Liquid Crystal Tied Into Knots to Advance Photonics

Using a miniature Möbius strip made from silica particles, scientists at the University of Warwick have shown how to tie liquid crystals in knots.

Researchers hope to harness crystals in the next generation of advanced materials and photonic devices by understanding the intricate configurations and properties of the substance.

Liquid crystal knots created around miniature Möbius strip particles (simulation). Different knots are produced by strips with different numbers of twists. The central part of the knot is shown in red around the strip in blue. Examples are shown for (a) two (Hopf link), (b) three (trefoil knot), (c) four (Solomon’s knot) and (d) five (cinquefoil knot) twists. Images courtesy University of Warwick.

In its normal state, the long, thin, rod-like crystal molecules align themselves to point in the same direction. To control the alignment of the structures, however, the scientists add a micron-sized silica particle, or colloid, to the liquid crystal, disrupting the orientation of the parallel molecules. A colloid in the shape of a sphere, for instance, will cause the molecules to align perpendicular to the surface of the sphere like the spikes on a hedgehog.

By adding colloids that have the knotted shape of a Möbius strip, three, four and five twists force the liquid crystal to take on the same structure of a trefoil knot, a Solomon’s knot, or a cinquefoil knot, respectively.

Visualization of the average configuration of the molecules in a liquid crystal knot (simulation). (a) A plane cross-section of the knot with the molecular alignment indicated by small cylinders. The grey rectangles correspond to a part of the particle and the red spots highlight central portions of the knot. (b) A full 3-D visualization with molecular orientation shown as a color map. Different colors correspond to different orientations as given in the inset.

“Recently it has been demonstrated that knots can be created in a variety of natural settings including electromagnetic fields, laser light, fluid vortices and liquid crystals,” said Gareth Alexander, assistant professor of Physics and Complexity Science. “Our research extends this previous work to apply to liquid crystal, the substance we use every day in our TVs, smartphones and computer screens. We are interested in this as creating and controlling these intricate knotted fields is an emergent avenue for the design of new metamaterials and photonic devices.”

The study, funded by the Engineering and Physical Sciences Research Council, appeared in PNAS (doi: 10.1073/pnas.1308225110).

For more information, visit: www.warwick.ac.uk