Elasticity-dependent self-assembly of micro-templated chromonic liquid crystal films.

We explore micropatterned director structures of aqueous lyotropic chromonic liquid crystal (LCLC) films created on square-lattice cylindrical-micropost substrates. The structures are manipulated by modulating the LCLC mesophases and their elastic properties via concentration through drying. Nematic LCLC films exhibit preferred bistable alignment along the diagonals of the micropost lattice. Columnar LCLC films, dried from nematics, form two distinct director and defect configurations: a diagonally aligned director pattern with local squares of defects, and an off-diagonal configuration with zig-zag defects. The formation of these states appears to be tied to the relative splay and bend free energy costs of the initial nematic films. The observed nematic and columnar configurations are understood numerically using a Landau-de Gennes free energy model. Among other attributes, the work provide first examples of quasi-2D micropatterning of LC films in the columnar phase and lyotropic LC films in general, and it demonstrates alignment and configuration switching of typically difficult-to-align LCLC films via bulk elastic properties.

Exploiting imperfections in the bulk to direct assembly of surface colloids.

We exploit the long-ranged elastic fields inherent to confined nematic liquid crystals (LCs) to assemble colloidal particles trapped at the LC interface into reconfigurable structures with complex symmetries and packings. Spherical colloids with homeotropic anchoring trapped at the interface between air and the nematic LC 4-cyano-4'-pentylbiphenyl create quadrupolar distortions in the director field causing particles to repel and consequently form close-packed assemblies with a triangular habit. Here, we report on complex open structures organized via interactions with defects in the bulk. Specifically, by confining the nematic LC in an array of microposts with homeotropic anchoring conditions, we cause defect rings to form at well-defined locations in the bulk of the sample. These defects source elastic deformations that direct the assembly of the interfacially trapped colloids into ring-like assemblies, which recapitulate the defect geometry even when the microposts are completely immersed in the nematic. When the surface density of the colloids is high, they form a ring near the defect and a hexagonal lattice far from it. Because topographically complex substrates are easily fabricated and LC defects are readily reconfigured, this work lays the foundation for a versatile, robust mechanism to direct assembly dynamically over large areas by controlling surface anchoring and associated bulk defect structure.

Topographically induced hierarchical assembly and geometrical transformation of focal conic domain arrays in smectic liquid crystals.

Controlling topological defects in 3D liquid crystal phases is a crucial element in the development of novel devices, from blue-phase displays to passive biochemical sensors. However, it remains challenging to realize the 3D topological conditions necessary to robustly and arbitrarily direct the formation of defects. Here, using a series of short pillar arrays as topological templates, we demonstrate the hierarchical assembly of focal conic domains (FCDs) in smectic-A liquid crystals that break the underlying symmetry of the pillar lattice, exhibit tunable eccentricity, and together develop a nontrivial yet organized array of defects. The key to our approach lies in the selection of the appropriate ratio of the size of focal domain to the dimension of pillars such that the system favors the "pinning" of FCD centers near pillar edges while avoiding the opposing effect of confinement. Our study unequivocally shows that the arrangement of FCDs is strongly influenced by the height and shape of the pillars, a feature that promotes both a variety of nontrivial self-assembled lattice types and the attraction of FCD centers to pillar edges, especially at regions of high curvature. Finally, we propose a geometric model to reconstruct the smectic layer structure in the gaps between neighboring FCDs to estimate the energetic effects of nonzero eccentricity and assess their thermodynamic stability.

Curvature-driven capillary migration and assembly of rod-like particles.

Capillarity can be used to direct anisotropic colloidal particles to precise locations and to orient them by using interface curvature as an applied field. We show this in experiments in which the shape of the interface is molded by pinning to vertical pillars of different cross-sections. These interfaces present well-defined curvature fields that orient and steer particles along complex trajectories. Trajectories and orientations are predicted by a theoretical model in which capillary forces and torques are related to Gaussian curvature gradients and angular deviations from principal directions of curvature. Interface curvature diverges near sharp boundaries, similar to an electric field near a pointed conductor. We exploit this feature to induce migration and assembly at preferred locations, and to create complex structures. We also report a repulsive interaction, in which microparticles move away from planar bounding walls along curvature gradient contours. These phenomena should be widely useful in the directed assembly of micro- and nanoparticles with potential application in the fabrication of materials with tunable mechanical or electronic properties, in emulsion production, and in encapsulation.

Pillar-assisted epitaxial assembly of toric focal conic domains of smectic-a liquid crystals.

SU-8 pillar-assisted epitaxial assembly of toric focal conic domains (TFCDs) arrays of smectic-A liquid crystals is studied. The 3D nature of the pillar array is crucial to confine and direct the formation of TFCDs on the top of each pillar and between neighboring pillars, leading to highly ordered square and hexagonal array TFCDs. Excellent agreement between the experimentally obtained critical pillar diameter and elasticity calculation is found.

Orientation and self-assembly of cylindrical particles by anisotropic capillary interactions.

In this research, we study cylindrical microparticles at fluid interfaces. Cylinders orient and assemble with high reliability to form end-to-end chains in dilute surfaces or dense rectangular lattices in crowded surfaces owing to capillary interactions. In isolation, a cylinder assumes one of two possible equilibrium states, the end-on state, in which the cylinder axis is perpendicular to the interface, or the side-on state, in which the cylinder axis is parallel to the interface. A phase diagram relating aspect ratio and contact angle is constructed to predict the preferred state and verified in experiment. Cylinders in the side-on state create distortions that result in capillary interactions. Overlapping deformations by neighboring particles drive oriented capillary assembly. Interferometry, electron microscopy, and numerical simulations are used to characterize the interface shape around isolated particles. Experiments and numerics show that "side-on" cylinders have concentrated excess area near the end faces, and that the interface distortion resembles an elliptical quadrupole a few radii away from the particle surface. To model the cylinder interactions for separations greater than a few radii, an anisotropic potential is derived based on elliptical quadrupoles. This potential predicts an attractive force and a torque, both of which depend strongly on aspect ratio, in keeping with experiment. Particle trajectories and angular orientations recorded by video microscopy agree with the predicted potential. In particular, the analysis predicts the rate of rotation, a feature lacking in prior analyses. To understand interactions near contact, the concentrated excess area near the cylinder ends is quantified and its role in creating stable end-to-end assemblies is discussed. When a pair of cylinders is near contact, these high excess area regions overlap to form a capillary bridge between the particles. This capillary bridge may stabilize the end-to-end chains. Finally, on densely packed surfaces, cylinder-covered colloidosomes form with particles arranged in regular, rectangular lattices in the interface; this densely packed structure differs significantly from assemblies reported for colloidosomes or particle-stabilized droplets in the literature.