High School Why are there no bubbles in a pipette when pressure is released?

[fresh_42]

TL;DR

How exactly does the capillary action work in this case?

I have observed the following phenomenon:
Step 1: Dunk the pipette into the solution.
Step 2: Press it and hold it pressed.
Step 3: Pull out the pipette, until the lower end is in the air.
Step 4: Release the pressure.

Now one would expect that the solution in the pipette sucks bubbles of air. However, it does not. The solution in the pipette climbs as if there was still a fluid to suck in. Why? O.k. it is presumably capillary action which is responsible, and the fluid has to climb until the pressures are equalized. But where are the bubbles?

 

[Swamp Thing]

Now one would expect that the solution in the pipette sucks bubbles of air.

If we expect to see bubbles, it's because we are assuming that the fluid wants to stick to a fixed place on the pipette's wall, and therefore the air has to force the liquid to bulge upwards in the middle.

But if the liquid doesn't mind sliding upwards, then it can just act as a piston. So I'd guess it depends on how much the liquid wets and adheres to the wall. Something to do with : https://en.wikipedia.org/wiki/Wetting

Another possibility that seems intuitively plausible is this: if the liquid layer is fairly thin, then it will bubble. If it's thick, it will slide like a piston. If this turns out to be true, then it might imply that the adhesion (resistance to sliding) is contributed by the interface between the wet and dry parts, while the tendency to slide easily is dominated by the wet part. If the liquid layer is thick, the latter might be stronger.

So the outcome (sliding vs bubbling) might depend on an interaction between wetting, surface tension and geometry. E.g. a larger diameter tube might show more bubbling tendency compared to a narrow one.