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

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The Cartesian diver experiment demonstrates buoyancy changes based on pressure and air volume. When the bottle is turned upside down, the diver sinks and does not float again, indicating a change in buoyancy. This may occur due to air escaping from the eyedropper, which alters its internal pressure and affects buoyancy. Despite the external air pressure being equal to atmospheric pressure when the bottle is opened, the diver remains submerged. Understanding these dynamics is crucial for explaining the behavior of the Cartesian diver in different orientations.
frensel
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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!
 
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Perhaps some air came out of the dropper when you turned it around.
 

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