Order Parameters and Algorithmic Approaches for Detection and Demarcation of Interfaces in Hydrate-Fluid and...

Some aspects of the use of order parameter fields in molecular dynamics simulations to delimit solid phases containing water, namely ice and hydrate, in both hydrophilic and hydrophobic fluids are examined; this includes the influences of rectangular meshes and of filtering on the quality of these parameters. Three order parameters are studied: the mass density, ρ; an angular tetrahedrality measure, Sg (Chau and Hardwick, Mol. Phys. 1998, 93, 511); and the water-dimer dihedral angle, F4 (Rodger et al. Fluid Phase Equilib. 1996, 116, 326). The parameters are studied to find their ability to distinguish between bulk phases, their consistency in different environments, their noise susceptibility, and their ability to demarcate the interface region. Spatial sampling and filtering are covered in detail, and some temporal features are illustrated by using autocorrelation maps. The parameters are employed to determine the position of interfaces as functions of time and, with the capillary wave fluctuation method (Hoyt et al. Phys. Rev. Lett. 2001, 86, 5530; Math. Comput. Simul. 2010, 80, 1382), to estimate solid-fluid interfacial stiffnesses, with partial success for the hydrophilic/hydrophobic-type interfaces.