We present a classical molecular-dynamics study of the collective dynamical properties of the coexisting liquid phase at equilibrium body-centered cubic (BCC) Fe crystal-melt interfaces. For the three interfacial orientations (100), (110), and (111), the collective dynamics are characterized through the calculation of the intermediate scattering functions, dynamical structure factors and density relaxation times in a sequential local region of interest. An anisotropic speed up of the collective dynamics in all three BCC crystal-melt interfacial orientations is observed. This trend differs significantly different from the previously observed slowing down of the local collective dynamics at the liquid-vapor interface [Acta Mater 2020;198:281]. Examining the interfacial density relaxation times, we revisit the validity of the recently developed time-dependent Ginzburg-Landau (TDGL) theory for the solidification crystal-melt interface kinetic coefficients, resulting in excellent agreement with both the magnitude and the kinetic anisotropy of the CMI kinetic coefficients measured from the non-equilibrium MD simulations.
By employing the non-equilibrium molecular dynamics (MD) simulations and the time-dependent Ginzburg-Landau (TDGL) theory for the solidification kinetics, we predict the kinetic coefficients for the BCC(100), (110), and (111) CMIs of the soft-spheres, which are modeled with the inverse-power repulsive potential, and compare with the previous reported data of the BCC Fe system. We confirm a universal-like behavior of the spatial integrations of the (density wave amplitudes) Ginzburg-Landau order parameter square-gradient for the BCC CMI systems. The TDGL predictions of the kinetic anisotropies for BCC soft-sphere and BCC Fe CMI systems are identical; both agree well with the MD measurement for the soft-sphere system but differ strongly with the MD measurement for the Fe system. This finding implies that the current TDGL theory reflects a preference of presenting the generic anisotropy relationship due to the interfacial particle packings but lacks the contribution parameter which addresses the specificities in the kinetic anisotropies owing to the particle-particle interactions. A hypothesis that the density relaxation times for the interface melt phases to be anisotropic and material-dependent is then proposed.
The direct measurement of the dielectric properties of the confined water is exceedingly challenging, result in the lack of a quantitative understanding of its critical roles in electrochemistry, interfacial reactivity and transport thermodynamics. In this paper, we employ the equilibrium molecular dynamics simulation and the linear response theory-based analytical expressions for the local permittivity tensor, to calculate the static and dynamic dielectric response properties of the monolayer ice and water confined in the 0.65 nm size hydrophobic slab pore under 5×10 8 Pa lateral pressure and different temperatures. We carry out a detailed comparative study on the performance of predicting the confined structure and dielectric response properties between two well known water molecule models, i.e., constant dipole moment SPC/E model and polarizable SWM4-NDP water model. We have analyzed the probability distributions of the instantaneous SWM4-NDP water molecular dipole moments and calculated the static structure factor, radial dipole-dipole correlation function, static dielectric tensor, total dipole autocorrelation function and Debye relaxation time of each simulation system. For the first time, we found the novel variation of the water molecular polarities, in the monolayer confined liquid and solid phase of water, due to the extreme confinement condition. The performance in describing the structural properties are found comparable between the two water models, and the enhancement of the confinement weakens the advantage of the SWM4-NDP model in predicting the static dielectric property. However, in the prediction of the dynamic properties such as dielectric relaxation time, SWM4-NDP water model is superior to the SPC/E model. Therefore, we suggest that using SWM4-NDP model in the future investigation of the structural phase transition kinetics, ionic transportation and solvation kinetics would be the better choice. The current achievement of the fundamental insight and computational data could potentially facilitate the theoretical advancements in designing new devices of energy storage, sensor, and medicine delivery based on confined water systems.
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