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Liquid crystal tied into knots to advance photonics

Photonics Spectra
Nov 2013
COVENTRY, England – Using a miniature Möbius strip made from silica particles allows liquid crystals to be tied in knots, which could lead to next-generation materials and photonic devices.


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


Long, thin, rodlike crystal molecules normally align themselves to point in the same direction. To control the alignment of the structures, however, 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, standing up like the spikes on a hedgehog.

By adding colloids in 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; the red spots highlight central portions of the knot. (b) A full 3-D visualization with molecular orientation shown as a color map. Photo courtesy of University of Warwick.


“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 at the University of Warwick. “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).


GLOSSARY
colloid
A particle that will not normally diffuse through animal or vegetable membrane. Larger than most inorganic compounds, colloids will remain suspended indefinitely.
liquid crystal
A type of material that possesses less geometrical regularity or order than normal solid crystals, and whose order varies in response to alterations in temperature and other quantities. Liquid crystals are characterized by phase varieties, including cholesteric, nematic and smectic. The optical properties of liquid crystals are familiar from their use in displays, known as LCDs.
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