History
In Reinitzer's reports from 1888 on the melting behaviour of cholesteryl benzoate, there is a note that the substance briefly turned blue as it changed from clear to cloudy upon cooling. This subtle effect however remained unexplored for more than eighty years until experimental results were published during the late 1960s and early 1970s that indicated that the blue color was due to at least two new and very different liquid crystalline phases.
For almost one hundred years, scientists assumed that the most stable cholesteric helical structure could be described by a single helical axis about which the director rotates. It turned out that in the new structure the director rotates in a helical fashion about any axis perpendicular to a line as illustrated in fig. 1. Although an unlimited number of helical axes are actually present, this structure was named double twist structure.
This double twist structure is more stable than the single twist structure (i.e., the normal helical structure of chiral nematics) only up to a certain distance from the line at the center. Since this distance is on the order of the pitch of the chiral nematic liquid crystal (typically 100 nm) and since the geometries of usual liquid crystal samples are much larger, the double twist structure occurs only rarely.
Blue phases are special cases when double twist structures fill up large volumes. When double twist structures are limited in all directions to the distance from the center line where the twist amounts to 45° a double twist cylinder results. Because of its small radius, such a cylinder is more stable than the same volume filled with a single twist chiral nematic liquid crystal.
A large structure can be composed from these double twist cylinders, but defects occur at the points where the cylinders are in contact as illustrated in fig. 5. These defects occur at regular distances and tend to make the structure less stable, but it is still slightly more stable than the single twist structure without defects, at least within a temperature range of about 1 K below the transition from the chiral nematic phase to an isotropic liquid.
The defects that occur at regular distances in three spatial dimensions form a cubic lattice just as we know it from solid crystals. Blue phases are thus formed by a regular three-dimensional lattice of defects within a chiral liquid crystal. Since the spacings between the defects of a blue phase are in the range of the wavelength of light (several hundred nanometers), for certain wavelength ranges of the light reflected from the lattice constructive interference occurs (Bragg reflection) and the blue phase reflects colored light (note that only some of the blue phases actually reflect blue light).
Read more about this topic: Blue Phase Mode LCD
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