Effects of controllable defects on the bistable properties of cholesteric liquid crystal materials and the potential application

The worldwide energy crisis necessitates the urgent need for energy conservation, especially in buildings whose energy consumption has already surpassed that of transportation and industrial sectors. Most of the energy loss in buildings is related to windows because their heat transfer coefficient is different from other building fabrics. Therefore, smart window technologies are proposed to eliminate energy usage, among which cholesteric liquid crystals show great potential due to their electrically controlled bistable properties. Cholesteric liquid crystals are characterized by two stable states: (1) the planar state that shows selective reflection or high light transmittance and (2) focal conic state that scatters incident light and shows opaque appearance. To realize a bistable state in which both the planar state and the focal conic state could be maintained without an external field is of vital importance for the application, since it could save considerable energy. In this study, oily streak defects and stability of the focal conic state are controlled by introducing a type of bent molecular materials. Effects of various factors on the bistable states are investigated, and the potential applications are explored. Results show that the bent molecule CB7CB could diminish the oily streak defects in the planar state and reduce the domain size of the focal conic state because it has a smaller bent elastic constant. The electro-optical test implies that bistable states are acquired in the composite containing CB7CB. Factors such as the anchoring strength of the alignment layer and cell thickness are then investigated and results reveal that strong anchoring and thinner cell thickness are not favored by the focal conic state. Moreover, bistable states could be stabilized by introducing a small amount of polymer followed by polymerization. Based on the above guidelines, colorless and colorful bistable light shutters are demonstrated, which could maintain both transparent and opaque states without any external electric field. The light shutter saves more energy than the one based on polymer-dispersed liquid crystals. Other potential applications, such as bistable displays and electronic label could also be realized based on the bistable property.