Coulomb Repulsion at the Nanometer-Sized Contact: A Force Driving Superhydrophobicity, Superfluidity, Superlubricity, and Supersolidity

Abstract

Superhydrophobicity, superfluidity, superlubricity, and supersolidity (4S) at the nanometer-sized liquid−solid or solid−solid contacting interfaces have long been issues of puzzling with the common characteristics of nonsticky and frictionless motion. Although the 4S occurrences have been extensively investigated, the mechanism behind the common characteristics remains unclear. From the perspectives of broken-bond-induced local strain and the skin-depth charge and energy quantum trapping and the associated nonbonding electron polarization, we proposed herewith that the Coulomb repulsion between the “electric monopoles or dipoles locked in the elastic solid skins or the solidlike covering sheets of liquid droplets” forms the key to the 4S. The localized energy densification makes the skin stiffer and the densely and tightly trapped bonding charges polarize nonbonding electrons, if exist, to form locked skin monopoles. In addition, the sp-orbit hybridization of F, O, N, or C upon reacting with solid atoms generates nonbonding lone pairs or unpaired edge electrons that induce dipoles directing into the open end of a surface. The monopoles and dipoles can be, however, demolished by UV radiation, thermal excitation, or excessively applied compression due to ionization or sp orbit dehybridization. Such a Coulomb repulsion between the negatively charged skins of the contacting objects not only lowers the effective contacting force and hence the friction but also prevents charge from being exchanged between the counterparts of the contact. Being similar to magnetic levitation, such Coulomb repulsion should be the force driving the 4S. Density function theory calculations, X-ray photoelectron spectroscopy, scanning tunneling microscopy/spectroscopy, and very low energy electron diffraction measurements have been conducted to verify the proposal. In particular, agreement between theory predictions and the measured size dependence of the elastic modulus, lattice strain, core−electron binding energy shift, and band gap expansion of nanostructures evidence the validity of the proposal of interface Coulomb repulsion.