Blue phases come of age: a review

If it is the exception that proves the rule, liquid crystalline blue phases prove the typical properties of liquid crystals: The latter are ordered fluids, which show birefringence because of the orientational order of their molecules. However, blue phases are optically isotropic in spite of local orientational order. Many liquid crystals form - similar to usual liquids - spherical or at least almost spherical droplets. But blue phase (BP) droplets can show facets that resemble crystal shapes. Typical liquid crystals appear in the bulk as translucent liquids. In contrast, blue phases show brilliant colors (due to Bragg scattering) - a remarkable property, which has probably lead to the discovery of liquid crystals some 120 years ago. Since then, blue phases attracted high academic interest but were not expected to be very useful, because the temperature range of their existence was rather small. But fortunately, there are very useful exceptions: During the last few years, systems have been developed which exhibit blue phases stable within temperature intervals as large as 50 °C. Even a promising prototype of a blue phase display was presented, last year.