Orientation, interaction and laser assisted self-assembly of organic single-crystal micro-sheets in a nematic liquid crystal

Abstract

Colloidal self-assembly has been one of the major driving themes in material science to obtain functional and advanced optical materials with complex architecture. Most of the nematic colloids reported so far are based on the optically isotropic spherical microparticles. We study organic single crystal micro-sheets and investigate their orientation, interaction and directed assembly in a nematic liquid crystal. The micro-sheets induce planar surface anchoring of the liquid crystal. The elasticity mediated pair interaction of micro-sheets shows quadrupolar characteristics. The average orientation angle of the micro-sheets in a planar cell and the angle between two micro-sheets in a homeotropic cell are supported by the Landau-de Gennes Q-tensor modeling. The self-assembly of the micro-sheets is assisted by a laser tweezer to form larger two-dimensional structures which have the potential for application of colloids in photonics.

Control of the Folding Dynamics of Self‐Reconfiguring Magnetic Microbots Using Liquid Crystallinity

“…To this end, we examined single microcubes dispersed in 5CB LC in planar optical cells between crossedpolarizers ( Figure 3) and made three key observations. First, as previously reported for cuboidal inclusions dispersed in LC, [33][34][35][36] we found that microcubes without and with DMOAP treatment diagonally aligned their bodies to n 0 and exhibited birefringent edges (Figure 3a,e), indicating that the near-surface n deviates from n 0 (and thus from the optical axes of the polarizers). Second, when viewed with a retardation plate of wavelength λ ¼ 530 nm with its slow axis (s) at 45 to the polarizers, a higher order interference color (blue; Figure 3b) was observed at the surfaces of native microcubes parallel to s, suggesting a planar alignment of n at the surface of the microbots (see inset in Figure 3b).…”

“…In the presence of (// x ), the nontreated microbots exhibited six point defects of m = −1/2 at the center of their surfaces (circles in Figure a(i) and open blue dots in Figure a(iii)). To satisfy the conservation law, we predicted that surface disclinations of m = +1/4 form along the edges of microbot surfaces in the y–z plane (red dots and lines in Figure a(iii)), although it is difficult to experimentally locate surface disclinations . When was removed, however, the nontreated microbots folded and no longer possessed the point defects, leading to the formation of disclinations of m = +1/4 and −1/2 (red and blue dots in Figure b(iii), respectively) along the edges perpendicular to n 0 .…”

“…To satisfy the conservation law, [24][25][26][27][28][29][30][31][32] we predicted that surface disclinations of m ¼ þ1/4 form along the edges of microbot surfaces in the y-z plane (red dots and lines in Figure 4a(iii)), although it is difficult to experimentally locate surface disclinations. [34] When H ⇀ was removed, however, the nontreated microbots folded and no longer possessed the point defects, leading to the formation of disclinations of m ¼ þ1/4 and À1/2 (red and blue dots in Figure 4b(iii), respectively) along the edges perpendicular to n 0 . The difference in the LC elastic and defect energies (ΔE 1 ) of the two states of a microbot in a , filled with LC-containing microcubes.…”

Liquid Crystal Colloids

“…Particles shaped as letters of the Latin alphabet (Figure 7k,l), on the other hand, exhibit well-defined orientations with respect to n 0 and induce surface boojums with  i s i =correlated with their shapes. Other colloidal shapes studied recently include pyramids, spirals, shape-morphing elastomeric rods, gourd-shaped dimers, particles with nanoscale surface roughness, and so on (98)(99)(100)(101)(102)(103)(104)(105)(106)(107)(108)(109)(110)(111)(112)(113), in all cases demonstrating that the geometric shape pre-defines locations of topological defects and colloidal interactions. Facile control of nanoparticle and molecular organization and the ensuing composite properties can be achieved by applying electric, magnetic, and optical fields (80).…”

Orientation Dependent Interaction and Self‐Assembly of Cubic Magnetic Colloids in a Nematic Liquid Crystal

“…Colloidal dispersions in nematic liquid crystals (NLCs) are very promising as they induce variety of topological defects and interact via long‐range elastic forces of the medium . They are especially interesting because the interaction is anisotropic and shape dependent, thereby providing a route to targeted colloidal assemblies . Spherically symmetric particles with homeotropic surface anchoring nucleates either a point defect (hedgehog) or a disclination ring (Saturn ring) in their vicinity.…”