Phonon-induced Anisotropic Dispersion Forces on a Metallic Substrate

Surfactant micelles (cetyltrimethylammonium chloride) adsorbed on Au(111) exhibit orientational order dictated by the gold crystal axes. To explain this phenomenon, we take into account the ionic contribution to the dielectric response of the metal. Since the motion of an ion inside the metallic lattice is restricted by its neighbors in an anisotropic way, the total dielectric response of the metal acquires directional dependence. A crystalline substrate is thus able to generate both torque and attraction on geometrically asymmetric objects. Numerical calculations show that the resulting anisotropic van der Waals force is indeed capable of orienting rod-like dielectric micelles on a Au(111) surface.