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Entropy 2013, 15(9), 3734-3745; doi:10.3390/e15093734
Article

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

1,2
,
1
 and
1,*
1 Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK 2 Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USA
* Author to whom correspondence should be addressed.
Received: 21 July 2013 / Revised: 2 September 2013 / Accepted: 3 September 2013 / Published: 6 September 2013
(This article belongs to the Special Issue Molecular Dynamics Simulation)
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Abstract

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.
Keywords: cloud data set; interfacial tension; coarse-graining; water; line tension; graphene cloud data set; interfacial tension; coarse-graining; water; line tension; graphene
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Santiso, E.E.; Herdes, C.; Müller, E.A. On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics. Entropy 2013, 15, 3734-3745.

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