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Knotting Particle Defect Lines
Jul 2011
WASHINGTON, July 6, 2011 — Loops and knots have been tied from microscopic topological defect lines that form when the ordering of a nematic liquid crystal is disrupted by the addition of colloidal particles.

Knots and links of arbitrary complexity are created and reconfigured from topological defect loops in the particle-stabilized chiral nematic liquid crystal using laser tweezers. The laser-induced micro-quenches locally rewire the disclination loops and essentially change the topology of entangled nematic braids. Designed by Simon Èopar. (Image: Science/AAAS)

When these lines are manipulated with laser tweezers, they can be woven into arbitrarily complex knots and links. The formation of these stable braided structures is enabled by using a sample cell similar to that commonly used in liquid crystal display technology. The findings could pave the way for the development of novel devices in the field of photonics.

In the experiment that was published by AAAS in the July 1 issue of Science, Igor Musevic and colleagues at the Jozef Stephan Institute in Slovenia saw that topological defect lines form when the normal order of molecules in a liquid crystal was disrupted. These lines normally become tangled and form knots sporadically, but the researchers discovered that knots from defect lines can be created by forcing liquid crystal molecules into a certain position. This action caused defect lines to form around each particle, like rope encircling a ball. Using laser tweezers the team tied the lines into complex knots.

Schematic presentation of made-to-order assembly of Borromean rings on a 4x4 particle array. The feasible tangle combinations were tested by the numerical algorithm based on the Jones polynomials. The selected configuration was identified by direct comparison with polynomials in the enumerated base of knot invariants and finally assembled using laser tweezers. Designed by Uroš Tkalec. (Image: Science/AAAS)

In a related perspectives piece, liquid crystals expert Randall D. Kamien of the University of Pennsylvania describes the findings and compares the knots formed in this study to knots found in biology, specifically in DNA.

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laser tweezers
A technique based on the principles of laser trapping and used to manipulate the position of small particles by gradually changing the position of the laser beam or beams once the particles are trapped. When the trap consists of a single focused beam, the optical tweezers can also be called a single-beam gradient trap. Also called optical tweezers.
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
AmericasBiophotonicscolloidal particlesDNAEuropeIgor MusevicJozef Stephan Institutelaser tweezersliquid crystal displaynanonematic liquid crystalphotonicsRandall D. KamienResearch & TechnologySloveniatopological defect linesUniversity of PennsylvaniaWashington DClasers

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