Effect of surface alignment layers on the configurational transitions in cholesteric liquid crystals

Abstract

To understand the mechanism behind the pitch change during the transient planar to the planar state transition of a cholesteric liquid crystal ͑while it is relaxing from the homeotropic state to the planar state͒, we studied a more controlled situation of pitch change in a thermochromic liquid crystal. We studied the pitch change in the liquid crystal, as a function of temperature, under various surface alignment conditions such as homogeneous and homeotropic surface alignments. In a cell with homogeneous surface treatment, the liquid crystal showed a discontinuous pitch change, while the pitch change was rather continuous in a cell with homeotropic treatment. The numerical simulations also showed an existence of continuous pitch change in a homeotropic cell, but not in a homogeneous one. This led to a prediction that when a cholesteric material is switched from the homeotropic state to the planar state, the transition may be much faster for the homeotropically treated cell as compared to the homogeneously treated cell. We performed experiments to verify this prediction with a chiral nematic mixture. Indeed, the homeotropic to planar state transition time of a homeotropic cell was 5 msec, much faster compared to about 200 msec for corresponding transition in a homogeneous cell.

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

“…As the temperature is changed, so-called pitch jumps occur which are easily observable as changes in the selective reflection wavelength. There have been several reports of thermal hysteresis in the pitch jumps [2][3][4], as well as theoretical treatments that relate the hysteresis to parameters including surface anchoring and cell thickness [5][6][7][8].…”

Effects of chiral dopant on electro-optical properties of nematic liquid crystal cells under in-plane switching and non-uniform vertical electric fields

“…When the temperature rises, the reflection wavelength of the cell will become shorter due to the thermodynamic vibrations effect on the original SH structure. The increased thermal vibration strength results in increased twisting power variation between the neighboring molecular structures, and the helical pitch length of the N*LC cell will be reduced [5,52]. Such a thermal vibration effect also decreases the light intensity of the Bragg reflection, which is attributed to the fact that the director of the helical axis is disturbed (i.e., it is no longer perpendicular to the substrate surface), leading to the light scattering.…”

P‐132: Influence of Polymerization Conditions on Response Time of Encapsulated Cholesteric LCs

“…It has been reported that the different alignment condition may result in a various response time from homeotropic state to planar state [3]. In this paper, we demonstrate the encapsulated CLC display via photo induced phase separation and investigate the influence of various UV curing conditions and cell gaps to response time of CLC display…”