A liquid floating upon another

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SUMMARY

The discussion centers on the behavior of a low-density liquid floating beneath a high-density liquid in a beaker, specifically addressing the forces that maintain this configuration. Participants clarify that while the liquids can remain in a metastable equilibrium, any perturbation, including thermal fluctuations, can disrupt this state. The Rayleigh-Taylor instability is identified as a key concept, illustrating that the interface between the two liquids is susceptible to disturbances that can lead to mixing over time. The lifetime of this equilibrium is inversely related to the ambient temperature, emphasizing the role of thermal dynamics in fluid behavior.

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Suni
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Hi all

Just a quick (relatively simple) question..
As high school physics/chem tought, a liquid of lower density will float uopn that of a higher density. This makes sense in most situations because of the pressure gradient in the higher-density liquid pushing the other fluid upwards. But what if we had a situation as so:

A low density liquid is sitting in a beaker. We then somehow place a high density liquid PERFECTLY upon the lower density so there is a point in time where the beaker would look like:

High density liquid
----------------- INTERFACE
Low density liquid

Now my question is what is it that would force the higher density liquid downwards? Because none of the liquid has actually penetrated into the low-density area yet (due to the interface) we can't use any pressure gradient differences. I was thinking it may have to do with Brownian motion causing a transfer of molecules across the interface which would eventually cause a total realisation of the pressure gradient. Does this make sense or is it wrong?

Thanks,
Dan
 
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Thanks for the link mate.. very helpful.. wasn't aware of that
So it is then right to say that a system like that can stay in equilibrium so long as it isn't touched/bumped etc? I guess that makes sense!
 
Suni said:
Thanks for the link mate.. very helpful.. wasn't aware of that
So it is then right to say that a system like that can stay in equilibrium so long as it isn't touched/bumped etc? I guess that makes sense!
That's close, but not exactly right. The RT phase is a metastable phase, meaning that any perturbation will destroy it. However, it doen't need touching or bumping for this to happen. Ordinary thermal fluctuations at the ambient temperature will kill such a phase given enough time. The lifetime is hence inversely proportional to the ambient temperature.
 
Salt domes to thunderstorms --- you don't really need the "liquid" constraint. This is also the situation in which "the butterfly effect" is manifested, the "butterflies" being the fluctuations G. mentions.
 
Last edited:
OK that makes sense.. thanks very much guys
 
so it's a complex system?
 

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