Abstract
Abstract Superlubricity means non-sticky and frictionless when two bodies are set contacting motion. Although this occurrence has been extensively investigated since 1859 when Faraday firstly proposed a quasiliquid skin on ice, the mechanism behind the superlubricity remains uncertain. This report features a consistent understanding of the superlubricity pertaining to the slipperiness of ice, self-lubrication of dry solids, and aqueous lubricancy from the perspective of skin bond-electron-phonon adaptive relaxation. The presence of nonbonding electron polarization, atomic or molecular undercoordination, and solute ionic electrification of the hydrogen bond as an addition, ensures the superlubricity. Nonbond vibration creates soft phonons of high magnitude and low frequency with extraordinary adaptivity and recoverability of deformation. Molecular undercoordination 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 magnetic levitation and hovercraft. O:H−O bond electrification by aqueous ions has the same effect of molecular undercoordination but it is throughout the entire body of the lubricant. Such a Coulomb repulsivity due to the negatively charged skins and elastic adaptivity due to soft nonbonding phonons of one of the contacting objects not only lowers the effective contacting force but also prevents charge from being transited between the counterparts of the contact. Consistency between theory predictions and observations evidences the validity of the proposal of interface elastic Coulomb repulsion that serves as the rule for the superlubricity of ice, wet and dry frictions, which also reconciles the superhydrophobicity, superlubricity, and supersolidity at contacts.
Highlights
Ice is most slippery of ever known at temperatures even below its melting limit at −22 °C under 2,000 atmospheric pressure (200 MPa) pressure
Instead of a quasiliquid layer, friction heating, or pressure melting, ice is covered with a supersolid skin that is elastic, polarized, less dense, and thermally more stable [11−13], as illustrated in Fig. 2: (1) Molecular undercoordination shortens and stiffens the H−O bond, and lengthens and softens the O:H nonbond with dual polarization of electron lone pairs on oxygen ions (H−O contraction polarizes the lone pair electrons in the first round and that enhances O−O repulsion in the second)
The mechanism of slipperiness of ice is analogous to the self-lubrication of metal nitride [99, 100] and oxide [101] skins with electron lone pairs coming into play
Summary
Ice is most slippery of ever known at temperatures even below its melting limit at −22 °C under 2,000 atmospheric pressure (200 MPa) pressure. All sorts of surfaces can get slick and slippery if ice and snow. Debating is still going on with the following possible mechanisms:. (1) Pressure melting creates the quasiliquid lubricant [2, 3]. (2) Friction heating melts ice [4]. (3) Quasiliquid skin forms due to molecular undercoordination [5]. (4) Low-frequency and high-magnitude vibrations associated of surface molecules [6] (2) Friction heating melts ice [4]. (3) Quasiliquid skin forms due to molecular undercoordination [5]. (4) Low-frequency and high-magnitude vibrations associated of surface molecules [6]
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