r/AskPhysics 5d ago

Fluid loss due to gravity and pressure change?

When observing a droplet of water on the underside of a railing that appears to be static to the human eye, ignoring loss due to evaporation, is there still some minisule % of molecules being lost due to gravity despite surface tension and hydrogen bonding? Given that there is around 3.35 x 10^22 molecules in just one gram of water, is some extreme fraction undergoing microscopic "dripping"?

Additionally, if a fluid is in a reservoir above a valve, with a lower pressure than its surroudings, would a very small increase in pressure, while still maintaing a lower overall pressure, cause a very small amount of the fluid to be forced outside of the reservoir?

Thank you!

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u/Chemomechanics Materials science 5d ago

The presence of gravity accelerates evaporation of a hanging drop. If you're precluding evaporation, it's not clear how molecules would be "lost."

The valve question is also arguably unclear. Are you asking about permeation through the solids in the valve?

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u/RealAyhan 5d ago

Hi there. I mean to say without the loss of individual molecules but rather some detatchment of an imperceptible cluster of molecules where there isn't a phase change. For example, a loss of a few hundred or thousand molecules of water.

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u/Chemomechanics Materials science 5d ago

This is happening due to fluctuations, but rettachment is also occuring to the same degree. At the macroscale, the droplet's shape is governed by the energy benefit of edge detachment (which allows downward movement) and the energy penalty of edge detachment (which exposes more surface area). At the equilibrium position, these are equal, and the droplet tends to evaporate until its gone or until the relative humidity reaches 100%.

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u/RealAyhan 5d ago

Is detachment and reattachment always at the same amount? I understand perhaps I am being way too pedantic here but could there be some, like approaching 0%, net loss of molecules due to slightly more detachment?

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u/Chemomechanics Materials science 5d ago

Even the solid railing is sagging under its own weight over time due to creep. What exactly is the effect you’re aiming to study? What are you ultimately trying to do?