The nematic liquid crystal phase is characterized by molecules that have no positional order but tend to point in the same direction (along the director). In the following diagram, notice that the molecules point vertically but are arranged with no particular order.
Ncmatic liquid crystals are subdivided into the ordinary nematic and the cholesteric-nematic. The molecules in the ordinary nematic structure maintain a parallel or nearly parallel arrangement to each other along the long molecular axes. They are mobile in three directions and can rotate about one axis. This structure is one-dimensional. When the nematic structure is heated, it is generally transformed into the isotropic liquid. The nematie structure is the highest-temperature mesophase in the thermotropic liquid crystals. The energy required to deform a ncmatic liquid crystal is so small that even the slightest perturbation caused by a dust particle can distort the structure considerably.
In the cholcsteric-nematic structure, the direction of the long axis of the molecule in a given layer is slightly displaced from the direction of the molecular axes of the molecules in an adjacent layer.
Smectic the most common arrangement, creates layers of molecules. The word “smectic” is derived from the Greek word for soap. This seemingly ambiguous origin is ‘explained by the fact that the thick, slippery substance often found at the bottom of a soap dish is actually a type of smectic liquid crystal.
The smectic state is another distinct mesophase of liquid crystal substances. Molecules in this phase show a degree of translational order not present in the nematics. In the smectie state, the molecules maintain the general orientational order of nematics, but also tend to align themselves in layers or planes. Motion is restricted to within these planes, and separate planes are observed to flow past each other. The increased order means that the smectic state is more “solid-like” than the’nematic. The term smevic covers all thermotropic liquid crystals that are not nematics. Many compounds are observed to form more than one type of smectic phase. As many as 12 of these variations have been identified. The alphabetic subscripts only indicate the order in which the smectic structures were first recognized and identified. The molecules (except in smectic D) are arranged in layers with their long axes parallel to each other. They can move in two directions in the plane and can rotate about one axis. Those within layers can be in neat rows or randomly distributed.
Smcctic liquid crystals may have structured or unstructured strata. Structured smectic liquid crystals have long-range order in the arrangement of molecules in layers to form a regular two-dimensional lattice. The most common of the structured liquid crystals is smectie B. Molecular layers are in well-defined order, and the arrangement of the molecules within the strata is also well-ordered. The long axes of the molecules lie perpendicular to the plane of the layers. In the smectic A structure, molecules are also packed in strata, but the molecules in a stratum are randomly arranged. The long axes of the molecules in the smectic A structure lie perpendicular to the plane of the layers. Molecular packing in the smectic C is the same as that in smectic A, except the molecules in the stratum are tilted at an angle to the plane of the stratum.
There is also a unique kind of liquid crystal known as the smectic D which is isotropic, but nevertheless shows three-dimensiontional order in the molecular packing of the structure. In the smectic-A mesophase, the director is perpendicular to the smectic plane, and there is no particular positional order in the layer. Similarly, the smectic-B mesophase orients with the director perpendicular to the smectic plane, but the molecules are arranged into a network of hexagons within the layer. In the smectic-C mesophase, molecules are arranged as in the smcctic-A mcsophase, but the director is at a constant tilt angle measured normally to the smectic plane.
As in the nematic, the smectic-C mesophasc has a chiral state designated C*. Consistent with the smectic C. the director makes a tilt angle with respect to the smeetic layer. The difference is that this angle rotates from layer to layer forming a helix. In other words, the director of the smectic C* mesophase is not parallel or perpendicular to the layers, and it rotates from one layer to the next