Design of Functional Materials based on Liquid Crystalline Droplets

Abstract

This brief perspective focuses on recent advances in the design of functional soft materials that are based on confinement of low molecular weight liquid crystals (LCs) within micrometer-sized droplets. While the ordering of LCs within micrometer-sized domains has been explored extensively in polymer-dispersed LC materials, recent studies performed with LC domains with precisely defined size and interfacial chemistry have unmasked observations of confinement-induced ordering of LCs that do not follow previously reported theoretical predictions. These new findings, which are enabled in part by advances in the preparation of LCs encapsulated in polymeric shells, are opening up new opportunities for the design of soft responsive materials based on surface-induced ordering transitions. These materials are also providing new insights into the self-assembly of biomolecular and colloidal species at defects formed by LCs confined to micrometer-sized domains. The studies presented in this perspective serve additionally to highlight gaps in knowledge regarding the ordering of LCs in confined systems.

Keywords: Liquid crystals; amphiphiles; chemically patterned microparticles; functional materials; micrometer-sized droplets; ordering transitions; self-assembly; topological defects.

Figure 1

(A–E) Bright field (left) and polarized light (middle) micrographs and corresponding schematic illustrations (right) of director configurations commonly observed for micrometer-sized droplets of nematic LC. The droplets displayed in the micrographs are 8-μm-diameter droplets of nematic 4′-pentyl-4-cyanobiphenyl (5CB) dispersed in either (A) water or (B–E) aqueous dispersions of the biological lipids. (F) Schematic illustrations of other possible director configurations of micrometer-sized LC droplets. The solid black lines within the droplet boundaries in the illustrations represent the orientation of the LC director and the black spots represent defects. Double headed arrows in polarized light micrographs indicate the orientation of the crossed polarizers. Reproduced with permission.

Figure 2

(A) Experimental phase diagram of the preferred configuration of nematic droplets of E7 dispersed in a polyurethane matrix as a function of temperature (T) and droplet radius (R). T_c is the nematic-to-isotropic clearing temperature (303 K for this system). (B) Schematic illustrations of the director configuration predicted as a function of decreasing R. Reproduced with permission.

Figure 3

(A) Procedure used to prepare LC droplets of predetermined sizes within polymeric multilayer capsules. Polymeric capsules were prepared by the sequential deposition of polyelectrolytes onto silica particles and the subsequent etching of silica. The resulting polymeric capsules were filled with LCs (see the text for details). (B–D) Chemical structures of the (B) 5CB and the polyelectrolytes used to create polymeric multilayer capsules ((C) PSS and (D) PAH). (E) Bright field micrographs of polymer-encapsulated 5CB droplets obtained using silica templates with diameters of 10 ± 0.22, 8 ± 0.20, 5 ± 0.19, 3 ± 0.18, 1 ± 0.04, and 0.7 ± 0.08 μm, respectively. All scale bars are 3 μm. Reproduced with permission.

Figure 4

Size-dependent ordering within LC. (A and B) Schematic illustrations of the bipolar (A) and uniform (B) ordering of LCs predicted from scaling arguments. (C, F, H, K) Polarized light and (D, G, I, L) bright field optical micrographs of polymer-encapsulated 5CB droplets with (C, D) diameters of 8.0 ± 0.2 μm and bipolar LC ordering, (F–I) diameters of 1.0 ± 0.2 μm and preradial LC ordering ((F and G) show the end on views of the preradial ordering; (H and I) show side views), and (K, L) diameters of 0.70 ± 0.08 μm and radial LC ordering. Point defects in the LCs are indicated by white arrows. Schematic illustrations (E, J, and M) show bipolar, preradial, and radial ordering of the LC droplets, respectively. The scale bars are 2 μm for (C, D, and F–I), and 1 μm for (K, L). Reproduced with permission.

Figure 5

Equilibrium director configurations observed in nematic LC droplets dispersed in a poly(vinyl butyral) matrix containing lecithin. The top row shows schematic illustrations of the configuration of the LC within each droplet, and the middle and bottom rows, respectively, show the corresponding bright field and polarized light micrographs of the 5CB droplets. The weight percent of lecithin doped into the polymeric matrix is indicated below each polarized light micrograph. Note that the scale-bar differs between figures. Double headed arrows in bright field micrographs indicate the orientation of the single polarizer, while double headed arrows in polarized light micrographs indicate the orientation of the crossed polarizers. Reproduced with permission.

Figure 6

(A) Concentration-dependent equilibrium director configurations induced by an adsorbate-driven change in the anchoring energy of LC droplets coated by polymeric multilayer capsules composed of alternating layers of PSS and PAH (Figure 3C and D respectively). The change in surface anchoring of the LC droplet (from tangential to perpendicular) was achieved by equilibrating 8.0 ± 0.2-μm-diameter, polymer-encapsulated 5CB droplets with aqueous solutions containing SDS at concentrations that ranged from 0 to 1 mM (as indicated). The top row shows schematic illustrations of the configuration of the LC within each droplet, and the middle and bottom rows, respectively, show the corresponding bright field and polarized light micrographs of the 5CB droplets. (B) Molecular structure of sodium dodecyl sulfate (SDS). Reproduced with permission.

Figure 7

SDS concentrations (c) in aqueous solution that caused LC droplets of the indicated size (d) to assume a radial configuration. Reproduced with permission.

Figure 8

Endotoxin-induced bipolar-to-radial ordering transitions in water-dispersed nematic 5CB droplets. (A–C) Molecular structure of (A) the glycolipid tail of endotoxin, lipid A, (B) 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine (DLPC) and (C) 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC). (D) Bulk concentrations of lipids (endotoxin, DLPC, or DOPC) or surfactants (SDS, Figure 6B) in aqueous solution required to induce bipolar-to-radial ordering transitions in 5CB droplets. (E) Pathway of transition states observed during endotoxin-induced ordering transitions. The top row shows schematic illustrations of the configuration of the LC within each droplet, and the middle and bottom rows, respectively, show the corresponding bright field and polarized light micrographs of the 5CB droplets. The ordering transitions were triggered by exposure of 8-μm-diameter 5CB droplets to 10 pg/mL of endotoxin. Contrary to the pathway observed for adsorbate-induced ordering transitions (Figure 6), no disclination rings were observed during the transition of the droplets from bipolar to radial configurations in the presence of 10 pg/mL of endotoxin. Reproduced with permission.

Figure 9

(A) Influence of LC droplet size on endotoxin-induced bipolar-to-radial ordering transitions in micrometer-sized droplets of nematic 5CB. The droplets were exposed to either an aqueous solution containing 100 pg/mL endotoxin (solid bar) or the same aqueous solution without endotoxin (open bar). No 5CB droplets were observed to exhibit a radial configuration in the absence of 100 pg/mL endotoxin above a diameter of 6 μm. (B) Confocal fluorescent micrograph showing localization of BODIPY-labeled endotoxin at the center of a 5CB droplet in the radial configuration. Reproduced with permission.^,

Figure 10

(A–D) Synthesis of non-spherical and chemical-patterned particles. (A) Emulsification of LC. (B) After emulsification, formation of bipolar nematic droplets with either one or two fluorescent PS colloids located at the poles. (C) After polymerization of the monomer within the droplets, formation of spherical particles. (D) Upon the extraction of the LC from the polymerized droplets, formation of non-spherical particles. (E, G) Combined fluorescence and bright field micrographs of bipolar nematic droplets exhibiting one or two fluorescent PS colloids adsorbed at their surfaces. (I, K) Combined fluorescence and bright field, of non-spherical particles exhibiting one or two fluorescent PS particles. Scale bars: 5 μm. (F, H, J, L) The corresponding schematic illustrations of the director field configuration (dark lines) within spherical nematic droplets and non-spherical particles; the blue spots represent the PS colloids (at the poles). Reproduced with permission.