Liquid Crystal Primer

(Go immediately to pictorial tutorial)

Ordinary fluids are isotropic in nature: they appear optically, magnetically, electrically, etc. to be the same from any perspective. Although the molecules which comprise the fluid are generally anisometric in shape, this anisometry generally  plays little role in anisotropic macroscopic behavior (aside from viscosity). Nevertheless, there exists a large class of highly anisometric molecules which gives rise to unusual, fascinating, and potentially technologically relevant behavior. There are many interesting candidates for study, including polymers, micelles, microemulsions, and materials of biological significance, such as DNA and membranes. Although at times we have investigated all of these materials, our primary effort centers on liquid crystals.

Liquid crystals are composed of moderate size organic molecules which tend to be elongated and shaped like a cigar, although we have studied, and the literature is full of variety of other, highly exotic shapes as well. Because of their elongated shape, under appropriate conditions the molecules can exhibit orientational order, such that all the axes line up in a particular direction. In consequence, the bulk order has profound influences on the way light and electricity behave in the material. For example, if the direction of the orientation varies in space, the orientation of the light (i.e., the polarization) can follow this variation. A well-known application of this phenomenon is the ubiquitous liquid crystal display, now comprising a $15b annual industry world-wide. Under other conditions the molecules may form a stack of layers along one direction, but remain liquid like (in terms of the absence of translational order) within the layers. As the system changes from one of these phases to another, a variety of physical parameters such as susceptibility and heat capacity, will exhibit "pretransitional behavior." Based solely on symmetry, this behavior may be related to other physical systems, such as superconductivity, magnetism, or superfluidity; this is the so-called "universality" of these phase transitions.

Using a battery of optical techniques, in addition to dielectric and certain surface probes, our research centers on the role of symmetry on liquid crystalline phases and phase transitions, how these systems behave in the presence of intense magnetic and electric fields, and the effects of confining these materials in spaces not much larger than the molecules themselves. By observing this behavior, we learn not only about the particular material under consideration, but about the global properties of anisotropic fluids and their relationships to other physical systems. Finally, we should point out that although our research is primarily fundamental in nature, determining critical exponents, surface potentials, induced polarizations, etc., a small but important component of our effort involves technology. For example, we have developed a new liquid crystal display architecture which is being developed for commercialization by American industry. This is a symbiotic approach to research, and has been an intellectual stimulation to our effort.


Typical Liquid Crystalline Molecules:

Methoxybenzilidene Butylanaline (“MBBA”)
p-decyloxybenzylidene p'-amino 2-methylbutylcinnamate ("DOBAMBC")
 

A Physicist’s View:

For many applications, a liquid crystal molecule is often pictured as a rod. This view will often provide important qualitative information about the macroscopic behavior of the system, but will overlook many nuances and sometimes important macroscopic features. For example, the chiral nature of DOBAMBC [above] permits the molecule to exhibit a "ferroelectric phase" [see below].  The size of the polarization and the response of the molecule to an applied voltage depends on the chemical structure of the molecule, largely the carbonyl [C=O] group near the chiral carbon.


Isotropic Phase

  • In the isotropic phase the moelcules are randomly aligned and exhibit no long range order.

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    The isotropic phase has a low viscosity and will often appear to be crystal clear. There is no long range positional or orientational order of the molecules, although this sort of order may exist on very short length scales of order tens of Angstroms, corresponding to a few molecular distances. For all practical purposes, the isotropic phase macroscopically appears to be like any other isotropic liquid such as water.


    Nematic Phase

     
    The molecules in the nematic phase are oriented on average along a particular direction. In consequence, there is a macroscopic anisotropy in many material properties, such as dielectric constants and refractive indices. This is the phase which is used in many liquid crystal devices (e.g., the "twisted nematic" cell), because the average orientation may be manipulated with an electric field, and the polarization of light will follow the molecular orientation as it changes through a cell. Typical response times are in the millisecond range.


    Smectic A Phase

    The smectic phase A, like the nematic, exhibits long range orientational order of the molecules. In addition, it exhibits a layer like structure in one dimension, and thus is often considered a two dimensional liquid (freedom of molecular motion within the layer) and a quasi one-dimensional solid (hindered translation from one layer to the next). The viscosity is rather high, and this phase is generally not useful for devices.


    (Tilted) Smectic C Phase

    In this phase the molecules are tilted with repsect to the layers, and the system is now "biaxial" in character.


    Smectic C* (Chiral) - Ferroelectric

    If the molecules are chiral, (lack inversion symmetry), Meyer, et al demonstrated on symmetry grounds that a polarization must exist parallel to the smectic layers and perpendicular to the molecules. The magnitude of the polarization is determined by molecular considerations, although its existence depends solely on symmetry. These materials can be used in rapidly switching electrooptic shutters, with response times in the microsecond range.


    Antiferroelectric LCs

    Antiferroelectric liquid crystals are similar to ferroelectric liquid crystals, although the molecules tilt in an opposite sense in alternating layers. In consequence, the layer-by-layer polarization points in opposite directions. These materials are just beginning to find their way into devices, as they are fast, and devices can be made "bistable."


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