Nature of dynamic friction in a humid hydrophobic nano-contact

TL;DR

This paper investigates the complex nature of dynamic friction in humid hydrophobic nano-contacts, revealing that water droplets can form within the contact and influence slip behavior, challenging previous assumptions.

Contribution

It introduces a new mechanism involving water droplet formation within hydrophobic nano-contacts, supported by experiments and molecular dynamics simulations.

Findings

Water molecules are not simply squeezed out but form droplets within the contact.

Full slip regime of the droplet explains experimental tribological behavior.

Refines understanding of water's role in hydrophobic friction.

Abstract

The physics of dynamic friction on water molecule contaminated surfaces is still poorly understood. In line with the growing interest in hydrophobic contact for industrial applications, this paper focuses on friction mechanisms in such interfaces. As a commonly used material, contact with graphite is considered in a twin-fold approach based on experimental investigations using the circular mode atomic force microscopy technique combined with molecular dynamic simulations. We demonstrate that an intuitive paradigm, which asserts that water molecules are squeezed out of a hydrophobic contact, should be refined. As a consequence, we introduce a mechanism considering a droplet produced within the sliding nano-contact by the accumulation of water adsorbed on the substrate. Then we show that a full slip regime of the droplet sliding on the hydrophobic substrate explains the experimental tribological behavior.