String-like Collective Motion and Diffusion in the Interfacial Region of Ice
Jack F. Douglas, Hao Zhang, Xuhang Tong, Xinyi Wang
We investigate the collective molecular motions and diffusion of water molecules in the mobile interfacial region of the secondary prismatic plane of hexagonal ice by molecular dynamics simulation based on the TIP4P/2005 water potential and metrologies of collective motion drawn from the field of glass-forming liquids. The width ξ of the mobile interfacial layer varies from a monolayer to a few nm as the temperature is increased towards the melting temperature Tm, in accord with recent simulations and high resolution measurements, and the dynamics within this layer is found to be dynamically heterogeneous in fashion similar to glass-forming liquids and the interfacial dynamics of metallic solids such as crystalline Ni and the grain boundary regions of Ni over a corresponding reduced temperature range, 2/3 < T / Tm < 1. In addition to exhibiting non-Gaussian diffusive transport of water molecules, decoupling between mass diffusion and structural relaxation, and stretched exponential relaxation of the self- intermediate scattering function, we find string-like collective molecular exchange motion in the interfacial zone and colored noise in the Debye-Waller factor, the mean square molecular displacement determined after a caging time of 1 ps. However, while the heterogeneous dynamics of ice is clearly similar in many ways to glass-forming materials, we find distinct trends between the average length of the strings L and the diffusion activation energy Ea and interfacial width ξ from glass-forming materials.