Here’s a tough one:
Are surface tension and viscosity related? If so, how?
Please, I’m looking for a real answer or a link to a legitimate source, such as an engineering textbook .
Please don’t send me google search results. I want to hear from someone with experience in fluid dynamics.
Sure, they’re related. Both are dependent on the strength of the intermolecular forces of attraction between the molecules of the substance in question.
Other physical properties dependent on the intermolecular forces of attraction include the melting point, boiling point, and vapor pressure.
Cite? See any college-level chemistry textbook.
This reminded me of a blog post from a vendor we use, entitled (appropriately enough) “How Does Surface Tension Relate to Viscosity?”
Not sure, but I don’t think they are that related, in the sense that there is not particularly a tendency for a certain surface tension to indicate a certain viscosity.
I think surface tension has more to do with preferential intramolecular attraction given that in the upward direction there are no liquid molecules available to interact with. I think viscosity, while it relies on intramolecular attraction, tends more to depend on molecular size.
For some examples:
Silicone oils all have pretty similar surface tensions (they are very low), but their viscosities vary over an enormous range. The surface tensions arise because of the attractions between the chains of dimethyl siloxane, and long and short chains are about equally attractive (the end groups constitute such a minor share of the total population). However, longer chain silicone oils have greater viscosity, and you can imagine these long ropelike molecules being more and more difficult to relocate the longer they are as they drag together.
Light alkanes and liquid metals both have low viscosities, but the metals have surface tensions a good order of magnitude higher than the alkanes.
Heavy oils and molten glass have similar high viscosities, but the glass has something like 5X greater surface tension.
This is off the top of my head, and I didn’t read the cite, but I do work with fluid dynamics and rheology and physical properties of liquids and gasses. I hope this is correct, and that it gets at what the OP cares about. If robby or anybody else can tell me what of this is wrong, I’d be glad to hear it.
That is a pretty fair summary, although the intramolecular forces are determined more by the natural and induced polarity of the molecules rather than just size. It just happens that larger unbound molecules (those that don’t form enclosed rings like benzene) tend to have more interactions that make them generally more “sticky”, like carbohydrates that form starches.
Stranger
Nitpick: you mean intermolecular forces (i.e. between molecules), not intramolecular forces (i.e. within molecules).
As for molecular size versus polarity, your point is well taken. For example, water (H[sub]2[/sub]O), being polar, has much stronger intermolecular forces than nonpolar methane (CH[sub]4[/sub]), which is well demonstrated by water’s much higher melting point and boiling point.
However, there are some high molecular weight nonpolar alkanes that have even stronger forces simply due to the size of the molecules. You have to get up into the 20+ carbon alkanes, however, before you start seeing molecules with higher melting/boiling points than than of water.
So on the whole, I agree. Polarity is generally a stronger factor than molecular size.