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On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics
Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK
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; in revised form: 2 September 2013 / Accepted: 3 September 2013 / Published: 6 September 2013
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
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MDPI and ACS Style
Santiso, E.E.; Herdes, C.; Müller, E.A. On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics. Entropy 2013, 15, 3734-3745.
Santiso EE, Herdes C, Müller EA. On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics. Entropy. 2013; 15(9):3734-3745.
Santiso, Erik E.; Herdes, Carmelo; Müller, Erich A. 2013. "On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics." Entropy 15, no. 9: 3734-3745.