Near-wall depletion and layering affect contact line friction of multicomponent liquids

TL;DR

This study uses Molecular Dynamics simulations to explore how near-wall glycerol depletion influences contact line friction in water-glycerol droplets on silica, revealing a sub-linear relationship with viscosity contrary to traditional theories.

Contribution

It introduces a correction to the Molecular Kinetic Theory by accounting for near-wall glycerol depletion effects on contact line friction.

Findings

Contact line friction scales sub-linearly with viscosity.

Glycerol depletion near the wall reduces effective interfacial friction.

A re-scaled interfacial friction coefficient improves theoretical predictions.

Abstract

The main causes of energy dissipation in micro- and nano-scale wetting are viscosity and liquid-solid friction localized in the three-phase contact line region. Theoretical models predict the contactline friction coefficient to correlate with the shear viscosity of the wetting fluid. Experiments conducted to investigate such correlation have not singled out a unique scaling law between the two coefficients. We perform Molecular Dynamics simulations of liquid water-glycerol droplets wetting silica-like surfaces, aimed to demystify the effect of viscosity on contact line friction. The viscosity of the fluid is tuned by changing the relative mass fraction of glycerol in the mixture and it is estimated both via equilibrium and non-equilibrium Molecular Dynamics simulations. Contact line friction is measured directly by inspecting the velocity of the moving contact line and the microscopic contact angle. It is found that the scaling between contact line friction and viscosity is sub-linear, contrary to the prediction of Molecular Kinetic Theory. The disagreement is explained by accounting for the depletion of glycerol in the near-wall region. A correction is proposed, based on multicomponent Molecular Kinetic Theory and the definition of a re-scaled interfacial friction coefficient.