  It is shown that the quantum fluctuations of the electromagnetic field
generally induce a torque at the interface of an anisotropic material with
another anisotropic or isotropic material. It is proven that this torque
depends on an interface zero-point energy determined by the dispersion of the
interface (localized and extended) modes. Our theory demonstrates that the
single-interface torque is essential to understand the Casimir physics of
material systems with anisotropic elements and determines the equilibrium
positions of the system.
