A liquid floating upon another

[Suni]

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

 

[Bystander]

http://en.wikipedia.org/wiki/Rayleigh-Taylor_instability

 

[Suni]

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!

 

[Gokul43201]

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.

 

[Bystander]

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.

 

[Suni]

OK that makes sense.. thanks very much guys

 

[alias25]

so it's a complex system?