Friction.pdf

Open Access

From Ice Superlubricity to Quantum Friction: Electronic Repulsivity and Phononic Elasticity

Xi Zhang，Yongli Huang，Zengsheng Ma，Lengyuan Niu， Chang Q Sun*

Abstract

Superlubricity means non-sticky and frictionless whentwo bodies are set contacting motion. Although this occurrence has beenextensively investigated since 1859 when Faraday firstly proposed a quasiliquidskin on ice, the mechanism behind the superlubricity remains uncertain. This report features a consistent understanding of the superlubricitypertaining to the slipperiness of ice, self-lubrication of dry solids, andaqueous lubricancy from the perspective of skin bond-electron-phonon adaptive relaxation.The presence of nonbonding electron polarization, atomic or molecularundercoordination, and solute ionic electrification of the hydrogen bond as an addition,ensures the superlubricity. Nonbondvibration creates soft phonons of high magnitude and low frequency withextraordinary adaptivity and recoverability of deformation. Molecularundercoordination shortens the covalent bond with local charge densification,which in turn polarizes the nonbonding electrons making them localized dipoles.The locally pinned dipoles provide force opposing contact, mimicking magneticlevitation and hovercraft. O:H-O bond electrification by aqueous ions has thesame effect of molecular undercoordination but it is throughout the entire bodyof the lubricant. Such a Coulomb repulsivity due to the negatively charged skins and elasticadaptivity due to soft nonbonding phonons of one of the contacting objects notonly lowers the effective contacting force but also prevents charge from being transitedbetween the counterparts of the contact. Consistency between theory predictionsand observations evidences the validity of the proposal of interface elastic Coulombrepulsion that serves as the rule for the superlubricity of ice, wet and dry frictions,which also reconciles the superhydrophobicity, superlubricity, andsupersolidity at contacts.