An experimental investigation of discrete changes in pitch in a thin, planar chiral nematic device

The phenomenon whereby sudden changes in helicoidal pitch occur in thin chiral nematic devices with strong surface anchoring has been studied experimentally. This has been done with the aim of examining the underlying process in the context of existing theory. Thin devices (similar to 2.5 mu m thick) containing two commercially available chiral nematic liquid crystals were employed in the study. The samples were chosen for their dependence of pitch on temperature; one exhibits a very slow increase in pitch with increasing temperature (95.6% of the Merck material BL131 in BL130), while the second shows the more common rapid reduction in pitch with increasing temperature (the Merck mixture TM1001). High resolution reflection spectra were obtained for the devices and a numerical fitting algorithm, based on the Berreman 464 matrix technique, provided accurate information on changes in the pitch, refractive indices, device thickness and changes in the surface director across the pitch jump region. We demonstrate that changes in refractive index and device thickness are negligible in analysis of the reflection spectra across the pitch jumps. We further show that the pitch changes discontinuously at the pitch jump, with no variation in its value as the process occurs. We find evidence that the surface directors also change by less than 10 degrees during this process in a manner analogous to coiling or uncoiling a constrained spring. This mechanism differs somewhat from others proposed in the literature.