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Metselaar L., Dozov I., Antonova K., Belamie E., Davidson P., Yeomans J. M., Amin Doostmohammadi A.,Electric-field induced shape transition of nematic tactoids, Phys. Rev. E, 96, 022706, 2017

If you mix oil and water, you will see two separate layers of fluid form. However, if you then shake the mixture, you will see oil droplets suspended in water, or vice versa. These droplets are perfectly spherical and under gravity the mixture will over time form two layers again.

The story becomes a tiny bit different when you start adding tiny rods to water. Initially they will happily suspend in the water, but if you add enough of them, droplets with a high concentration of rods will emerge. These droplets are called tactoids, and the overall material of rods suspended in a solution is called a liquid crystal. Tactoids can be observed only under the microscope and, strikingly, are not spherical. The tactoids are elongated, and have sharp tips on the long ends.

This characteristic shape is due to the fact that the suspension inside the tactoids is elastic (all rods prefer to lie next to each other), that the surface tension between the tactoid and the water background is small (for oil in water this is large) and that the rods want to lie aligned to the interface.

Most interestingly, when an electric field is applied along the long axis of the tactoids, they will stretch until the long axis is up to 15 times as large as the short axis and the tactoids are cigar-shaped. This elongation process is entirely reversible when the electric field is turned off again. The strong elongation happens because the rods are strongly anchored to the interface: when you try to rotate the rods with an electric field, they will drag the interface with them.

The results contribute to developments in improving the properties and processing of liquid crystal materials. Subjecting tactoids to shear flow improves the optical properties of liquid crystal films, by improving the internal alignment of the rods inside the tactoids. Our results demonstrate that using electric fields can achieve this same effect, but within an environment that is much more easily controlled. We therefore expect that these new findings will be of interest to a broad range of physicists, chemists and engineers concerned with the science and technology of liquid crystal materials.

An abstract of the article can be found here.

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