Stabilized electrically induced Helfrich deformation and enhanced color tuning in cholesteric liquid crystals

Abstract

When subjected to an AC electric field perpendicular to its layers, the cholesteric planar state may undergo a periodic layer undulation, known as the Helfrich deformation, which generates a color change of the reflected light. The Helfrich deformation of regular cholesteric liquid crystals is, however, unstable and easily taken over by the focal conic state, and thus the color tuning range is narrow. We demonstrated that the Helfrich deformation can be stabilized in a large electric field region by doping a bent dimer with an anomalously small bend elastic constant and dispersing a small amount of polymer network. By varying the dimer concentration, we were able to systematically change the bend elastic constant and thus verify the theoretical prediction of the undulation threshold electric field and periodicity. We also achieved reflectance tuning with electric field less than 2 V μm and color tuning covering the entire visible light region with electric field as low as 4 V μm.

Hierarchical organization of structurally colored cholesteric phases of cellulose via 3-d printing

“…[38] This observation is consistent with the behavior found in other polymers and LCs, that were subjected to broad volume changes, applied strain, and electric fields, in which shear-induced alignment and/or periodic controllable wrinkling have been demonstrated. [37][38][39][40][41][42][43][44][45][46] This wrinkling can be used to modify the structure of a sample, targeting properties which can be as simple as physical surface roughness or more complex like diffraction of light. Such features can be used to homogenize optical behavior, a phenomenon which was observed in the presented case.…”

“…The 3-d printed HPC filaments exhibited remarkable color uniformity and low angular dependence through a wide range of angles (Figure 3 We attributed the formation of these wrinkles to the Helfrich-Hurault deformation where the cholesteric layers went through periodic patterning due to the applied gradients (e.g., thermal, electrical and mechanical). [16,17,[37][38][39] In our system, such deformation was a result of the competition between the polymer chains that were forced to follow the shear direction and the cholesteric layers that tried to retain their helicoidal structure through intermolecular interactions. This deformation of the chains is enabled by the polymer chain elasticity of the HPC and relatively low viscosity of the suspension.…”

Control of Large-Area Orderliness of a 2D Supramolecular Chiral Microstructure by a 1D Interference Field

“…This 2D microgrid structure looks like a Helfrich deformation, which often occurs in pure CLC systems under the application of a proper electric field. 39,40 However, this phenomenon has never been discovered in polymerizable CLC systems during photopolymerization without an external electric field.…”

Nanoscale hyperspectral imaging of tilted cholesteric liquid crystal structures

“…, however it appears that the topic of planar films with a spatially-variable orientation of the helical axis gained some revival in 2017-18 by inspiring different research. [20][21][22][23][24][25][26][27] In 2011, tuning the orientation of the twist and, in return, of the macroscopic color of a CLC slab with a polygonal texture was demonstrated. 28 Polygonal textures (Figure 2(a)), made of an array of micrometer-scale polygonal cells, can be observed in flat CLC films when the helical axis is strongly tilted with respect to the substrates.…”