Frictionless nanohighways on crystalline surfaces

TL;DR

This paper provides a unified, geometry-based framework for understanding nanoscale friction on crystalline surfaces, revealing a new type of directional superlubricity with high anisotropy.

Contribution

It introduces a novel categorization of contact types based on geometry, including a new directionally lubric contact, applicable to various materials.

Findings

Identifies three contact types: lubric, corrugated, and directional lubric.

Discovers a stable, highly anisotropic frictional state.

Applicable to a wide range of crystalline and layered materials.

Abstract

The understanding of friction at nano-scales, ruled by the regular arrangement of atoms, is surprisingly incomplete. Here we provide a unified understanding by studying the interlocking potential energy of two infinite contacting surfaces with arbitrary lattice symmetries, and extending it to finite contacts. We categorize, based purely on geometrical features, all possible contacts into three different types: a structurally lubric contact where the monolayer can move isotropically without friction, a corrugated and strongly interlocked contact, and a newly discovered directionally structurally lubric contact where the layer can move frictionlessly along one specific direction and retains finite friction along all other directions. This novel category is energetically stable against rotational perturbations and provides extreme friction anisotropy. The finite-size analysis shows that our categorization applies to a wide range of technologically relevant materials in contact, from adsorbates on crystal surfaces to layered two-dimensional materials and colloidal monolayers.