The video clip below is an animated graphic showing how a liquid moisture barrier penetrates the pores in a concrete slab. The clip was produced by Protect Crete to demonstrate the way their moisture barrier products are able to diffuse throughout the slab by travelling downwards through the tiny capillaries.

As you watch the animated clip, think about the reverse action that would take place if there was no moisture barrier underneath the slab. That is, try to visualise the water travelling upwards from the wet soil through the same capillaries, and reaching the top of the slab.

This is how capillary action works. It doesn't need gravity, because it relies on the 'attractive force' between the water molecules and the 'adhesive force' of the tiny cavities.