Researchers discover a novel liquid crystal phase of banana-shaped particles

Publication in Science with Utrecht researchers Marjolein Dijkstra and Massimiliano Chiappini

Scanning electron microscopy image of the colloidal bananas. Some particles are coloured in, to emphasize their shape. The scale bar is 5 micrometers.

A team of researchers from Oxford and Utrecht has developed a new system of micrometer-sized banana-shaped particles. With these bananas they experimentally confirm the existence of the so-called ‘splay-bend nematic’ liquid crystal phase, which was predicted forty years ago, but had remained elusive until now. These results, published today in Science, provide the cornerstone for further development of new banana-shaped liquid crystals. Such liquid crystals can switch their orientation under electric fields at ultra-fast speeds, making them ideal candidates for new highly responsive displays.

Liquid crystals have unique properties that make them easily respond to external stimuli such as electric fields, and are therefore often used in displays of TVs and computers. The molecules that form these liquid crystals are typically elongated in shape. While simple rod-like molecules form just five liquid crystal phases, banana-shaped molecules form more than fifty phases. These ‘banana phases’ were discovered twenty years ago and have since then initiated a true ‘banana-mania’ in the field of liquid crystals.

Confocal microscopy image of the splay-bend nematic phase. The wavy nature of this phase arises from the banana-shaped of the particles when packing close together. The scale bar is 10 micrometers.

Extremely challenging

However, despite their importance, to date nobody had managed to look inside these banana phases and directly visualize the way the banana particles pack or move. This is largely due to the fact that molecular systems are extremely small and move very fast, making their direct imaging extremely challenging even when using the most advanced microscopes. The researchers from Oxford and Utrecht have now managed to study,  visualize, and analyze, for the first time, the inner details of banana-shaped liquid crystals with single particle resolution.

Bananas in the splay-bend nematic phase, coloured according to the particle orientation as indicated by the white arrows in the inset.

Different banana phases

Using image analysis techniques, the researchers directly determined the positions and orientations of the banana-shaped particles, which enabled the identification of a range of different banana phases. Moreover, with their colloidal bananas they experimentally confirmed the existence of the so-called splay-bend nematic liquid crystal phase, which was predicted forty years ago, but had remained elusive until now.


Shaping colloidal bananas to reveal biaxial, splay-bend nematic and smectic phases
Carla Fernández-Rico, Massimiliano Chiappini*, Taiki Yanagishima, Heidi de Sousa, Dirk G. A. L. Aarts, Marjolein Dijkstra*, Roel P. A. Dullens
Science, 20 August 2020, DOI 10.1126/science.abb4536
* researchers affiliated with Utrecht University