(Go immediately to pictorial tutorial)
Ordinary fluids are isotropic in nature: they appear optically,
electrically, etc. to be the same from any perspective. Although the
which comprise the fluid are generally anisometric in shape, this
generally plays little role in anisotropic macroscopic
(aside from viscosity). Nevertheless, there exists a large class of highly
anisometric molecules which gives rise to unusual, fascinating, and
technologically relevant behavior. There are many interesting
for study, including polymers, micelles, microemulsions, and materials
of biological significance, such as DNA and membranes. Although at
we have investigated all of these materials, our primary effort centers
on liquid crystals.
Liquid crystals are composed of moderate size organic molecules
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
molecules can exhibit orientational order, such that all the axes line
up in a particular direction. In consequence, the bulk order has
influences on the way light and electricity behave in the material. For
example, if the direction of the orientation varies in space, the
of the light (i.e., the polarization) can follow this variation. A
application of this phenomenon is the ubiquitous liquid crystal
now comprising a $15b annual industry world-wide. Under other
the molecules may form a stack of layers along one direction, but
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
of physical parameters such as susceptibility and heat capacity, will
"pretransitional behavior." Based solely on symmetry, this behavior may
be related to other physical systems, such as superconductivity,
or superfluidity; this is the so-called "universality" of these phase
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
in the presence of intense magnetic and electric fields, and the
of confining these materials in spaces not much larger than the
themselves. By observing this behavior, we learn not only about the
material under consideration, but about the global properties of
fluids and their relationships to other physical systems. Finally, we
point out that although our research is primarily fundamental in
determining critical exponents, surface potentials, induced
etc., a small but important component of our effort involves
For example, we have developed a new liquid crystal display
which is being developed for commercialization by American industry.
is a symbiotic approach to research, and has been an intellectual
to our effort.
Typical Liquid Crystalline Molecules:
Methoxybenzilidene Butylanaline (“MBBA”)
p-decyloxybenzylidene p'-amino 2-methylbutylcinnamate
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
the macroscopic behavior of the system, but will overlook many nuances
and sometimes important macroscopic features. For example, the chiral
of DOBAMBC [above] permits the molecule to exhibit a "ferroelectric
[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.
In the isotropic phase the moelcules are randomly aligned and exhibit
long range order.
The isotropic phase has a low viscosity and will often appear to be
clear. There is no long range positional or orientational order of the
molecules, although this sort of order may exist on very short length
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.
The molecules in the nematic phase are oriented on average
a particular direction. In consequence, there is a macroscopic
in many material properties, such as dielectric constants and
indices. This is the phase which is used in many liquid
crystal devices (e.g., the "twisted nematic" cell), because the
orientation may be manipulated with an electric field, and the
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
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
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
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
rapidly switching electrooptic shutters, with response times in the
Antiferroelectric liquid crystals are similar to ferroelectric liquid
although the molecules tilt in an opposite sense in alternating layers.
In consequence, the layer-by-layer polarization points in opposite
These materials are just beginning to find their way into devices, as
are fast, and devices can be made "bistable."