Fine Structure Defects in Liquid Crystals
Principal Investigator - E.C. Gartland, Jr.

Topological defects appear in a vast array of physical situations, ranging from cosmological Big Bang scenarios to superfluids. Liquid crystals are unique systems where topological structures are accessible for detailed studies. Under the polarizing microscope, defects appear as singularities of the texture. One of the principal problems is the fine structure of the defect cores (scales 10 -100 molecular lengths). Computer simulations of the variation of tensorial order parameter reveal a surprising result -- the core of point defects is a ring rather than a singular point.   
(Top) Microscopic texture of point defects in nematic liquid crystal.
(above) Computer-generated tensorial order paramater represents the cross-section of the ring defect core. The magnification is x104 in comparison with the optical image.   

Oily Streak Defects at Work in New Displays
Principal Investigators - Oleg D. Lavrentovich and Deng-Ke Yang

A new technology of bistable cholesteric displays employs an electric field induced change in the cholesteric structure. The transformation starts with a buckling instability of the oily streaks: above some voltage threshold, the straight lines of defects suddenly lose their tension and start to expand and buckle as if their line tension were negative. Eventually the oily streaks transform the planar structure into a focal conic structure that strongly scatters light. A curious feature of the effect is that the buckling streaks preserve their width. A theoretical model explains the basic features of the phenomenon as a balance between the dielectric, elastic, and anchoring forces; according to the model, the width of the buckling streaks is fixed by the surface anchoring. With an understanding of the "oily streaks" we are able to pre-select liquid crystals and display cells to enhance the performance of bistable cholesteric reflective displays.     

Microscopic texture of "oily streaks" defects in a cholesteric liquid crystal before (above) and after (below) the electric field is applied.   

Magneto-Optic Response of Liquid Crystals
Principal Investigator - Peter Palffy-Muhoray

(Left) Photograph of magnet and nematic cell illuminated
from below. (Right) Computer simulation of pattern. Fortran
source code for the simulation is available via the ILCS web
server at http://alcom.kent.edu/ILCS   

Unlike most organic materials, liquid crystals respond readily to magnetic fields. The response originates in the anisotropic susceptibility of nematics, and the relative ease with which they undergo orientational deformations. Because of the large birefringence, these deformations are easy to observe.

Classroom Demonstration:
An interesting classroom demonstration is to place a small rare earth magnet on a nematic cell between crossed polarizers on an overhead projector. As the magnetic field reorients the liquid crystal, beautiful interference colors appear, indicating director deformations.

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