How square ice helps lubrication

TL;DR

This study uses molecular dynamics simulations to show that square ice forms under certain conditions between graphene sheets, significantly reducing friction and potentially explaining low-friction behavior in wet graphite systems.

Contribution

It demonstrates the formation of square ice in confined water between graphene and its role in superlubricity during sliding.

Findings

Square ice forms at specific pressure and speed conditions.

Square ice exhibits structural similarity to cubic ice VII and ice X.

Sliding on square ice results in extremely low friction due to superlubricity.

Abstract

In the context of friction we use atomistic molecular-dynamics simulations to investigate water confined between graphene sheets over a wide range of pressures. We find that thermal equilibration of the confined water is hindered at high pressures. We demonstrate that, under the right conditions, square ice can form in an asperity, and that it is similar to cubic ice VII and ice X. We simulate sliding of atomically flat graphite on the square ice and find extremely low friction due to structural superlubricity. The conditions needed for square ice to form correspond to low sliding speeds, and we suggest that the ice observed in experiments of friction on wet graphite is of this type.