Topographically induced homeotropic alignment of liquid crystals on self-assembled opal crystals

The surface of multilayered opal crystals resulted in homeotropic alignment of liquid crystal (LC), originated from the surface topography of opal crystals rather than a chemical nature of the nanoparticles. The polar anchoring energy (5.51 × 10^-5 J/m²) of the crystal surface for nematic LC molecules was in a similar range to the conventional polyimide alignment layer (2.11 × 10^-5 J/m²) used for commercial applications. The critical length scale for anchoring transition was approximately L_w = ∼1 μm. If a diameter of particle d ≪ 1 μm for opal crystals, LC molecules preferred to anchor vertically to the surface to minimize elastic free energy of bulk LCs. The LC favored a planar anchoring if d > 1 μm. The results provide crucial insights to understand the homeotropic alignment of LCs on solid surfaces and therefore offer opportunities to develop novel materials for a vertical alignment of LCs.