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Author Topic: Intermolecular forces from maximum drop size ?  (Read 5872 times)

Igor

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Intermolecular forces from maximum drop size ?
« on: 09/06/2006 14:44:34 »
I noticed that when steam condeses on the underside of a horizontal surface it forms drops which only drip off when they grow to a diameter of around 10-15mm.

Is it possible to calculate the Van der Waals force between water molecules using this observation ?.

[The drop falls when the gravitational force exceeds the Van der Waals force].
« Last Edit: 12/06/2006 14:08:23 by Igor »

daveshorts

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Re: Intermolecular forces from maximum drop size ?
« Reply #1 on: 09/06/2006 18:19:04 »
The force between water molecules is mostly from something called hydrogen bonds, which are because the oxygen tends to grab the electrons away form the hydrogens so the oxygen is slightly positive and the hydrogens slightly negative, so the hydrogens from one molecule are attracted to the oxygen of another.

The size the drop falls off at is dependent on the surface tension of the water (as well as in a very colplexy way to the geometry of the situation), this is related to the strength of the bonds between the atoms, but also any detergents in the water, dissolved gasses, and very definitely the temperature (surface tension goes to zero at the boiling point).

So it would be difficult. An easy way to get an idea of the strength of the bonds between the molecules is the amount of energy it requres to break them - this can be easily worked out by taking the amount of energy it takes to boil a mole of water (18g) and dividing this by the number of molecules in a mole (6.023 * 10^23)

Igor

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Re: Intermolecular forces from maximum drop size ?
« Reply #2 on: 12/06/2006 14:08:44 »
Thank you for your post Daveshorts.
My mistake calling the intermolecular bonds in water "Van der Waals",
I have altered the title of this thread accordingly.
« Last Edit: 12/06/2006 14:09:06 by Igor »

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Re: Intermolecular forces from maximum drop size ?
« Reply #3 on: 12/06/2006 14:12:34 »
Would altitude affect this?

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daveshorts

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Re: Intermolecular forces from maximum drop size ?
« Reply #4 on: 12/06/2006 15:07:32 »
Altitude will affect the boiling point, and therefore the surface tension for a given temperature, so yes.

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Re: Intermolecular forces from maximum drop size ?
« Reply #5 on: 12/06/2006 15:53:13 »
Would relative gravity play a part too?

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daveshorts

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Re: Intermolecular forces from maximum drop size ?
« Reply #6 on: 12/06/2006 16:00:49 »
Yep the density and the local gravitational field, will affect ho hard a given sized drop is being pulled. and therefore how large it has to be to break the surface tension.

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Re: Intermolecular forces from maximum drop size ?
« Reply #7 on: 12/06/2006 16:18:48 »
I would imagine that ambient temperature would also effect things as would humidity.

What about the surface its self and its properties?

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daveshorts

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Re: Intermolecular forces from maximum drop size ?
« Reply #8 on: 12/06/2006 19:51:28 »
Yep the whole thing is hideously complex, temperature definitely affects the surface tension, and often the viscosity of the liquid (which also is a huge factor), the details of the surface are again extreemly important.

I met a guy called Norman McMillan who has spent the last 20years developing a system for characterising liquids by bouncing likght through a drop as it forms and drops off on a nozzle, this process is very dependent on a huge number of variables which makes it very useful, but a right pain to do. He was having to make the heads the drops came out of from fused silica, so you could wash them at a high enough temperature that they were reliably clean.

ROBERT

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Re: Intermolecular forces from maximum drop size ?
« Reply #9 on: 15/06/2006 15:04:24 »
quote:
Originally posted by daveshorts

(surface tension goes to zero at the boiling point).

According to this site the surface tension of water at 100 Celcius is only 21% lower than at 0 Celcius :-

" Surface Tension of Water in Contact with Air - SI units

oC x10-2 (N/m)

0  7.56
5  7.49
10 7.42
20 7.28
30 7.12
40 6.96
50 6.79
60 6.62
70 6.44
80 6.26
90 6.08
100 5.89 "
http://www.engineeringtoolbox.com/water-surface-tension-d_597.html
« Last Edit: 15/06/2006 15:08:45 by ROBERT »

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Re: Intermolecular forces from maximum drop size ?
« Reply #9 on: 15/06/2006 15:04:24 »