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Physics

Overcoming the Limit in Liquid Crystals

It is well known that cholesteric liquid crystals never reflect more than 50% of ambient light. CNRS researchers(1) have recently discovered a way to push this limit up to 80%.

Cholesteric liquid crystals are abundant in nature and can be found in the shells of arthropods, the organization of DNA molecules, plant cell walls, biopolymers, and even human bone. The name “cholesteric” dates back to the 19th century when they were first discovered in natural substances derived from cholesterol.

A cholesteric liquid crystal is composed of molecules that take a helical structure, which reflects light. Only light that has a wavelength close to the helix “pitch”–the distance over which the helix rotates 360 degrees–will be reflected. This is what gives the shells of some beetles, for example, their shiny, metallic appearance. Moreover, since the pitch varies with temperature, pressure, or the angle of observation, such liquid crystals are routinely used in a variety of applications. These range from sensors that can detect hotspots in electronic circuits, to forehead thermometers or even banknotes.

 

texture

© M. Mitov/CNRS Photothèque

Fingerprint-like texture of a cholesteric liquid crystal.


A cholesteric liquid crystal is usually organized in helices of only one orientation. Circularly polarized light2 is either 100% reflected or 100 % transmitted by such liquid crystals, depending on the right or left orientation of the helical structure it is sent through. In contrast, ambient light, which is not polarized, is broken down into two waves: one that is reflected and one that is transmitted. This is why only 50% of ambient light is reflected by liquid crystals.

To increase this percentage, Michel Mitov and Nathalie Dessaud have exploited the capacity that some cholesterics have of changing their sense of helicity with temperature. This means that when heated, these liquid crystals can go from being left-handed to right-handed, for example.

The researchers have added a small percentage of a light-sensitive monomer to the cholesteric liquid crystal. This monomer is capable of forming a polymer network of defined helicity, left or right, inside the liquid crystal when exposed to ultraviolet (UV) light.3 They then heated this mix at a temperature defined to get a given pitch and a right-handed helical structure, then submitted it to UV light for polymerization.  

The resulting gel is comparable to a sponge (the polymer) filled with water (liquid crystal). When cooled down to a temperature defined to keep the same pitch but a reversed helicity, the liquid crystal becomes constituted of two sets of molecules: those that are close to the polymer and tend to take the polymer structure, thus being similarly helically oriented; and those that are further away from the polymer and thus remaining free to move and change structure with temperature. With this procedure, the researchers managed to obtain a liquid crystal structured in both left- and right-handed helices, resulting in more than 50% of reflected ambient light.

The two scientists have shown that their technique works in the infrared range, a region of the electromagnetic spectrum that is important for temperature regulation and telecommunications applications among others. “Our results could also pave the way for creating 'smart windows' in buildings, vehicles, greenhouses, and any other places where controlling light is an issue.” Such windows, made up of a cholesteric film sandwiched between two glass panels, could control the amount of heat and light transmitted. The reflection of light could be adjusted as needed–by subjecting the material to an electrical voltage and by varying its value. On a hot day, a window could be set to reflect more infrared (and eventually visible) light back to the outside, so that less heat enters the room. In winter, the reverse would be possible to allow more light through the window. These windows could one day render traditional, energy-hungry air-conditioning systems obsolete.

 

Isabelle Dumé

 

 

 

 

Notes :

1. Centre d'Elaboration de Matériaux et d'Etudes Structurales (CEMES).
2. For more on polarized light: www.physicsclassroom.com/Class/light/U12L1e.htm3. M. Mitov and N. Dessaud, “Going beyond the reflectance limit of cholesteric liquid crystals,” Nature Materials. 5: 361. 2006.

Contacts :

Michel Mitov
mitov@cemes.fr
www.cemes.fr


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