Solidification and superlubricity with molecular alkane films

TL;DR

This study investigates the shear properties and phase transitions of molecular alkane films confined between mica surfaces, revealing conditions for superlubricity and the role of surface alignment and water in friction behavior.

Contribution

It provides new insights into the phase behavior and ultralow friction mechanisms of nanoscale alkane films under controlled surface alignments.

Findings

Friction coefficients as low as 0.001 observed for incommensurate dry interfaces.

Surface alignment and water presence significantly influence friction levels.

Liquid-solid phase transitions are characterized at the molecular scale.

Abstract

Hydrocarbon films confined between smooth mica surfaces have long provided an experimental playground for model studies of structure and dynamics of confined liquids. However fundamental questions regarding the phase behavior and shear properties in this simple system remain unsolved. With ultra-sensitive resolution in film thickness and shear stress, and control over the crystallographic alignment of the confining surfaces, we here investigate the shear forces transmitted across nanoscale films of dodecane down to a single molecular layer. We resolve the conditions under which liquid-solid phase transitions occur and explain friction coefficients spanning several orders of magnitude. We find that commensurate surface alignment and presence of water at the interfaces each lead to moderate or high friction, whereas friction coefficients down to 0.001 are observed for a single molecular layer of dodecane trapped between crystallographically misaligned dry surfaces. This ultralow friction is attributed to sliding at the incommensurate interface between one of the mica surfaces and the laterally ordered solid molecular film, reconciling previous interpretations.