I think this article ignored the major reason that large particles rise to the surface. The laws of inertia and momentum indicate that larger particles don’t react as quickly as smaller particles do at the end of each back-and-forth shake of the container. This means there is a force for relative motion between different-sized particles. Just as a basketball will roll over a tennis ball when they collide, so larger particles when they collide will tend, despite their weight, to ride up over smaller ones.
The convection effect can also be explained by momentum and inertia. As the container stops at each end of the back-and-forth shake, the contents tries to keep on going, thus pressing against the leading side of the container. This leaves a slight gap at the trailing side, into which only small particles can fall. As for rising faster in a vacuum, it would seem that air probably has a cushioning or even binding effect. Remove it, and it becomes easier for inertia and momentum to work their magic.
Richard S. Blake
East Falmouth, Mass.