The same is not true for solids, e.g. an iron block kept over a wooden block.

  • 3
    $\begingroup$ Because liquids flow... $\endgroup$
    – Jon Custer
    Mar 31 at 13:05
  • $\begingroup$ ... and the gravitational field. $\endgroup$
    – Bill N
    Mar 31 at 17:30

Jon Custer's comment is sufficient: we define fluids (gas and liquid) as different states of matter from solids specifically because fluids do not have a fixed shape.

I'll add that, if you are careful, you can in fact fill a glass (optimistically or pessimistically :-) ) such that the denser fluid remains on top. Any small perturbation will probably cause the fluid to break the interface barrier and flow to bottom, of course.

Such a phenomenon is seen in nature with (gas) atmospheric inversion layers. Again, this is a transient phenomenon.

  • $\begingroup$ Furthermore, the greater the viscosity, the slower the settling process. In the limit of infinite viscosity (the ideal solid, with creep ignored), a denser solid atop another lies away from equilibrium but is kinetically prevented from reaching equilibrium. $\endgroup$ Mar 31 at 20:11

It is a general rule that the equilibrium state of a system is the state of lowest energy. Taking your example, the state of lowest potential energy is obtained, if the denser liquid occupies the "bottom half" and the less dense liquid occupies the "upper half" of the container. If we invert the two liquids the potential energy increases by $E_{pot} = \Delta m g h = \Delta \rho V g h$, where $h$ is the distance between the two center of masses. This idea is also called Archimedes' principle.

The argument naturally breaks down, if gravity were absent. Hence, if we leave the earth the isotropy of space demands that the two liquids can be arbitrarily oriented. This should not come as a surprise, because without gravity the directions "up" and "down" have no meaning.

  • $\begingroup$ Can you apply and demonstrate the same principle for a centrifuge? $\endgroup$ Apr 8 at 10:53
  • $\begingroup$ You can apply this principle to every system. One example would be a centrifuge. An other example would be a ballon filled with helium, which is located in a moving car. If you step onto the breaks the ballon move to the back of the car. To understand this just apply the above principle. $\endgroup$
    – Semoi
    Apr 8 at 15:34

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