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

Abstract

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 (,2.5 mm 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 10u during this process in a manner analogous to coiling or uncoiling a constrained spring. This mechanism differs somewhat from others proposed in the literature.

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

“…The refractive index was determined from analysing the reflection spectra by the Berreman method [38]. During the experiment the rubbing direction of the cell was parallel to the plane of polarization of the incident light.…”

Capillary and winding transitions in a confined cholesteric liquid crystal

“…Previous and more recent works have analysed the problem using simple elastic models [17][18][19][20][21][22][23][24]. There are various experimental works that deal with these problems [25][26][27][28][29][30][31][32][33].…”

Light-induced pitch transitions in photosensitive cholesteric liquid crystals: Effects of anchoring energy

“…In particular, these discontinuities manifest themselves in a jump-like temperature dependence of selective light transmission spectra [9][10][11][12]. Different mechanisms behind the temperature variations of the pitch in CLC cells and hysteresis phenomena were discussed in Refs.…”