String-like collective motion and diffusion in the interfacial region of ice

XY Wang and XH Tong and H Zhang and JF Douglas, JOURNAL OF CHEMICAL PHYSICS, 147, 194508 (2017).

DOI: 10.1063/1.5004177

We investigate collective molecular motion and the self-diffusion coefficient D-s of water molecules in the mobile interfacial layer of the secondary prismatic plane (11 (2) over bar0) of hexagonal ice by molecular dynamics simulation based on the TIP4P/2005 water potential and a metrology of collective motion drawn from the field of glass- forming liquids. The width xi of the mobile interfacial layer varies from a monolayer to a few nm as the temperature is increased towards the melting temperature T-m, in accordance with recent simulations and many experimental studies, although different experimental methods have differed in their precise estimates of the thickness of this layer. We also find that the dynamics within this mobile interfacial ice layer is "dynamically heterogeneous" in a fashion that has many features in common with glass-forming liquids and the interfacial dynamics of crystalline Ni over the same reduced temperature range, 2/3 < T/T-m < 1. In addition to exhibiting non-Gaussian diffusive transport, decoupling between mass diffusion and the structural relaxation time, and stretched exponential relaxation, we find string-like collective molecular exchange motion in the interfacial zone within the ice interfacial layer and colored noise fluctuations in the mean square molecular atomic displacement < u(2)> after a "caging time" of 1 ps, i.e., the Debye- Waller factor. However, while the heterogeneous dynamics of ice is clearly similar in many ways to molecular and colloidal glass-forming materials, we find distinct trends between the diffusion coefficient activation energy E-a for diffusion D-s and the interfacial width xi from the scale of collective string-like motion L than those found in glass-forming liquids. Published by AIP Publishing.

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