Thin Nematic Films: Liquid Beauty

Image source: arXiv:1010.0832 [cond-mat.soft]

No, it's not a moon passage in front of an exoplanet. It's a thin nematic film. Let me explain.

Between condensed matter physics and chemistry, between solids and liquids, there is soft condensed matter. Soft condensed matter deals with the behavior of materials like gels, glasses, surfactants, or colloids. Typically these are fairly large molecules, possibly floating in some substrate, and can assemble to even larger structures. Understanding this assembly, the existence of different phases, and also the motion of the molecules is mathematically challenging due to the complexity of the system.

But taking on this challenge is rewarding: Soft matter is all around you, from toothpaste over body lotion to salad dressing. It is even quite literally in your veins. One of the best known examples for soft matter however is probably not blood, but liquid crystals.

Liquid crystals are rod-like molecules whose chemical structure encourages them to collectively align. How well this works depends on variables in the environment, for example temperature and magnetic fields. Liquid crystal have different phases; the transition between them depends on these environmental variables. In the so-called nematic phase molecules are locally aligned but still free to move around, and the orientation might change over long distances.

To make the molecule orientations visible, one uses polarized light on a thin film of liquid crystals on some type of substrate and a polarization filter to take the image. The liquid crystal molecules change the polarization of the light depending on the molecules' orientation, so different light intensities become a measure for the orientation of the molecules.

For the images we are looking at here we have the substrate below the liquid crystal and air above it. These two different surfaces causes a conundrum for the molecules in the liquid crystal, because they would prefer to align parallel to the substrate, but vertical to the air surface. Now if the film is fairly thick - "thick" meaning a μm or more - the molecules manage to align along threads that bend to achieve this orientation, though there are the occasional topological defects in this arrangement, places where the molecules change orientation abruptly. This is what you see for example in the image blow

[Picture Credits: Oleg Lavrentovich from the Liquid Crystal Institute at Kent State University, for more pictures see here.]

But this behavior changes if the film becomes very thin, down to a tenth of a μm or so. Then, the competing boundary conditions from the two interfaces start getting in conflict with the molecules' desire to align, which breaks the symmetry in the plane of the liquid and leads to the formation of periodic structures, like the ones you see in the first image. In this example, the nematic film does not cover the whole area shown, but it's a drop that covers only the parts where you see the periodic structures. This has the merit that one can see that the orientation of the structure to the boundary is always perpendicular.

The typical molecules in these films are not very large. In the example here, it's 6CB with the chemical structure C₁₉H₂₁N. The size of this molecule is much smaller than the width of the film when the effect sets in, so this cannot be the relevant scale. The question at which width the instability sets in has been studied in this paper, where also the image was taken from. It's an intriguing effect that can teach us a lot about the behavior of these molecules, not to mention that it's pretty.