopits, where DNA strands are placed. When the system is in equilibrium, the DNA is trapped.
"This molecule is randomly jiggling around in the pit – randomly selecting different configurations to be in – and the number of possible configurations is a measure of the molecule's entropy," Stein explained. "It could, at some point, land on a configuration that's thin enough to fit into the channel outside the pit, which would allow it to move from one pit to another. But that's unlikely because there are so many more shapes that don't go through than shapes that do. So the pit becomes an 'entropic barrier.'"
The researchers used a pump to push fluid down these channels, creating a non-equilibrium situation. Some molecules were able to escape the pits but others remained trapped.
"It wasn't at all clear how this experiment would come out," Stein said. "This is a non-equilibrium phenomenon that requires barriers, but our barriers are entropic and we don't understand entropy away from equilibrium."
This research raised a lot of questions about two fundamental aspects of physics – entropy and non-equilibrium states. However, the team believe that there are also potential applications for GAD. For example, it could be used to quickly make molecular mixtures.