Researchers from the group of Professor Roel Dullens at Oxford’s Department of Chemistry have developed a new system of micrometre sized banana-shaped particles (Fig. 1). With these bananas they experimentally confirm the existence of the so-called ‘splay-bend nematic’ liquid crystal phase (see Fig. 2 and 3), which was predicted 40 years ago, but had remained elusive until now. These results, published in Science, provide the cornerstone for the further development of new banana-shaped liquid crystals.
Liquid crystals are a fascinating state of matter that we encounter in our everyday life. For example, the cell membranes in our bodies are in a liquid crystalline state - and so are the functional materials used in displays of TVs and computers. The reason this state of matter is a cornerstone of both living and technological materials is inherent to its unique properties: it shows order in a preferred direction, like a solid, but it also flows, like a liquid, and as such, it easily responds to external stimuli such as electric fields.
The shape of the liquid crystal forming building blocks, which are typically elongated molecules, has an enormous impact on the way they pack into liquid crystals. While simple rod-like molecules form just five liquid crystal phases, banana-shaped molecules form more than fifty phases. These ‘banana phases’ were discovered 20 years ago and have since then initiated a true ‘banana-mania’ in the field of liquid crystals. The fact that even a small molecular curvature results in a new library of banana phases is not only fascinating from a fundamental point of view, but also from an industrial perspective, as they can switch their orientation under electric fields at ultra-fast speeds, making them ideal candidates for new highly responsive displays.
Despite the importance of banana phases, to date nobody has managed to look inside them 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.
Carla Fernandez-Rico and co-workers used colloidal bananas and optical microscopy to study and visualize, for the first time, the inner details of banana-shaped liquid crystals with single particle resolution. Using image analysis techniques, they directly determined the positions and orientations of the banana-shaped particles, which enabled the identification of a range of different banana phases (see Fig.2). Moreover, with their colloidal bananas they experimentally confirmed the existence of the so-called splay-bend nematic LC phase (see Fig.3), which was predicted 40 years ago, but had remained elusive until now.
Fig.1: Scanning electron microscopy image of the colloidal bananas. Note the false colouring emphasizes the shape of the particles. The scale bar is 5 micrometers.
Fig.2: Bananas in the splay-bend nematic phase colored according to the particle orientation as indicated by the white arrows in the inset.
Fig.3: Confocal microscopy image of the splay-bend nematic phase. Note the wavy nature of these phase arises from the banana-shaped of the particles when packing close together. The scale bar is 10 micrometers.