Interdisciplinary Applied Mathematics

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Effect of Structure and Thermal Motion of the Wall Atoms

Smooth walls (Toxvaerd, 1981; Somers and Davis, 1992) as well as walls with atomistic structure (Travis and Gubbins, 2000; Sokhan et al., 2001; Somers and Davis, 1992) have been widely used in the MD simulation of fluids confined in nanoscale channels. For a smooth wall, the wall-fluid potential depends only on the normal distance between the fluid atom and the channel wall, while for a wall with atomistic structure, the wall-fluid potential depends on the relative distance between the fluid atom and each atom in the wall. Typically, only the first fluid layer is significantly influenced by the wall structure, and the rest of the fluid layers are not significantly affected by the structure of the wall. In many simulations, the wall atoms are either frozen to their lattice sites (Heinbuch and Fischer, 1989; Zhang et al., 2001) or constrained to their lattice sites by a spring (Thompson and Robbins, 1990; Travis and Gubbins, 2000; Sokhan et al., 2001). The former

FIGURE 10.5. Number density profiles across (a) and along (b) a 4.0<r-wide slit channel for three different cases where the fluid-fluid and fluid-wall interactions are modeled differently (System A: WCA system, filled circles, system B: LJ system, open circles, and system C: WCA-LJ system, open triangles). (Courtesy of K. P. Travis.)

enables the use of a larger time step in MD, since the thermal vibration of the solid atoms is not resolved, while the latter seems to be more realistic. The thermal oscillation of wall atoms introduces further corrugations into the potential felt by the fluid atoms near the wall, and therefore leads to a reduced density oscillation near the channel wall (Thompson and Robbins, 1990; Sokhan et al., 2001).

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