The orientational assembly of glass rods (3 x similar to 15 mu m) in nematic, twisted nematic, and cholesteric liquid crystal cells was observed and quantified with optical microscopy. At this size, the rods were affected strongly by gravity and sedimented to the bottom of the cells. Temporal visualization of the sedimentation process (induced by flipping the cell over) shed insight into the effect the liquid crystal order had on the glass rod orientation. For nematic and twisted nematic geometries, the glass rods were aligned parallel to the local director orientation. Control experiments indicate that the rod alignment is not due to capillary flow induced artifacts from fabrication of the sample or due to interactions with the buffed substrates. As evidence, the glass rods rotated 90 degrees as they fell from the top to the bottom of a twisted nematic cell. More complex behavior was observed for cholesteric cells depending on the pitch length. A computational model was developed to predict the elastic energy of the system as a function of the angle between the long axis of the glass rod and the cholesteric liquid crystal director. The model predicted that the elastic energy of the system was minimized when the glass rods remained parallel to the cholesteric liquid crystal director when the pitch was sufficiently long, which agrees with experimental results. (C) 2011 Optical Society of America
Surface Limitations to the Electro-Mechanical Tuning Range of Negative Dielectric Anisotropy Cholesteric Liquid Crystals01/01/2012
Recent work on negative dielectric anisotropy cholesteric liquid crystals (CLCs) showed that externally applied dc voltages resulted in blue tuning of the reflection band position up to 20% of its original position. These results also showed that the observed shift in reflection band position was not caused by a direct interaction between the CLC and the applied voltage, but indirectly through electromechanical stresses that deformed the conductive glass substrates, in turn deforming the liquid crystal. In this work, the goal is to clarify that the major limiting factors on the tuning range limit result from the magnitude of the surface anchoring energy and surface induced hysteresis effects. An analytic solution for the tuning range limit and its dependence on the surface and bulk properties is derived that agrees well with the experimental data. Using this model, it was shown that tuning range limits in excess of 35% of the notch position should be expected with typically available alignment materials, and that with proper CLC/surface optimizations, values in the range of 75% are possible.