Anchoring strength measurements of a lyotropic chromonic liquid crystal on rubbed polyimide surfaces

Techniques to achieve planar anchoring of the director of a lyotropic chromonic liquid crystal (LCLC) have been developed only recently. These techniques range from the very weak anchoring achieved by simply scratching the glass with a very fine abrasive to the very strong anchoring achieved with lithographic techniques. A possible alignment technique is to use rubbed polyimide (PI) alignment layers as is routinely done to align thermotropic liquid crystals (TLCs). The anchoring strength of the LCLC disodium cromoglycate (DSCG) on different rubbed PI alignment layers is measured using a π/2 twist cell and optical polarisation techniques. Although the anchoring strength is not as large as can be achieved with lithographic techniques, it is larger than scratched glass for one PI. Just as important, rubbed PI provides a much more uniform surface than is possible with scratched glass. Interestingly, addition of a small amount of surfactant to the LCLC, which results in improved alignment, does not increase the anchoring strength. Weak anchoring of LCLCs can also be achieved with a photo-patterned dye surface layer. Finally, atomic force microscopy measurements of the surface combined with prior theoretical work linking surface features and anchoring strength predict anchoring strengths consistent with experiment.