Density oscillations, simulation

The principal effect of the presence of a smooth wall, compared to a free surface, is the occurrence of a maximum in the density near the interface due to packing effects. The height of the first maximum in the density profile and the existence of additional maxima depend on the strength of the surface-water interactions. The thermodynamic state of the liquid in a slit pore, which has usually not been controlled in the simulations, also plays a role. If the two surfaces are too close to each other, the liquid responds by producing pronounced density oscillations. 

Fig. 5(a) contains the oxygen and hydrogen density profiles it demonstrates clearly the major differences between the water structure next to a metal surface and near a free or nonpolar surface (compare to Fig. 3). Due to the significant adsorption energy of water on transition metal surfaces (typically of the order of 20-50kJmoP see, e.g., [136]), strong density oscillations are observed next to the metal. Between three and four water layers have also been identified in most simulations near uncharged metal surfaces, depending on the model and on statistical accuracy. Beyond about...

The results in Table V illustrate that MD studies, compared to the MC results in Table IV, facilitate the investigation of transport and time-dependent properties. Also, they show that use of the MCY potential leads to very large density oscillations and increasing water density near the surfaces. This appears to be a serious drawback to the use of the MCY potential in simulations of interfacial water. Results from the investigations using the ST2 potential show that interfacial water density is approximately 1.0 g/cc, with a tendency for decreased density and hydrogen bonding near the surfaces. As in the MC simulations, orientations of the water dipole moment are affected by the presence of a solid/liquid interface, and an... 

Monte Carlo and Molecular Dynamics simulations of water near hydrophobic surfaces have yielded a wealth of information about the structure, thermodynamics and transport properties of interfacial water. In particular, they have demonstrated the presence of molecular layering and density oscillations which extend many Angstroms away from the surfaces. These oscillations have recently been verified experimentally. Ordered dipolar orientations and reduced dipole relaxation times are observed in most of the simulations, indicating that interfacial water is not a uniform dielectric continuum. Reduced dipole relaxation times near the surfaces indicate that interfacial water experiences hindered rotation. The majority of simulation results indicate that water near hydrophobic surfaces exhibits fewer hydrogen bonds than water near the midplane. 

A wide variety of different models of the pure water/solid interface have been investigated by Molecular Dynamics or Monte Carlo statistical mechanical simulations. The most realistic models are constructed on the basis of semiempirical or ab initio quantum chemical calculations and use an atomic representation of the substrate lattice. Nevertheless, the understanding of the structure of the liquid/metal surface is only at its beginning as (i) the underlying potential energy surfaces are not known very well and (ii) detailed experimental information of the interfacial structure of the solvent is not available at the moment (with the notable exception of the controversial study of the water density oscillations near the silver surface by Toney et al. [140, 176]). 

Mathematical formalism has been developed using semi-empirical considerations [36, 37]. Computer simulation smdies show that resulting equation predicts oscillations. Attempt has been made to provide justification on the bases of Navier-Stokes equation but it is open to question. Dimensional analysis has recently been employed for investigating the phenomena [31]. Flow dynamics and stability in a density oscillator have been examined by Steinbock and co-workers [38], They have related it to Rayleigh-Taylor instability of two different dense viscous liquids. A theoretical description has been presented which is based on a one-fluid model and a steady state approximation for a two-dimensional flow using Navier-Stokes equation. However, the treatment is quite complex and cannot explain the generation of electric potential oscillations. 

The comparison to the Monte Carlo simulations is good, this method yields for the test case with the observed density oscillations in the profile of the counterions. Another integral equation is derived from the one particle direct correlation function ri(l) Eq.(1.72) by introducing relative coordinates for all field in the diagram representation and taking the derivatives with respect to those coordinates. This yields and exact hierarchy of equations that is related to thy BGY hierarchy. The first member of the Wertheim-Lovett-Mou-Buff (WLMB) equation is... 

The domain is discretized with a Cartesian grid of 128 cells with a length of 6.25 X10 m in each spatial direction, resulting in a total of about two million cells. The gravity is set to zero and continuous (Neumann) boundary conditions are selected for all sides. The droplet is initialized quiescent at the center of the domain as an oblate ellipsoid with an aspect ratio of the semiaxis to the two short semiaxis of 5tl and a volume corresponding to the volume of a sphere with the diameter >0 = 2 mm. For the surrounding medium we take air with density 1.1894 kg/m and dynamic viscosity 1.82 x 10 Pa s. The computational domain is shown in Fig. 17.14. For the droplet oscillation simulations only, we use the balanced CSF model by BrackbUl [37] instead of the CSS model for the calculation of the surface... 

Near strongly attractive surfaces, liquid structure differs noticeably from the bulk one. This is caused by the packing effect due to the localization of molecules in a plane(s) parallel to the wall and by specific fluid-wall interactions, such as H-bonds. Density oscillations of liquids near solid substrates were observed in experiments [143, 144, 417-419] and in numerous computer simulations of confined fluids. Besides, fluids with strongly anisotropic interactions (such as water) unavoidably undergo orientational ordering near the wall. It is important to know the character of this ordering and its intrusion into the bulk liquid. In the present section, we consider structural properties of adsorbed water layers in the liquid, bilayer, and monolayer phases. 

Another feature seen in the fine-scaled interfacial density profiles is that the crystal layers tend to relax outward into the fluid region, so that the spacing of the peaks in the density oscillations increases. This phenomenon has also been observed in recent and MD simulations of the singlecomponent hard sphere interface, where it appears to be related to the preference of the fluid to order at the interface in a way that is more consistent with a [111] face. 

It is generally known, and our simulations confirm, that liquids in contact with a hard surface exhibit density oscillations over the first few molecular layers, which can he described by the distribution function p(z), where z is the distance from the surface. This function has a pronounced maximum not far from the sum of the radii of the surface atoms and those of the liquid molecules. After this maximum there is a minimum, then a more shallow second maximum, etc., the oscillations petering out after a few molecular cross sections. 

Mavri, J., Berendsen, H.J.C. Dynamical simulation of a quantum harmonic oscillator in a noble-gas bath by density matrix evolution. Phys. Rev. E 50 (1994) 198-204. 

Stelzer et al. [109] have studied the case of a nematic phase in the vicinity of a smooth solid wall. A distance-dependent potential was applied to favour alignment along the surface normal near the interface that is, a homeotropic anchoring force was applied. The liquid crystal was modelled with the GB(3.0, 5.0, 2, 1) potential and the simulations were run at temperatures and densities corresponding to the nematic phase. Away from the walls the molecules behave just as in the bulk. However, as the wall is approached, oscillations appear in the density profile indicating that a layered structure is induced by the interface, as we can see from the snapshot in Fig. 19. These layers are... 

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