Particle tracking is based on the classical understanding of molecules. Each molecule follows a certain path and has a well defined position and velocity at any time. Each molecule has an identity. This picture is correct from a quanum mechanical point of view as long as densities are low and temperatures are high. These conditions are fulfilled in rarefied gases.
As the name suggests, particle tracking can be used to trace the path of a molecule.
This is interesting for example, when a molecules travels through a reactor. The image below shows a simple vacuum aparatus.
The gas flows from region 1 through regions 2-5 to the outlet. The regions 2-5 act as a gas distribution system. The geometry of regions 2-5 definetely has an effect on the path molecules take on their way through the aparatus.
If a surface reation occurs at the lower side of the aparatur, the spatial distribution of the surface reaction rate will depend on the geometry of regions 2-5. In order to study the influence of the geometry on the reaction rate particle tracking can be used. The image below shows the contribution of region 2-5 to the total reaction rate. Region 2 contributes most. The contribution of the other regions decreases as the pressure in the inlet pipe decreases.