Molecular dynamic simulation of liquids

Fluid flow in very small systems on the nanoscale, having typical lengthscales of (10 -1000 nanometers), behaves differently from fluid flows on more anthropomorphic scales. As the length scale becomes smaller several scaling effects may occur. The governing dimensionless numbers such as the Reynolds number or the surface-to-volume ratio formally tend to zero or infinity, respectively. Ultimately, the continuum assumption will break down. Not all physical processes involved on the nanoscale have yet been identified and/or fully understood, because of the great difficulties associated with experiments and the theoretical modeling on the nanoscale.

Molecular dynamic simulation of the capillary-driven flow in a pore

Figure 1: Initial condition for the simulation of a capillary-driven flow down a pore.

Rather than extending the continuum to smallest scales, the method of molecular dynamic simulation (MD) tackles the problem from a particle point of view. This technique has turned out to be an useful tool for the modeling and understanding of smallest-scale fluid flows. Using MD the behaviour of atoms and molecules in space and time is simulated directly, using Newton's equations of motion together with appropriate models for the intermolecular interactions. This tool resembles a "perfect" experiment and it is, therefore, particularly suited to understand the processes which occur on scales which separate the atomic and the collective (fluid) motion.

In this project we employ MD to simulate systems which involve fluid-fluid and fluid-solid interafaces which are driven by an applied thermal gradient along the surface. On the macroscopic scale this usually induces surface-tension gradient which can drive sigificant fluid motion (see thermocapillary convection). Interesting effects associated with interfaces on the atomic scale are layered structures of the fluid near solid walls, compressibility effects owing to high Laplace pressures, slip of the velocity on solid walls, random walks of the contact line, thermal creep, viscous heating, and modifications of the heat conduction and diffusion processes.