Site-dipole field and vortices in confined water

We use molecular dynamics simulations to examine how the spatial patterns formed by the diffusive motion of water molecules are influenced by confinement between hydrophilic and hydrophobic surfaces. For bulk liquid water, Higo et al. found that the long-range orientational order of spatial dipole moments can form vortex-like spatial patterns for as long as 300 ps [Higo et al., Proc. Natl. Acad. Sci. USA 98, 5961 (2001)]. We perform a similar analysis for confined water and we find that the existence of vortices in these systems is dependent on both the surface separation and the surface hydrophilicity. Vortices perpendicular to the surface normal disappear when the surface separation is reduced to a thickness where the system is comprised of mostly interfacial water molecules. Vortices exist at slightly smaller separations for hydrophobic surfaces than for hydrophilic surfaces because the dipoles are less aligned at the hydrophobic surfaces. The dipole alignment that is induced by the hydrophilic surfaces is counter to the direction required by vortices.