A liquid floating upon another

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    Floating Liquid
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Discussion Overview

The discussion revolves around the behavior of two immiscible liquids with differing densities, specifically focusing on the stability of a high-density liquid placed atop a low-density liquid and the factors that might influence their interface. The scope includes conceptual understanding and theoretical implications of fluid dynamics.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant, Dan, questions what would force a high-density liquid downwards when it is perfectly placed on a low-density liquid without any penetration, suggesting that Brownian motion might play a role in creating a pressure gradient over time.
  • Another participant provides a link to the Rayleigh-Taylor instability, which relates to the discussion of stability in such liquid systems.
  • There is a clarification that while the system can remain in a metastable phase, it is susceptible to perturbations, including thermal fluctuations, which can disrupt the equilibrium without external disturbance.
  • A further comment suggests that the phenomenon can be observed in various contexts beyond liquids, referencing the "butterfly effect" in complex systems.

Areas of Agreement / Disagreement

Participants express varying views on the stability of the liquid interface, with some agreeing on the metastable nature of the system while others highlight the role of thermal fluctuations and perturbations. The discussion remains unresolved regarding the exact mechanisms at play.

Contextual Notes

The discussion does not fully resolve the assumptions regarding the conditions necessary for the stability of the liquid interface and the implications of thermal fluctuations on the system's behavior.

Who May Find This Useful

Individuals interested in fluid dynamics, thermodynamics, and the behavior of complex systems may find this discussion relevant.

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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