When water reaches 100 °C, it turns into a gas phase. At sea level, that is. If you take away some air pressure, water will boil already at colder temperatures. It is clear that materials made up of ordinary molecules take on a phase depending on temperature and pressure. Leiden theoretical physicist Prof. Vincenzo Vitelli wondered what happens if materials have self-spinning dimers as building blocks instead.

Simulation
To this end, first authors Benny van Zuiden and Jayson Paulose simulate self-spinning dimers on their computer and study how they organize themselves. When they apply a gradually increasing pressure on them, they see the system change from an ordered state to a very chaotic state.
In the figure below we see on the left a beautifully ordered state, with dimers neatly forming a triangular crystal lattice. Moreover, the relative orientation of nearby particles are locked as they spin.
At the far right, the concentration is so high that the system gets stuck in a glassy phase. Remarkably, there is a liquid phase in between. Usually a substance will become more solid as its density increases. Here the opposite happens.

Liquid
So how can there be a liquid state? With low density, the dimers have plenty of room to move as they wish and stay in sync, like a group of stage dancers. When the stage is too small, dancers will bump in to each other and they chaotically move around, as particles in a liquid. If the stage however gets so tiny that dancers are unable to move, they get stuck in a disordered configuration reminiscent of a glass.
 
Contact
Erik Arends
Physics Outreach Officer, Leiden University
arends [at] physics.leidenuniv.nl
+31 (0)71 527 5471