Molecular dynamics simulations of the contact angle between water droplets and graphite surfaces

TL;DR

This study uses molecular dynamics simulations to analyze how water droplets of various sizes contact graphite surfaces, examining the transition from nanoscale to macroscopic wetting behavior and assessing the accuracy of the Young equation.

Contribution

It introduces a detailed simulation approach for nanoscale droplets and evaluates the applicability of the modified Young equation in this regime.

Findings

Larger droplets are nearly spherical, smaller ones deviate from circular profiles.

The modified Young equation accurately predicts contact angles for droplets around 40 Å.

Evidence of a transition to macroscopic wetting behavior at certain length scales.

Abstract

Wetting is a widespread phenomenon, most prominent in a number of cases, both in nature and technology. Droplets of pure water with initial radius ranging from 20 to 80 [\AA] spreading on graphitic surfaces are studied by molecular dynamics simulations. The equilibrium contact angle is determined and the transition to the macroscopic limit is discussed using Young equation in its modified form. While the largest droplets are almost perfectly spherical, the profiles of the smallest ones are no more properly described by a circle. For the sake of accuracy, we employ a more general fitting procedure based on local averages. Furthermore, our results reveal that there is a possible transition to the macroscopic limit. The modified Young equation is particularly precise for characteristic lengths (radii and contact-line curvatures) around 40 [\AA].