Seminarium Zakładu Mechaniki i Fizyki Płynów Dynamika molekularna przeplywow Knudsena 14:15!! prof. dr hab. Marek Cieplak, Instytut Fizyki PAN, Warszawa środa, 16 stycznia 2002

Novel technological applications often involve fluid flows in the Knudsen regime in which the mean free path is comparable to the system size. The boundary conditions at the wall-fluid interface in Knudsen flows are studied. The wall is modelled by atoms tethered to a lattice that interact by Lennard-Jones forces with the fluid atoms. Monatomic and polymeric Lennard-Jones fluids are considered and Couette and gravity-driven flows are studied. The scenarios of behavior envisioned by Maxwell, more than 100 years ago and still commonly used, are found not to be valid in general. For instance, there are novel effects related to a non-zero residence time of the fluid molecules in the wall vicinity. In the limiting case of strongly attractive fluid-wall interactions, the velocity ditribution of the outcoming atoms is indeed thermal. However, when the attractive tail in the fluid-wall interactions is weak, there are significant deviations from Maxwell's hypothesis. Striking many body effects are found as one interpolates between the dilute gas and the dense fluid regime. The molecular nature of the viscous and thermal slip phenomena are elucidated. The molecular dynamics simulations provide a unique window on several fundamental issues pertaining to the bridging of scales between atomic-scale and continuum physics.