Imagine we have to liquids with densities $\rho_2$ and $\rho_1$ and $\rho_2>\rho_1$. Suppose we pour $\rho_1$ into a container. And then we pour $\rho_2$ on it. As a rule, the denser liquid ($\rho_2$) moves downwards and $\rho_1$ moves upwards. This process is caused by the buoyancy force. But how does the $\rho_2$ buoyancy force push $\rho_1$ up while it is above it?
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1 Answer
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In principle the dense fluid could remain above the lighter fluid - as long as the boundary remains perfectly horizontal. This is of course unstable, and any vibration will cause the boundary to move up in one place and down in another. Gravity then pulls the dense fluid down where the boundary is lower, and the lighter fluid is pushed out of the way and upward through the places where the boundary is higher.
Interestingly, if you can prevent waves that are too long in the boundary, the dense liquid can stay above the lighter one. The actual length depends on the densities, gravity and the surface tension. This is one way of explaining why you can keep water in a straw by putting your finger over the top end.
