Van der waals torque coupling between slabs composed of planar arrays of nanoparticles

We present a theoretical study of the van der Waals torque between plates that are composed either of anisotropic material or of arrays of nanoparticles. The torque is calculated within the Barash and Ginzburg formalism in the nonretarded limit, and is quantified by the introduction of a Hamaker torque constant. Calculations are conducted between anisotropic slabs of materials including BaTiO₃ and arrays of Ag nanoparticles, and it is found that measurable torques occur for 200 nm slabs separated by 15 nm. Depending on the dielectric function, in particular the zero frequency contribution, the Hamaker torque constant can be big enough to induce the orientation of arrays of nanoparticles.