Looking At Liquid Crystal Anomalies

On a similar track as yesterday’s post today’s news is about research carried out on liquid crystals. The concept of liquid crystals is that they share properties associated with standard liquids but also solid crystals. For instance they may retain the order and structure similar to a crystal but flow as if they were a fluid. When viewed under a microscope in a polarised light source liquid crystals form some of the most interesting, at least aesthetically, images produced by nature (this is in fact one of the more monochromatic examples and often the images are a lot more vibrant than this):

Thank you to Wikipedia and to Minutemen for this image

For a long time it was believed that liquid crystals were completely or almost completely homogeneous with some slight distortion where surfaces were in contact with it. However it was theorised and then proved that their were common defects that could form within the crystal structure that were essential for understanding some of the properties of liquid crystals. The excitations and defects that exist in the structures of liquid crystals are of particular interest as they share many analogues to the behaviour of other fluids such as the plasmas found in stars or nuclear experiments. The interaction of these defects with other entities is also important for liquid crystal’s uses. For instance lipids actually interact specifically with the anomalous parts of the liquid to produce extremely high sensitivity sensors. This recent study aims to use molecular dynamic models to simulate the activity of the crystal defects down to the atomic length scale. The crystal used was a mixture of two separate liquids called 5CB and 8CB. It was found that the density and structure of the liquid crystal are significantly different at their defects then in the majority of the material. For instance, the larger 8CB molecules for parallel layers near surfaces and so block off the passage of 5CB molecules. Near defects, however, their tends to be gathering of 5CB molecules which push out the 8CB and make the defects drop in density. So far the model that has been presented correctly matches with the limited experimental evidence and theoretical predictions that have been made. Since 5CB and 8CB mixed together is one of the most common liquid crystals the results of this are widely applicable to many studies that are likely to happen in the future.

Paper links: Segregation of liquid crystal mixtures in topological defects


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