 |
Charles Rosenblatt
Professor of Physics and Macromolecular Science, Case Western Reserve University S.B., Massachusetts
Institute
of Technology (1974) E-mail: rosenblatt@case.edu |
Most people are first drawn to liquid crystals by their beautiful optical textures: stars, curves, splotches, and zig-zags, all in a palette of colors rivaling the most ostentatious paintings of modern art. On closer and more scientific inspection one finds that liquid crystals are generally composed of rod-shaped molecules, which exhibit an intermediate degree of order between solid and liquid. For example, in the nematic phase the elongated molecules are orientationally ordered along a particular axis, with no long range positional order. Since the molecules are also optically anisotropic, the polarization state of light may be altered as it passes through the liquid crystal, facilitating the brilliant display of colors. At lower temperatures partial positional order may set in, such that the molecules behave like a liquid in two dimensions, and are nearly solid-like in the third direction. This smectic phase has more than a dozen variations, each with its own characteristic symmetry.
The study of liquid crystals touches on a multitude of physics disciplines. One of my central interests has been phase transitions from one liquid crystalline state to another. The goal of this work is to elucidate the fundamental character of these transitions, and their relationships to other phase changes in nature. For example, the transition from a completely isotropic phase to the nematic phase is ordinarily discontinuous and involves a latent heat. Under certain conditions, however, the transition may become continuous, such that the pretransitional behavior associated with the specific heat, the order parameter susceptibility, etc. is characteristic of a rather specialized model: a five component spin in three spatial dimensions. We need not limit ourselves to three spatial dimensions, however. Ultra-thin, free-standing liquid crystal films can be drawn across an open frame, comparable to a single piece of paper stretched across a football field! In this configuration my group investigates not only phase transitions in two spatial dimensions, but surface phenomena as well. The crossover region from two to three spatial dimensions may be achieved by investigating thicker films or liquid crystals in highly porous glass.
One of the most important factors in the physics of liquid crystals is symmetry. For example, a certain class of molecules that lacks inversion symmetry may exhibit ferroelectric or even antiferroelectric behavior, as well as a phenomenon known colorfully as the Devil's Staircase. Individual liquid crystalline molecules may also be chemically attached to form a long linear chain, radically altering the viscoelastic properties of the material. If the molecules are instead attached in a cyclic arrangement, the structure can exhibit a biaxial nematic phase, displaying fascinating optical, magnetic, and electrical traits.
Liquid crystals exhibit fascinating properties on very short length scales, the so-called "nanoscopic" scale. Using the stylus of an atomic force microscope, we scribe patterns onto a polymer-coated substrate as small as 10 nanometers in length, approximately one-ten thousandth the width of a human hair. The liquid crystal molecules are forced to align parallel to the scribing direction, allowing us to study phase transitions and elastic behavior on very tiny length scales. One of the more exciting aspects of this work is our use of near field scanning optical microscopy to create three dimensional images of the liquid crystal (“nanotomography”), with resolution of order 10 nm.
In addition to understanding the basic science of these materials, we are also involved in technology. For example, we are using liquid crystals as a means of steering laser light from one optical fiber to another, as well as to create images with sub-micrometer pixel sizes. During the early '90s, my colleagues Rolfe Petschek, Mike Fisch, Karl Crandall and I created a new liquid crystal display architecture known as the "vertically aligned cholesteric" display.
To study these myriad liquid crystalline properties we possess a battery of high resolution and often unique experimental tools. For example, we use quasi-elastic light scattering to examine viscoelastic properties and nonhydrodynamic behavior, critical order parameter fluctuations, and ionic diffusion. Intense magnetic and electric fields, combined with optical techniques such as ellipsometry, are used to explore intermolecular interactions, interactions with surfaces, and novel effects arising from symmetry considerations. Atomic force, scanning tunneling, and near field optical microscopy are used to examine interfacial phenomena, and we use dielectric and magnetic susceptibility measurements to determine the elastic properties as a function of parameters such as molecular architecture and temperature.
A very different sort of scientific program involves microgravity of fluids. Using a "Faraday" magnet, which is capable of generating a uniform magnetic force over a region of several cubic centimeters, we are able to apply an upward force on a fluid that exactly cancels out the downward gravitational force. For all practical purposes, the fluid becomes weightless, allowing us to study a variety of phenomena. For example, a fluid that is tethered to two solid supports is known as a "liquid bridge." Real-life examples include the fluid in the lungs, oil in porous rock, and water that wets a fabric. By studying these fluids in an effective zero-gravity environment, one learns about fluid stability, surface tension, and dynamics. Because the magnet force is controllable with time, we have performed experiments where we have oscillated "gravity," and have examined the collapse of a cylindrical liquid bridge when gravity is suddenly "turned on." Another research topic involves the instability that occurs when one places a dense fluid on top of a lighter fluid. Under ordinary gravity this arrangement is unstable, but one can stabilize this configuration by use of magnetic levitation of the (magnetic) heavier fluid. On turning off the field, an instability develops, and the heavier fluid falls to the bottom in a very complex manner. This phenomenon, which is known as a “Rayleigh-Taylor instability,” occurs in exploding supernovae, inertial confinement in fusion processes for purposes of energy generation, and even in vinegar and oil salad dressing!