| dc.description.abstract | The nematic phase represents the most fluid liquid crystalline phase. The high shape anisotropy and long range orientational order of constituent particles in this phase yields wide-ranging applications from high strength fibers even to liquid crystal display technology. This wide relevance makes the investigation of the assembly of liquid crystals structures into such ordered domains of paramount importance in soft matter phenomena. External fields have been proposed as a means to effectively control nematic ordering. Discotic liquid crystals, in the presence of external fields, are hypothesized to form a unique, higher-ordered, biaxial nematic phase. Herein, we experimentally demonstrate for the first time, the phase transitions of colloidal nanoplates that possess negative anisotropic polarizability in an electric field. We synthesize α-Zirconium Phosphate nanoplatelets by the hydrothermal method, and these disks are exfoliated in aqueous medium using tetra-(n)-butyl hydroxide to yield very low aspect ratio monolayers. In the absence of any fields, we observe an isotropic-nematic transition as concentration increased. We demonstrated that small cell thicknesses around 120 μm provided clear schlieren textures for the disclinations of the nematic phase. Experiments investigating the phase behavior of these nanoplatelets under the effect of an external electric field yielded results consistent with theoretical extensions of the Onsager theory. The phase diagram in the field strength vs concentration plane reveals the existence of a tricritical point where a second-order transition meets with a first-order phase transition. The first order transition is characterized by a shift from a uniaxial nematic phase (characterized by quad-isogyre disclinations) to the biaxial nematic phase (characterized by bi-isogyre disclinations). The second order transition is characterized by a shift from the paranematic phase to the highly anisotropic biaxial nematic phase. Our experiments demonstrate that electric fields offer a vital tool to control the self-assembly of nanoplate liquid crystals and could be translated other 2D systems such as graphene oxide by scaling with respect to factors including aspect ratio, polydispersity, anisotropy. Furthermore, the biaxial nematic phase could be useful to 3D imaging technology by wielding the ability to control the alignment of particles along two axes with a unidirectional electric field. | en |
