18 August 2005: Liquid crystals enter an extended blue phase
Liquid crystals with blue phases that exist over a surprisingly wide temperature range will lead to new applications in photonics, say researchers.
There are three so-called blue phases in liquid crystals, corresponding to a disordered, amorphous structure, or fluid three-dimensional lattice structures. In the ordered phases, the lattice dimensions are the same order as the wavelength of visible light, producing strong light reflection that is useful in photonics applications. However, blue phases are severely limited by their temperature instability - they exist over a narrow range, generally 0.5-2°C.
Now researchers in the UK have produced a set of novel blue phase liquid crystals with a widely extended stable range of 16-60°C. Harry Coles and Mikhail Pivnenko at the University of Cambridge used a series of bimesogens as building blocks, in which two rigid mesogenic structures were linked by a flexible alkyl chain. This unusual molecular structure for a liquid crystal imparts the exceptional thermal stability.
A typical blue-phase mixture contained mixtures of three bimesogens with different alkyl chain lengths. A chiral additive was also included, its concentration altering the reflectance of the system to give red, green and blue base reflected colours. In all, 30 different mixtures with extended blue phases were prepared.
The base colour of the reflected light can be switched to any colour by applying an external electric field that distorts the lattice, with a typical response time of 10 milliseconds.
The researchers predict that these new materials will help to create a new type of bright, reflective liquid crystal display with low power consumption, and will be useful in tuneable optical filters and organic lasers. Steve Down
H J Coles and M N Pivnenko, Nature 2005, 436, 997