On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics

Entropy

Erik E. Santiso¹  Carmelo Herdes¹ 

[1] Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK; E-Mail:

关键词: cloud data set;    interfacial tension;    coarse-graining;    water;    line tension;    graphene;   

DOI  :  10.3390/e15093734

来源: mdpi

【 摘 要 】

A methodology for the determination of the solid-fluid contact angle, to be employed within molecular dynamics (MD) simulations, is developed and systematically applied. The calculation of the contact angle of a fluid drop on a given surface, averaged over an equilibrated MD trajectory, is divided in three main steps: (i) the determination of the fluid molecules that constitute the interface, (ii) the treatment of the interfacial molecules as a point cloud data set to define a geometric surface, using surface meshing techniques to compute the surface normals from the mesh, (iii) the collection and averaging of the interface normals collected from the post-processing of the MD trajectory. The average vector thus found is used to calculate the Cassie contact angle (i.e., the arccosine of the averaged normal z-component). As an example we explore the effect of the size of a drop of water on the observed solid-fluid contact angle. A single coarse-grained bead representing two water molecules and parameterized using the SAFT-γ Mie equation of state (EoS) is employed, meanwhile the solid surfaces are mimicked using integrated potentials. The contact angle is seen to be a strong function of the system size for small nano-droplets. The thermodynamic limit, corresponding to the infinite size (macroscopic) drop is only truly recovered when using an excess of half a million water coarse-grained beads and/or a drop radius of over 26 nm.

【 授权许可】

CC BY   
© 2013 by the authors; licensee MDPI, Basel, Switzerland.