Mechanochromic Responses of Cholesteric Liquid Crystal Droplets with Nanoscale Periodic Helical Structures Showing Reversible and Tunable Structural Color

Abstract

Cholesteric liquid crystal (CLC) droplets with nanoscale periodic helical structures are omnidirectional under planar anchoring, and light is selectively reflected from the droplet center at all directions. However, when CLC droplets are too small, random light scattering dominates. In this regard, we present a facile method to increase the selective Bragg reflection of CLC droplets and demonstrate the unique mechanochromic responses of CLC-droplet-dispersed polymer films. The films are originally opaque and white, when the films are bidirectionally stretched, spherical CLC droplets are squeezed into oblate shape and the point defect at the center of each CLC droplet transitions to a ring defect, greatly increasing the selective Bragg reflection region and resulting in the appearance of structural color. By tuning the helical pitches via the chiral dopant concentration from 1.7 to 3.0 wt %, the desired structural color is achieved. Smart sensors show color changes in response to hydration and dehydration are designed, paving the way for the developments of CLC-based functional materials and smart devices.

Mechano-Optical Response Behavior of Polymer-Dispersed Cholesteric Liquid Crystals for Reversible and Highly Sensitive Force Recorders

“…Here, we developed a reversible mechanochromic recorder based on a polymer-dispersed cholesteric liquid crystal (PDCLC) as the sensing layer (Figure b). PDLC, i.e., liquid crystals anchoring in the voids of polymer network, have been extensively studied and used in a wide variety of applications, especially in displays, , smart windows , and intelligent devices. , However, none has reported the quantitative relationship between force and optical response or the application as a force recorder. As illustrated in Figure c, the device based on PDCLC is able to both store the color data with the removal of force and recover to the initial state via an electric field.…”

Polymer Dispersed Cholesteric Liquid Crystal Mixtures for Optical Time–Temperature Integrators

“…[21] As only a small reflecting spot is observed for spherical droplets, the transformation to oblate spheres improves the reflectivity as the flattened area of the oblate spheroids induced planar CLC alignment. [22] Although such photonic emulsions and PDCLC coatings have primarily been deployed for sensing, [25,26] lasing, [21,27] display applications, [13,28] printed temperature-responsive photonic coatings, and freestanding films, [29,30] even commercial products [1] have been realized based on thermosensitive photonic emulsions. Despite the stable performance of these emulsified PDCLC coatings, the use of a single CLC mixture limits the application range, hence combining multiple CLC mixtures is required to develop photonic PDCLC coatings with more complex optical responses.…”

Polymer-Dispersed Cholesteric Liquid Crystal under Homeotropic Anchoring: Electrically Induced Structures with λ1/2-Disclination

“…The application multiplicity of CLC dispersions is due to the vast diversity of the formed orientational structures, which are determined by the boundary conditions, the droplet’s shape, the CLC elasticity constants, the ratio of the droplet’s size d and the cholesteric intrinsic helix pitch characterized by the relative chiral parameter [ 16 , 17 , 18 , 19 , 20 , 21 ]. For example, luminescence or Bragg reflection of light by cholesteric depends on both the CLC pitch [ 22 , 23 , 24 , 25 ] and the orientation of the cholesteric layers in the droplet [ 3 , 26 , 27 ].…”