Applications for photonic crystals and metamaterials put stringent requirements on the characteristics of advanced optical materials, demanding tunability, high Q factors, applicability in visible range, and large-scale self-assembly. Exploiting the interplay between structural and optical properties, colloidal lattices embedded in liquid crystals (LCs) are promising candidates for such materials. Recently, stable two-dimensional colloidal configurations were demonstrated in nematic LCs. However, the question as to whether stable 3D colloidal structures can exist in an LC had remained unanswered. We show, by means of computer modeling, that colloidal particles can self-assemble into stable, 3D, periodic structures in blue phase LCs. The assembly is based on blue phases providing a 3D template of trapping sites for colloidal particles. The particle configuration is determined by the orientational order of the LC molecules: Specifically, face-centered cubic colloidal crystals form in type-I blue phases, whereas body-centered crystals form in type-II blue phases. For typical particle diameters (approximately 100 nm) the effective binding energy can reach up to a few 100 kBT, implying robustness against mechanical stress and temperature fluctuations. Moreover, the colloidal particles substantially increase the thermal stability range of the blue phases, for a factor of two and more. The LC-supported colloidal structure is one or two orders of magnitude stronger bound than, e.g., water-based colloidal crystals.
COBISS.SI-ID: 2318436
Colloidal assembly in strongly confined cholesteric structures is demonstrated using phenomenological modeling. Particle trapping sites and trapping potentials, which are intrinsically imposed by the strongly anisotropic orientational profile of the confined blue phases, are calculated. Locations of the trapping sites and profiles of the trapping potentials are shown to depend importantly on the particle size, and the array of trapping sites can even change symmetry. Trapping sites provide robust binding of various colloidal structures with binding energy of ~ 100kT for ~100 nm particles. Maximizing the filling of the trapping sites by particles proves to lower the full free energy of the system, offering means for thermodynamic stabilization of confined blue phases. Finally, we present formation of disclination cages, formed as a three-dimensional closed network of defect lines surrounding sufficiently large particles with strong homeotropic anchoring.
COBISS.SI-ID: 2377828
We show numerically that the interaction potential or force mediated by a liquid crystalline blue phase (BP) between two parallel plates exhibits oscillatory behavior with the variation of the interplate distance, when they impose strong normal anchoring. Its periodicity is about half of the unit cell dimension of the bulk BP. The interaction arises from the deformation of the confined BP structure around the midplane of the system. The oscillatory interaction can be regarded as a clear manifestation of the BP ordering, because the cholesteric helical alignment adopted by a chiral liquid crystal cannot yield an oscillatory interaction.
COBISS.SI-ID: 2374500