Som general principles for cartridge material (please correct this if you know any better):
1. Surface tension is the main force you need to manipulate. In other terms, the tendency of a liquid to adsorb to the mesh - low surface tension - vs. the tendency of a liquid to cohere to itself - high surface tension.)
2. Both propylene glycol and glyerine have a lower surface tension than water. I would imagine there would also be an emulsifier in there that would reduce it further.
3. The viscosity of the liquid defines how quickly the forces will act to distribute the liquid. E-liquid clearly has a higher viscosity than water.
4. All things being equal, the eliquid will adhere to the mesh. It will therefore appear to move from areas of lower surface area to areas of higher surface area.
5. If the cartridge mesh is finer than the atomizer mesh, it will tend to pull the liquid out of the atomizer mesh, and vice versa. ("Dry" smoke.)
6. If the cartridge mesh is coarser than the atomizer mesh, the atomizer mesh will tend to pull the liquid out of the cartridge mesh. (What we want.)
7. A cartridge mesh that is too coarse will allow the e-liquid to cohere into droplets large enough that you end up sucking them out through the mouthpiece ("wet" smoke).
8. If the atomizer mesh and the cartridge mesh are not touching, the liquid will not transfer. This is a real problem for all-metal cartridge designs, since the metal tends to permanently deform when pushed down by the atomizer.
9. The mesh right at the top of the cartridge is the most important part to get right if you want to avoid liquid in the mouth (empricial.) My feeling is that probably a coarse mesh underneath and a fine mesh cap at the top would be ideal, but I will need to try this.
10. Using a spring to maintain contact between the cartridge and atomizer meshes only works if you put it between the mouthpiece and the cartridge itself (filing down the ridges in the cartridge). Even here, it tends to squeeze on the cartridge so that the eliquid pools and you get eliquid in the mouth. There may be a spring weak enough that it doesn't cause pooling, but strong enough that it maintains contact.
11. Cartridges work better to begin with (empirical). You cannot use the first few hours of use to evaluate them. Use them at least a day before you evaluate. I presume this has something to do with the mesh surface.
12. You will always cut yourself working with metal mesh.