Electric-field-induced shape transition of nematic tactoids

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.

Understanding the self-assembly of cholesteric liquid crystals

Tortora M. M. C., Doye J. P. K., Perturbative density functional methods for cholesteric liquid crystals, J. Chem. Phys. 146, 184504, 2017

What is the common point between living cells, soap bubbles, Romanesco broccolis and DNA? All of these objects result from a physical process known as self-assembly, through which atoms and molecules spontaneously organise themselves into functional structures of many shapes and sizes. The diversity and complexity of this ordering phenomenon is beautifully illustrated by the seemingly endless variety of patterns exhibited by falling snowflakes, reflecting the subtle dependence of their crystalline arrangement on the local conditions of their formation in the atmosphere.

Another everyday example of such self-assembled materials may be found within the screens of our phones, TVs and computers. At the heart of every pixel of most modern display devices lies a thin liquid layer comprised of rod-like molecules, which possess the surprising ability to organise in a helical fashion in the absence of external cues. These phases are known as cholesteric liquid crystals (CLCs), and their fascinating optical properties extend far beyond the screen of your digital watch; a number of birds, insects and even some fruits owe their shiny, colourful appearance to similar CLC structures.

We have introduced a novel numerical method to predict the ways in which molecules can assemble into such cholesteric phases based on many microscopic parameters such as their shape, concentration and chemical properties. This approach enables us to study the liquid-crystal behaviour of a number of experimental systems that are very challenging for theoreticians to understand, and shed some light on the mechanisms through which tiny individual molecules can organise into complex structures many orders of magnitudes larger than their own size.

An illustration of a cholesteric phase of cigar-shaped particles is shown on the figure. The distance P over which their direction of alignment performs a full turn is known as the cholesteric pitch. It determines, among other things, the beautiful colours reflected by peacock feathers.

A full version of the article can be found here.