Why does the Cartesian diver sink when the bottle is turned upside down?

[frensel]

Hi,

I follow the instruction in wikipedia to make a Cartesian diver (Coca Cola bottle + eyedropper), and it works well. I press the bottle, the eyedropper sinks; I release, it floats.

However, if I turn the bottle upside down, the eyedropper sinks and it never floats again. That means the buoyancy of the eyedropper is somewhat changed but I cannot understand the mechanism. If the buoyancy of the eyedropper decreases, the air pressure inside the bottle should increase, but it seems that it is not the case. I can open the bottle and the air pressure inside the bottle is equal to the atmospheric pressure, the eyedropper still sinks.

Can anyone repeat the experiment and explain why it happens? Thank you!

 

[JDługosz]

Perhaps some air came out of the dropper when you turned it around.