Molecules taken up
The movie shows a zeolite slab in the center (red balls = oxygen atoms, yellow balls = silicon atoms), which gives rise to pores (blue channels) that are initially empty. The pores are filled with gas molecules (here: butane displayed as orange chains) via the green subboxes (called control volumes) located at either end of the simulation box. The special trick is that the control volumes are constantly replenished with new molecules. That is, as soon as too many molecules have left the control volume to keep the concentration at a constant level new molecules will be inserted to ensure the constant supply. The diagram on the bottom, the so-called uptake curve, measures the progress of the filling. The related article, which can be found [here], highlights that guest transport into exceptionally thin porous materials (nanosheets, nanospheres, etc.) can violate Fick's laws if the pores are as smooth as in the movie.