Free-energy model for nanoparticle self-assembly by liquid crystal sorting

Minimal Model of Solitons in Nematic Liquid Crystals

Solitons in liquid crystals have generated considerable interest. Several hypotheses of varying complexity have been advanced to explain how they arise, but consensus has not emerged yet about the underlying forces responsible for their formation or their structure. In this work, we present a minimal model for solitons in achiral nematic liquid crystals, which reveals the key requirements needed to generate them in the absence of added charges. These include a surface inhomogeneity, consisting of an adsorbed particle capable of producing a twist, flexoelectricity, dielectric contrast, and an applied ac electric field that can couple to the director's orientation. Our proposed model is based on a tensorial representation of a confined liquid crystal, and it predicts the formation of "butterfly" structures, quadrupolar in character, in regions of a slit channel where the director is twisted by the surface imperfection. As the applied electric field is increased, solitons (or "bullets") become detached from the wings of the butterfly, and then propagate rapidly throughout the system. The main observations that emerge from the model, including the formation and structure of butterflies, bullets, and stripes, as well as the role of surface inhomogeneity and the strength of the applied field, are consistent with experimental findings presented here for nematic LCs confined between two chemically treated parallel plates.

STEM Tomography of Au Helical Assemblies

Composite, helical nanostructures formed using cooperative interactions of liquid crystals and Au nanoparticles were studied using a scanning transmission electron microscopy (STEM) mode. The investigated helical assemblies exhibit long-range hierarchical order across length scales, as a result of the crystallization (freezing) directed growth mechanism of nanoparticle-coated twisted nanoribbons and their ability to form organized bundles. Here, STEM methods were used to reproduce the 3D structure of the Au nanoparticle double helix.

Direct Observation of Liquid Crystal Droplet Configurational Transitions using Optical Tweezers

Liquid crystals (LCs) are easily influenced by external interactions, particularly at interfaces. When rod-like LC molecules are confined to spherical droplets, they experience a competition between interfacial tension and elastic deformations. The configuration of LCs inside a droplet can be controlled using surfactants that influence the interfacial orientation of the LC molecules in the oil-phase of an oil in water emulsion. Here, we used the surfactant sodium dodecyl sulfate (SDS) to manipulate the orientation of 5CB molecules in a polydisperse emulsion and examined the configuration of the droplets as a function of SDS concentration. We triggered pronounced morphological transitions by altering the SDS concentration while observing an individual LC droplet held in place using an optical tweezer. We compared the experimental configuration changes to predictions from simulations. We observed a hysteresis in the SDS concentration that induced the morphological transition from radial to bipolar and back as well as a fluctuations in the configuration during the transition.

Field-triggered vertical positional transition of a microparticle suspended in a nematic liquid crystal cell

In this paper, based on the numerical calculation of total energy utilizing the Green's function method, we investigate how a field-triggered vertical positional transition of a microparticle suspended in a nematic liquid crystal cell is influenced by the direction of the applied field, surface anchoring feature, and nematic's dielectric properties. The new equilibrium position of the translational movement is decided via a competition between the buoyant force and the effective force built on the microparticle by the elastic energy gradient along the vertical direction. The threshold value of external field depends on thickness L and Frank elastic constant K and slightly on the microparticle size and density, in a Fréedericksz-like manner, but by a factor. For a nematic liquid crystal cell with planar surface alignment, a bistable equilibrium structure for the transition is found when the direction of the applied electric field is (a) perpendicular to the two plates of the cell with positive molecular dielectric anisotropy or (b) parallel to the two plates and the anchoring direction of the cell with negative molecular dielectric anisotropy. When the electric field applied is parallel to both plates and perpendicular to the anchoring direction, the microparticle suspended in the nematic liquid crystal tends to be trapped in the midplane, regardless of the sign of the molecular dielectric anisotropy. Such a phenomenon also occurs for negative molecular dielectric anisotropy if the external field is applied perpendicular to the two plates. Explicit formulas proposed for the critical electric field agree extremely well with the numerical calculation.

Shaping Liquid Crystals with Gold Nanoparticles: Helical Assemblies with Tunable and Hierarchical Structures Via Thin-Film Cooperative Interactions

The availability of helical assemblies of plasmonic nanoparticles with precisely controlled and tunable structures can play a key role in the future development of chiral plasmonics and metamaterials. Here, a strategy to efficiently yield helical structures based on the cooperative interactions of liquid crystals and gold nanoparticles in thin films is developed. These nanocomposites exhibit exceptional long-range hierarchical order across length scales, which results from the growth mechanism of nanoparticle-coated twisted nanoribbons and their ability to form organized bundles. The helical assembly formation is governed by the presence of rationally functionalized nanoparticles. Importantly, the thickness of the achieved nanocomposites can be reversibly reconfigured owing to the polymorphic nature of the liquid crystal. The versatility of the proposed approach is demonstrated by preparing helices assembled from nanoparticles of different geometries and dimensions (spherical and rod-like). The described strategy may become an enabling technology for structuring nanoparticle assemblies with high precision and fabricating optically active materials.

Nanoparticle-based hollow microstructures formed by two-stage nematic nucleation and phase separation

Rapid bulk assembly of nanoparticles into microstructures is challenging, but highly desirable for applications in controlled release, catalysis, and sensing. We report a method to form hollow microstructures via a two-stage nematic nucleation process, generating size-tunable closed-cell foams, spherical shells, and tubular networks composed of closely packed nanoparticles. Mesogen-modified nanoparticles are dispersed in liquid crystal above the nematic-isotropic transition temperature (TNI). On cooling through TNI, nanoparticles first segregate into shrinking isotropic domains where they locally depress the transition temperature. On further cooling, nematic domains nucleate inside the nanoparticle-rich isotropic domains, driving formation of hollow nanoparticle assemblies. Structural differentiation is controlled by nanoparticle density and cooling rate. Cahn-Hilliard simulations of phase separation in liquid crystal demonstrate qualitatively that partitioning of nanoparticles into isolated domains is strongly affected by cooling rate, supporting experimental observations that cooling rate controls aggregate size. Microscopy suggests the number and size of internal voids is controlled by second-stage nucleation.