Atomic motion, interfacial dynamics

Extrapolating continuous description of fluid motion to a molecular scale might be conceptually difficult but unavoidable as far as interfacial dynamics is concerned. Long-range intermolec-ular interactions, such as London-van der Waals forces, still operate on a mesoscopic scale where continuous theory is justified, but they should be bounded by an inner cut-off d of atomic dimensions. Thus, distinguishing the first molecular layer from the bulk fluid becomes necessary even in equilibrium theory. In dynamic theory, the transport in the first molecular layer can be described by Eq. (60), whereas the bulk fluid obeys hydrodynamic equations supplemented by the action of intermolecular forces. Equation (61) serves then as the boundary condition at the solid surface. Moreover, at the contact line, where the bulk fluid layer either terminates altogether or gives way to a monomolecular precursor film, the same slip condition defines the slip component of the flow pattern. 

Many of the fiindamental physical and chemical processes at surfaces and interfaces occur on extremely fast time scales. For example, atomic and molecular motions take place on time scales as short as 100 fs, while surface electronic states may have lifetimes as short as 10 fs. With the dramatic recent advances in laser tecluiology, however, such time scales have become increasingly accessible. Surface nonlinear optics provides an attractive approach to capture such events directly in the time domain. Some examples of application of the method include probing the dynamics of melting on the time scale of phonon vibrations [82], photoisomerization of molecules [88], molecular dynamics of adsorbates [89, 90], interfacial solvent dynamics [91], transient band-flattening in semiconductors [92] and laser-induced desorption [93]. A review article discussing such time-resolved studies in metals can be found in... 

An important message carried in this chapter is that friction and adhesion are connected by the similarity in the dynamic feature that they both are accompanied by unstable motion of interfacial atoms, i.e., jump in and out of contact, and it is the energy loss in approach/ separation cycle that determines the magnitude of friction. 

Application of this construction reveals that the mobile particle motion at the NP interface indeed takes the form of strings, as in GF liquids [23] and the GB of polyciystalline materials [16,69] and we next characterize these structures more precisely to see how their geometry compares to their GF and GB counterparts. We will examine the nature of the collective motion and average local particle displacement dynamics of the Ni atoms in the interfacial region of NP using the same numerical metrologies as in our previous complementary study of the dynamics of GB in polycrystalline Ni, as described in Section II.D. 

In MD simulation, atoms and molecules are allowed to interact for a period of time by approximations of known physics in order to explore the physicochemical properties of solutions and structures such as interfacial phenomena and the dynamics of water molecules and ions, thus providing detailed information and fundamental understanding on relationships between molecular structure, movement, and function (Brossard et al. 2008 Du and Miller 2007a, 2007b Du et al. 2007a, 2007b Lazarevic et al. 2007 Miller et al. 2007 Nalaskowski, et al. 2007). With MD simulation, scientists are able to examine the motion of individual atoms and molecules in a way not possible in laboratory experiments. 

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