Nanoparticle doping in nematic liquid crystals: distinction between surface and bulk effects by numerical simulations.

Doping nematic liquid crystals with small amounts of nanoparticles can significantly alter the electro-optic response of the nematic host. Some of these effects result from nanoparticles influencing the liquid crystal/substrate interface, while other effects are caused by nanoparticles in the bulk. So far, little attention has been paid to the influence of surface interactions on the determination of bulk properties. In the present study, these effects are investigated experimentally and confirmed by numerical simulations. The splay-type Fréedericksz-transition of the nematic liquid crystal 5CB doped with CdSe quantum dots is investigated, as these dispersions are known from earlier studies to affect the initial alignment layers. In comparison, dispersions of chemically and thermally stable silanized gold nanoparticles in the apolar nematic host FELIX-2900-03 are analyzed, which are expected to be bulk-active only. A data fitting routine is presented which allows a distinction between bulk and surface effects of nanoparticle doping. For the quantum dots, an increase of pretilt angle proportional to the doping concentration is found, as well as a slight decrease of the anchoring energy of molecules at the confining substrates. The silanized gold particles show no influence on the boundary conditions up to doping concentrations of 2.5 % (w). For higher concentrations an increase of pretilt angle is reported.