Optical singularities induced in a nematic-cell by carbon nanotubes

Optical and electrical conductivity properties of multi-walled carbon nanotubes (MWCNTs) and nematic liquid crystal (LC) composites are investigated. The MWCNTs with high aspect ratio L/d ≈ 300 ÷ 1000 and nematic LC 5CB (4- pentyl-4-cyanobiphenyl) are used. The composites are prepared by introduction of MWCNTs (0.0001÷0.1 % wt) into LC solvent with subsequent sonication. The increase of MWCNT concentration (between 0.0025÷0.05 % wt) results in selfaggregation of MWCNTs and formation of up to 200 micron-sized 3d aggregates with fractal boundaies. The visually observed formation of spanning MWCNT networks near the percolation threshold at ~0.025 % wt is accompanied with transition from non-conductive to conductive state and generation of optical singularities. The observed effects are explained by the strong interactions between MWCNTs and LC medium and planar orientation of 5CB molecules on the lateral surface of MWCNTs. As a result, an interfacial 5CB micro size shell with inhomogeneous structure appears around each nanotube cluster. Diffraction of laser beam on clusters fractal boundaries creates speckle field with multitude of optical vortices. The irregular birefringence of interfacial shells induces polarization singularities in propagating laser beam. Growth of the LC interfacial shell thickness in external electric field and its disappearance during transition of the nematic to isotropic liquid are measured and discussed. It is shown that the formation of 5CB interfacial shell allows fixing existence of nanostructures beyond space resolution of microscope.