Computational Study On The Dynamics Properties Of The Interfacial Liquids At Solid(Liquid)-Liquid Interfaces

Interfacial liquids are widely present in modern production and daily life,includ-ing various types of interfaces,such as solid-liquid and liquid-vapor interfaces,as well as novel liquid-liquid interfaces formed by liquid polymorphism.The liquid behavior at these interfaces profoundly affects production and lifestyle.For example,the liq-uid nature at the solid-liquid interface affects the crystal growth process,and liquid polymorphism is closely related to the next generation memory devices,phase-change material memory.Therefore,understanding interfacial liquid behavior is crucial for improving material performance.However,because the liquid at the interface is often sandwiched between two condensed matter phases,it is difficult to observe its behavior,especially its dynamic properties,using experimental means.Compared to experimen-tal studies,using molecular dynamics simulations to characterize interfacial liquids is a more mature field.However,most research has focused on characterizing the liq-uid structure at the interface,with little research on studying the dynamic properties at the interface.In particular,the characterization of the liquid-liquid interface system formed by liquid polymorphism remains largely unexplored.Therefore,this paper em-ploys molecular dynamics simulations as the primary tool to systematically investigate the dynamic properties of the liquid at three different types of interfaces:homogeneous solid-liquid interfaces（BCC-Fe,BCC-Soft Sphere）,heterogeneous solid-liquid inter-faces（Cu（111）/Pb（l））,and liquid-liquid interfaces（Si O₂）formed by liquid polymor-phism.The main results are presented below.（1）We developed a method to characterize the liquid collective dynamics at the solid-liquid interface.This method was applied to the three orientations（100）,（110）,and（111）for the equilibrium body-centered-cubic（BCC）Fe and Soft-Sphere（SS）crystal-melt interface to obtain the profiles of the liquid density relaxation time at the interface.The results show that the interfacial liquid density relaxation time is anisotropic and varies with the material.In addition,the crystal–melt interface solidification kinetic coefficients can be predicted using the time-dependent Ginzburg-Landau theory with the interfacial liquid density relaxation time.The predictions of the kinetic coefficients were corrected by substituting the values of the interfacial liquid density relaxation times into the time-dependent Ginzburg Landau theory.The updated results showed a high agreement with the results of the non-equilibrium molecular dynamics simulations.（2）We utilized the collective dynamics characterization method proposed in this study in combination with del Rio and González’s method[del Rio and González,Acta Mater.198,281（2020）]to investigate the collective dynamics behavior of liquids at Cu（111）/Pb solid-liquid interfaces.We have identified a new mechanism for interfacial liquid relaxation that is different from the“de Gennes narrowing”effect observed in bulk liquids,where the maximum interfacial liquid relaxation time does not coincide with the wave vector corresponding to the maximum peak of its structure factor.Additionally,we obtained the thermophysical parameters（such as adiabatic speed,longitudinal vis-cosity,dispersion relation,specific heat ratio,etc.）of the interfacial liquid by fitting the intermediate scattering function（ISF）in the full wave vector range using a hydro-dynamic model.Comparison of the fitted values of interfacial liquid with those of the corresponding thermophysical parameters of the bulk liquid reveals that the adiabatic sound speed,longitudinal viscosity,and dispersion relations of the liquid in both direc-tions are greater than those of the bulk liquid under the same wave vector conditions,without exhibiting anisotropy between the two directions.However,the specific heat ratio value is significantly greater than that of the bulk liquid in one direction and similar to that of the bulk liquid in the other direction,indicating a more obvious anisotropy.（3）The equilibrium liquid-liquid interface between Si O₂high-density liquid（HDL）and low-density liquid（LDL）phases was constructed using molecular dynamics sim-ulations.A characterization method was developed for the structural,thermodynamic,and dynamic properties of the liquid-liquid interface system.The results were compared with previous studies on the liquid-liquid phase transition（LLPT）system for bulk liq-uids and conventional crystal-melt interfaces.It was found that the calculations of the dynamic properties of the Si O₂HDL-LDL interface showed a spatial transition from fragile to strong liquid and hybrid three types of dynamic within the HDL-LDL inter-face.From the results of the structural properties,we observed that the coordination number ratio of Si/O jumped to an unexpectedly large value as the interface crossed from HDL to LDL,defining an interfacial region where HDL and LDL exhibited sig-nificant mixing.The concentration profile defined by the particle structure is obtained from the calculation of the tetrahedral order parameter.This concentration profile is compared with traditional crystal-melt interface studies to explain the coexistence of the LLPT phase in the framework of traditional alloy thermodynamics and phase equi-libria.