Carbon nanotube-induced chirality and macroscopic helical twist in achiral liquid crystals

A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal, and the mixture was found to exhibit a weak degree of chirality both in the smectic and nematic phases. The induced chirality in the LC was probed by means of the electroclinic effect in the liquid crystal's smectic-$A$ and nematic phases, which showed significant pretransitional behavior on approaching the smectic-$A$ -- smectic-$C$ and the nematic -- smectic-$A$ transition temperatures, respectively, from above. The carbon nanotubes also were found to induce a bulk twist over macroscopic dimensions in an achiral nematic matrix. The nanotube-induced chiral pitch length $P$ was determined as a function of average nanotube concentration by measuring the radii of curvature of reverse twist disclination lines in 90\r{ } twist nematic cells. The results reveal information about the nanotubes' spatial distribution inside the cells. A concentration for the onset of significant aggregation of the nanotubes can be quantified from the apparent saturation of $P^{-1}$ at higher concentrations. The macroscopic helical twisting power of the nanotubes has been estimated from the results. The results indicate that there is a net chirality associated with the carbon nanotubes, which is transmitted into the achiral liquid crystal.