Microscopic Marangoni flows cannot be predicted on the basis of pressure gradients

TL;DR

This paper demonstrates that microscopic Marangoni flows are accurately predicted by chemical potential gradients, while pressure gradient-based predictions do not match direct molecular dynamics simulations.

Contribution

It shows that chemical potential gradients, not pressure gradients, correctly predict microscopic Marangoni flows in molecular simulations.

Findings

Chemical potential gradient approach aligns with simulations.

Pressure gradient approach fails to predict flows accurately.

Microscopic flow predictions depend on the correct thermodynamic description.

Abstract

A concentration gradient along a fluid-fluid interface can cause flow. On a microscopic level, this so-called Marangoni effect can be viewed as being caused by a gradient in the pressures acting on the fluid elements, or as the chemical-potential gradients acting on the excess densities of different species at the interface. If the interfacial thickness can be ignored, all approaches should result in the same flow profile away from the interface. However, on a more microscopic scale, the different expressions result in different flow profiles, only one of which can be correct. Here we compare the results of direct non-equilibrium Molecular Dynamics simulations with the flows that would be generated by pressure and chemical potential gradients. We find that the approach based on the chemical potential gradients agrees with the direct simulations, whereas the calculations based on the pressure gradients do not.