The compounds that tend to stick to the interfaces are not necessarily the same compounds that create a stable foam. I wouldn't worry about the large bubbles in the riser section. This is not the region where the bubble size is critical. You just want to collapse the foam asap into the collection cup. This will allow for more foam to take its place. One of my thoughts throughout all of this is that one should skim as 'wet' as possible, without losing an unreasonable amount of fluid over time. The skimmate will be less concentrated, but there ought to be more total junk in there. We want the skimmer to remove as much as possible, right (barring overskimming debates)? Raise the skimmer riser, and it has more time to evaporate water and concentrate surfactants at the interface. This will lead to a more stable, dry foam, but it only needs to be stable to the point where it is over the spillover ridge (point of no return).
The region right above where the air/water ratio increases dramatically is different than both the main skimmer body and the foam region. Here, the water is rapidly draining via gravity. You'd waste a lot of water by spilling at this point of course. After that, I'd say good to go. The wet-to-dry foam transition is usually characterized by a wall thickness of 10-100 microns. At a certain point, the water is a very small percentage of the total volume of the foam. This foam is called a persistent or residual foam, and is more or less indefinitely stable. I'd guess that this might be a designation for the smaller bubbles at the very top of the riser you noticed. They would collapse, of course, when rehydrated in the collection cup. Just some tidbits.
All in all, I think you should wait a few days before trying to tweak things around. Let it become more consistent. I'll be jumping into your situation with the air slimmer in a few weeks time, hopefully.
Cheers,
G1
