On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics
AbstractA 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.
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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.
Santiso EE, Herdes C, Müller EA. On the Calculation of Solid-Fluid Contact Angles from Molecular Dynamics. Entropy. 2013; 15(9):3734-3745.Chicago/Turabian Style
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.