Prolate and oblate chiral liquid crystal spheroids

Abstract

Liquid crystals are known to exhibit intriguing textures and color patterns, with applications in display and optical technologies. This work focuses on chiral materials and examines the palette of morphologies that arises when microdroplets are deformed into nonspherical shapes in a controllable manner. Specifically, geometrical confinement and mechanical strain are used to manipulate orientational order, phase transitions, and topological defects that arise in chiral liquid crystal droplets. Inspired by processes encountered in nature, where insects and animals often rely on strain and temperature to alter the optical appearance of dispersed liquid crystalline elements, chiral droplets are dispersed in polymer films and deformation induced by uniaxial or biaxial stretching. Our measurements are interpreted by resorting to simulations of the corresponding systems, thereby providing an in-depth understanding of the morphologies that arise in these materials. The reported structures and assemblies offer potential for applications in smart coatings, smart fabrics, and wearable sensors.

Fig. 1. Phase transitions of high-chirality LC droplets in aqueous polymer solutions.

(A) Molecular structure of the chiral dopant, S-811; (B) chiral LC droplets (MLC 2142 and 36.25 wt % S-811) dispersed in 10 wt % PVA aqueous solution. (C) Cross-polarized reflection mode micrographs presenting phase transitions of a 25-μm chiral LC droplet in 10 wt % high-molecular weight (HMw) PVA solution cooled from the isotropic phase at 0.2°C/min. (D) Effect of polymer type on the phase-transition temperatures of chiral LC droplets [MLC 2142/S-811 63.75/36.25 (wt %)] dispersed in aqueous solutions (Chol, cholesteric (chiral); BPI, blue phase I; and BPII, blue phase II). Photo credit: Monirosadat Sadati, The University of Chicago.

Fig. 2. Chiral LC droplets embedded in polymer films.

(A and B) SEM micrographs of the cross section (side view) of high-chirality LC droplets embedded in dry PAA and PVA films, respectively. (C and D) Polarized optical microscopy (POM) of high-chirality LC droplets embedded in dry PAA and PVA films, respectively. (E) POM of a low-chirality LC droplet (6 wt % chiral dopant, N = 5) in a dry PVA film. (F) POM and (G and H) directors obtained from simulations for a low-chirality nematic oblate with D_x = D_y = 4 μm, D_z = 2 μm and pitch = 630 nm (colors are from blue to red according to $\widehat{n}\bullet \widehat{z}$). The splay and bend elastic distortions are shown in blue (S_SB > 0.002) and in yellow (S_SB < −0.002), respectively. Defects are shown in black (isosurface for S = 0.5). (I) POM of a 60-μm high-chirality LC droplet in dry PVA films. (J and K) POM, directors, and defects from simulations for a nematic oblate with D_x = D_y = 4 μm, D_z = 2 μm and pitch = 258 nm. (H) and (L) represent side views of the directors along the y axis. (C) to (F), (I), and (J) are taken in the xy plane.

Fig. 3. Transformation of the RSS structure to the helical configuration upon uniaxial deformation.

(A) Cross-polarized reflection mode micrographs of a uniaxially stretched low-chirality LC oblate (N = 5) droplet embedded in a dry high–molecular weight PVA film. (B) Polarized light micrographs and (C) director fields obtained from simulations for chiral nematic ellipsoids with pitch = 630 nm during uniaxial stretch along the x axis. The axis lengths for the ellipsoids from top to bottom are D_x = D_y = 4 μm, D_z = 2 μm; D_x = 2D_y, D_z = 1.59 μm; D_x = 3D_y, D_z = 1.39 μm; and D_x = 4D_y, D_z = 1.287 μm, respectively. The RSS-like structure becomes destabilized upon uniaxial deformation of the oblate, and a helical configuration becomes energetically more favorable from D_x = 3D_y. First and last columns in panel (C) represent the top view and side view of the director fields, receptively. The director field is colored according to its projection on the x axis. Defects are shown in black (isosurface for S = 0.5). The experimental and simulated cross-polarized micrographs presented in images (A) and (B) are taken in the xy plane.

Fig. 4. Uniaxial deformation of the high-chirality LC destabilizes the RSS structure and shifts the light of the reflection band to shorter wavelengths.

(A) Cross-polarized reflection mode micrographs and the reflectance spectra of a high-chirality LC sphere droplet embedded in a PAA polymer film before and after strain (ε = 2.0). a.u., arbitrary units. (B) Cross-polarized reflection mode micrographs and the reflectance spectra of a high-chirality LC oblate embedded in a PVA polymer film before and after strain (ε = 2.75). (C and D) Cross-polarized images of the high-chirality LC confined in an oblate and uniaxially stretched droplet obtained by simulations. (E) Top and side view (with defect line) of the director field configuration along the x axis. Director is colored based on its projection onto the z axis, and the order parameter defect lines (black) correspond to S = 0.6. Blue regions correspond to a director alignment on the xy plane; near the border, the director field shows the typical cholesteric behavior (deformed RSS).

Fig. 5. BP transitions in an initially deformed highly chirality droplet.

(A) Cross-polarized reflection mode micrographs of a stretched high-chirality LC droplet (ε = 2.5) at different phases: cholesteric, BPI, and BPII. (B) Measured reflectance spectra of the stretched high-chirality LC droplet in different phases: cholesteric, BPI, and BPII. Snapshots along the z axis of the simulated defect structure of a uniaxially stretched (C) BPI and (D) BPII, from an initial oblate geometry to x = 3y. (E) Scalar order parameter along the z axis for BPI and BPII, respectively. The scalar order parameter of the initial oblate-like geometry (solid black line) has been compared with two representative stretched geometries (red and blue dashed lines).