Tangential electrostatic field at metal surfaces

In electrodynamics, it has been long believed that when the electrostatic equilibrium is reached the electrostatic field outside a metal is always perpendicular to the metal surface. However, the tangential electrostatic field (TEF) may be discovered at the metal surface through the mechanical-electric coupling in the work. The TEF can lead to new knowledge and more accurate modification on electrostatics of metals including the electrostatic equilibrium conditions, uniqueness theorem, method of image charges, electrostatic shielding, Thompson's theorem, and Green's reciprocation theorem. The TEF and the mechanical-electric coupling can also indicate that an intrinsically intensive electrostatic field may exist at the nanoparticle surface. Thereby a unified model could be constructed for the surface-enhanced Raman scattering (SERS) which has been a long-standing problem in physics and chemistry in recent several decades. Furthermore, when the micro-electro-mechanical systems (MEMS) work, the deformation of the metal plate can induce an additional electrostatic field and a newly attractive electrostatic force between the metal plates. They may be important for the design and fabrication of high-performance MEMS devices. Overall, the revealed TEF may update the physical knowledge of the electrostatics of metals in electrodynamics and may acquire widespread applications in the areas of SERS, MEMS, and so on.