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April 24th 2019

Researchers from this programme, working at Utrecht University, have discovered that plastic crystals can be made with the help of rod-shaped particles. Plastic crystals are the 'little brothers' of the better-known liquid crystals. In plastic crystals the elongated particles are arranged in a regular three-dimensional lattice, just like in a solid. However the particles can also freely rotate just like in a liquid. That is because the particles are electrically charged as a result of which they repel each other. That creates just enough freedom of movement to be able to rotate. The outcome is an unusual aggregation state between liquid and solid. The substance is solid, but at the same time so motile that some plastic crystals collapse under their own weight.

Glass phase
The research also led to the creation of a plastic glass phase. In this phase particles can still rotate but they are not arranged in a regular lattice. Nevertheless, the material still behaves like a solid. Researchers discovered it is possible to switch this phase on and off with the help of an electric field. The electric field first ensures that all of the free rotations disappear.  Furthermore, this unexpectedly led to the rods arranging themselves in a regular 3D lattice as well. The process is reversible: as soon as electric field disappears plastic glass phase returns.

This work was published in Nature Communications.  It could lead to applications in the area of electronic screens (electronic ink), but the new phase is also providing new insights into the role of rotations during crystallisation or the glass transition.

In these images g indicates the direction of gravity (in the first two images the gravity points forwards, out of the image). E indicates the direction of the electric field.