Introduction to Liquid Crystals
LCs | Twisted Nematic | FLCs | SSFLCs
What are Ferroelectric Liquid Crystals?
The Nematic and Smectic A (SA) liquid crystal phases are too symmetric to allow any vector order, such as ferroelectricity. The tilted smectics, however, do allow ferroelectricity if they are composed of chiral molecules. The pictures below show the original ferroelectric LC, DOBAMBC and a modern compound, W 314:
In the simplest case, the Smectic C (SC), the average long molecular axis is tilted from the layer normal z by a fixed angle but the molecules are free to rotate on the so-defined tilt cone. The phase has a C2 symmetry axis perpendicular to both the molecular director and the layer normal. The molecules exhibit a net spontaneous polarization along this axis. The magnitude of the polarization depends on temperature, generally decreasing as the tilt angle goes to zero at the SC - SA phase transition. The following Figure shows the geometry of the chiral SC phase:
Ferroelectric liquid crystals (FLCs) also exhibit a sponteous helixing of the polarization, so that over macroscopic distances (a few microns, say) the polarization averages to zero.
Since the coupling of the polarization to applied fields is linear in the field, this means that FLCs can be made to switch quickly (typically within a few microseconds) and in a bipolar manner. This makes FLCs ideally suited to electrooptic applications. FLCs are now included in several display technologies, the most popular of which use the surface-stabilized (SSFLC) geometry.