DiStruc

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.

Colloidal rods in square confinement

Cortes L. B. G., Gao Y., Dullens R. P. A., Aarts D. G. A. L., "Colloidal liquid crystals in square confinement: isotropic, nematic and smectic phases", J. Phys.: Condens. Matter 29, 2017

Imagine you’ve got a bunch of matches and you need to pack them into a matchbox. That’s an easy challenge, unless each match is constantly moving around (due to Brownian motion) and they need to align along any of the walls of the matchbox. If you only need to fit a handful of rods it’s still ok, but at higher densities the rods will all want to point in one direction, which is impossible given the boundary conditions. At even higher concentrations they want to pack in layers, which forms another complicating factor.

To study this problem experimentally we have developed a system of small silica rods, which are around 5 micrometer long and 0.5 micrometer thick. Under gravity they slowly settle into square boxes of varying dimensions, from several times the rod length up to tens of times, and they form a range of patterns that we can quantify using microscopy down to the almost single particle level. Some of the patterns observed were already predicted in computer simulations, but some other structures are novel and we are currently working on a theory to understand these observations better.

Importantly, such packing problems occur in nature and technology: for example, the packing of DNA or the confinement of actin filaments in biological cells, and liquid crystals in each pixel of your phone’s screen. Our experiments shine light on the possible patterns which form and on the applicability of continuum theories down to these small lengthscales.

A full version of the article can be found here.

The flow – orientation coupling of rods

Christian Lang, Joachim Kohlbrecher, Lionel Porcar and Minne Paul Lettinga, "The Connection between Biaxial Orientation and Shear Thinning for Quasi-Ideal Rods", Polymers 2016

Like every common liquid, water possesses a certain objection to flow. This results from an inner friction of the molecules and their complicated interactions. Since the molecules are very small compared to, for example, a glass of water and also very quickly moving due to thermal motion, the objection of molecules to flow in the glass is always the same, no matter how fast we stir the glass.

Rods, with a length almost a million times the water molecule size and a width only 28 times the water molecule size, can be added to the water. This results in a much higher inner friction and a larger objection to flow. One finds such mixes almost everywhere in our daily consumables. Examples are food and health care products.

Interestingly enough, the resulting mix of water and rods is harder to move in a slow flow than it is in a faster flow. This means that the inner friction of the mix is dominated by the motion of rods and not by the flow of the water molecules between the rods.

In our research, we have, therefore, tried to observe the rods under different flow conditions in order to see the cause of this phenomenon. Since the rods are much too small to be seen, we have chosen to shoot neutrons at the mix and investigate the changes in the resulting scattering pattern of neutrons scattered from the flow aligning rods. The scattering pattern, thereby, serves as a fingerprint of the interaction of rods and, hence, gives us the necessary microscopic information.

In this study, we have observed that the rods are under thermally induced motion even if the mix is not brought to flow by us, resulting in a completely disordered state with a high objection to flow. As soon as we bring the whole mix to flow, the situation drastically changes. The rods align with the flow direction into a very ordered state and the friction between rods is strongly reduced, resulting in a friction very close to pure water. The thermal motion, thereby, is dominated by the flow alignment. 

On top of this orienting behavior, we were able to detect for the first time that the order is disparate along different directions in our sample. This phenomenon is usually called biaxiality and it is caused by the difference in momentum transfer under shear in different directions inside the sample. So far, this phenomenon had been only described by theory but not directly observed in an experiment.

With this knowledge, we are now enabled to review the microscopic theory of rod-dynamics under shear. We found an overall qualitatively correct description of the experiment by theory. However, certain very profound discrepancies between theory and measurement are present. In our experiment, we observe that the transport of rods is more hindered by the surrounding rods in a slow flow than it is in a faster flow, while the interaction of rods in a faster flow is much stronger compared to the slow flow case and also compared to theory. Additionally, the mass transport is direction dependent, or in mathematical terms, a tensorial quantity.

Based on these observations we can now aim on the development of an improved theory, taking the mentioned points into account. Also, we are enabled to understand the flow of food and health care products, containing rod-like objects, better than before.    

A full version of the article can be found here