As a concrete example, consider boiling water. As water boils, bubbles form which rise to the surface. I know that things rise because of the Archimedes principle, however as far as I understand it, it works because water from below pushes the bubbles upwards.
My question is, (*) how did the water get below the bubble in the first place? After some thinking, I came up with the following explanation. At first, water near the bottom of the vessel heats up, releases dissolved gases and these gases expand, while in contact with the bottom of the vessel.
The bubbles continue expanding, which decreases its pressure. I am assuming that the increase in pressure due to rise in temperature is compensated because the gases expand rapidly (is this correct?). Also, I take it that (in other situations) any bubbles formed will try to expand even if the water is not being heated.
As the bubble expands, and pressure decreases, the water close to the bottom of the vessel is able to "break into" the bubble because (1) water is at a higher pressure at the bottom, so it is the first place where the water can "break into" the bubble and (2) the decrease in the pressure inside the bubble allows the surrounding water to enter it.
This way, the surrounding water gets under the bubble and then lifts the bubble to the surface.
Now, I am not sure if this is a correct explanation. The first question I have is whether (1) is valid, i.e., why can't the surrounding water enter the bubble from all sides, does the pressure difference I mention account for this?
Secondly, what happens if there is no water to enter from the sides of the bubble. To test this, I have the following experiment in mind. Consider a test tube with water and heat it uniformly from the sides and bottom. In this case, what I expect to happen is that the released gases build up at the bottom of the test tube and keep expanding until they pour the water out of the test tube (like milk boiling over).
However, I don't have the equipment to perform such an experiment and I think the experiment above might be very sensitive in that any slight asymmetry might let some water slide to the bottom of the air bubble.
To conclude, does there need to be water below the bubble to push it upwards, i.e., is (*) a valid question? If so, is the explanation above correct and what is the outcome of the experiment I have outlined above.