  This work discusses optical singularities and electrical conductivity
behavior in a thin electrooptical cell filled with composites including
multi-walled carbon nanotubes (MWCNTs) and nematic liquid crystal (LC). The
MWCNTs with high aspect ratio $L/d\approx 300\div1000$ and nematic LC 5CB
(4-pentyl-4-0-cyanobiphenyl) were used. The composites were prepared by
introduction of MWCNTs ($0.0001\div0.1$ % wt) into LC solvent with subsequent
sonication. The increase of MWCNT concentration (between $0.005\div0.05$ % wt)
resulted in self-organization of MWCNTs and formation of micron-sized
aggregates with fractal boundaries. The visually observed formation of spanning
MWCNT networks near the percolation threshold at ~0.025 % wt was accompanied
with transition from non-conductive to conductive state and generation of
optical singularities. The observed effects were explained by the strong
interactions between MWCNTs and LC medium and planar orientation of 5CB
molecules near the lateral surface of MWCNTs. It was speculated that optical
singularities arose as a results of interaction of an incident laser beam with
LC perturbed interfacial shells covering the MWCNT clusters. Behavior of the
interfacial shell thickness in external electric field and in the vicinity of
the nematic to isotropic transition was discussed.
