The knowledge about the precise behaviour of gases is need for many technical applications. But under certain circumstances, the gas cannot be treated as a continuum. A different point of view is needed.
When lowering the pressure, less molecules are present. They collide less frequently with each other. The average distance travelled between two collisions is called mean free path.
When the mean free path becomes large the probability of colliding with the wall instead of colliding with other gas molecules increases. Thus, the interaction between the gas molecules and the wall surface becomes more important. The same situation arises when instead of low pressure the tube diameter becomes small.
For a general description the Knudsen number is used. This is the ratio of the mean free path to a characteristic length, e.g. the tube inner diameter.
Experiments have shown that with rising Knudsen number, the gas flow through a tube is larger than suspected. This is accounted for by removing the no slip boundary condition that sets the velocity of the gas to zero directly at the wall. Instead, a “slip” is introduced which is quantified by empirical correlations.
In this project we try to describe this phenomenon by surface diffusion instead of a slip coefficient. For this we measure the gas flow (helium and carbon dioxide) in silicon micro channels of different sizes at different pressures. To change the gas-surface interaction, we functionalize the surface. Based on this data, a model will be set up for describing and predicting the given and other systems.
This project is a cooperation with the ITEP (KIT) where the experiments and DSMC simulations are performed.