Liquid crystal colloids: targeted design and self-assembly of optical composites
By: Miha Ravnik
From: Univ. Oxford
At: Complexo Interdisciplinar, Anfiteatro
[2010-06-09]
Structuring in nematic colloids allows for mechanisms such as entanglement, hierarchical ordering, and self-assembly. Strong, highly anisotropic, and spatially dependent inter-particle potentials bind colloidal composites. In addition to inter-particle interactions, colloidal assembly can be driven also by introducing inclusions into non-homogeneous liquid crystalline profiles or cholesteric blue phases. Combined with well controllable response of liquid crystals to external fields, surface geometry, and surface anchoring, liquid crystal colloids offer a highly promising route for production of complex optical patterns at micrometer and sub-micrometer scale to be used in photonic and plasmonic applications.
Here we present strategies for assembly of colloidal particles into 2D and 3D structures, using uniform nematic, twisted nematic, and cholesteric blue phases. Our work is based on numerical minimization of phenomenological Landau-de Gennes free energy, with full link to experimental realization of these structures. In uniform nematic, we show that regular 2D colloidal crystals can be assembled by configuring elastic dipoles and quadrupoles. Elastic dipoles are shown to form also stable 3D colloidal crystals. Using temperature quench, entangled colloidal structures can be built. These structures are characterized by robust string-like binding potentials that offer novel mechanisms for assembling of superstructures. Twisted nematics additionally stabilize entangled colloidal structures and allow for formation of regular 2D entangled structures. 3D colloidal crystals are demonstrated in cholesteric blue phases. Finally, we discuss the role of material activity as means for colloidal manipulation.