forces originating from the same physical basis, but having different names due to historical reasons. The van der Waals force is the short-distances asymptotic of this general force, for which one can neglect the retardation of electromagnetic fields, but the Casimir force is realized at larger distances where the retardation is important. The common origin of the forces is nicely explained by the Lifshitz theory [@Lif56], which is able to describe the transition between the two regimes. It predicts transition between the Casimir and van der Waals forces at separations $d\sim
\lambda_p/10$, where $\lambda_p=2\pi c/\omega_p$ is the plasma wave length and $\omega_p$ is the plasma frequency [@Gen04; @Lam00]. Keeping in mind the common origin of the forces, in what follows we will also use a generalized name [*dispersive forces*]{}.

At separations below $100\;nm$ the Casimir force is very strong and becomes comparable to electrostatic forces corresponding to voltages in the range $0.1-1\;V$ [@Ser97; @Cle03; @Cha01], while for separations below $10\;nm$ the van der Waals force dominates any attraction [@Tab69; @Ede00; @Zwo08a]. These properties make the dispersive force an important player in nanosciencies. Moreover, from the fundamental point of view, measurements of the forces from nano to microscales have attracted strong 