Colloidal particles with a limited number of interactive sites are called colloidal molecules, and their assemblies have been intensively studied to reveal complex micro-structures. In this study, we examine colloidal particles in nematic liquid crystals, so-called nematic colloids, as colloidal molecules and fabricated some non-close-packed assemblies. Micrometer-sized particles with homeotropic surface anchoring of liquid crystal in a homeotropic cell interact with each other through dipolar-type anisotropic interactions arising from the elastic deformation of the nematic field around the particles. Using optical tweezers, we have built two-dimensional colloidal assemblies with low packing densities, including polygon-rings, chains of polygon-rings, and lattices composed of octagon-rings in a hierarchical way from smaller structure units. Because the nematic field is sensitive to the electric field, the response of the polygon-rings to an alternative electric field has been studied. They exhibited homogeneous reversible shrink as large as 15%-22% to their original sizes under several volts.
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