Cholesteric Flakes in Motion Driven by the Elastic Force from Nematic Liquid Crystals

Abstract

The paper presents a methodology to control the motion and orientation of suspended reflective cholesteric flakes in a nematic liquid crystal (LC) matrix. The flakes exhibit a dielectric anisotropy which controls their alignment with their in-plane axes parallel to an external electrical dc field. The elastic forces imposed by the LC host affect the switching behavior of the flakes and take care of the realignment to the planar state as soon as the dc field is switched off. When the LC host has a positive dielectric anisotropy, the switching voltage of the flakes is reduced by a factor of 2 in comparison with a LC host with negative dielectric anisotropy or in comparison with an isotropic host. We discovered that the LC host further regulates the back relaxation of cholesteric to return to the planar state upon retrieving the electric field. Whereas, in the isotropic fluid, flakes do not exhibit a preferred orientation when relaxed. Based on this newly proposed principle, we demonstrated its application as an optical switch for smart windows. Depending on the pitch of the cholesteric helix of the flakes, the light of a preset wavelength is reflected. Upon application of an electric field, the embedded flakes rotate their planes perpendicular to the substrate and consequently the incident light becomes fully transmitted without reflection or scattering of light.

Responsive Photonic Liquid Crystalline Flakes Produced by Ultrasonication

“…In all previously reported cases where ChLC flakes are used as optically active components for flexible reflective displays, [ 17,19,21,26,37 ] windows, [ 29 ] circular polarization‐dependent security labels, [ 11 ] or as effect pigments for decorative coatings, [ 14 ] the cholesteric material is based on a densely crosslinked polymer network with a static photonic structure.…”

“…LC network flakes have found use in display technology [ 17–21 ] as well as decorative elements. [ 22,23 ] A variety of manufacturing methods have been described, including cryogenic [ 22 ] and mechanical [ 24,25 ] fracturing, and soft lithography.…”

“…[ 22,23 ] A variety of manufacturing methods have been described, including cryogenic [ 22 ] and mechanical [ 24,25 ] fracturing, and soft lithography. [ 17,21,26 ] Ultrasonication, a technique used in cellular biology and chemistry to release cell contents by rupturing cell membranes [ 27 ] and as method to deaggregate nanoparticles, [ 28 ] has been presented once, but not for responsive photonic LC networks. [ 29 ] Herein, we present ultrasonication as a viable method of reducing the ChLCN film dimensions while preserving the stimulus‐responsive color change capabilities of the particles and show thorough characterization of the material along all steps of fabrication and incorporation in a binder material.…”