I don't understand your question exactly, but basically whichever direction the molecules are able to flow they can. Blow smoke against the wall, you'll see all the smoke spread out in a circle. Depending on the type of smoke the particles may settle or they will just flow through the air in whatever direction until opposed by another force
Density can work in any direction
So then, at the very edge of the flat world, wouldn't something be pulled sideways by the density of the world being only on one side?
I don't understand your question exactly, but basically whichever direction the molecules are able to flow they can. Blow smoke against the wall, you'll see all the smoke spread out in a circle. Depending on the type of smoke the particles may settle or they will just flow through the air in whatever direction until opposed by another force