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At a basic level, both surface tension and viscosity are the result of forces between particles. Usually we'd talk about intermolecular forces, because liquids are usually molecules, but obviously that doesn't work for mercury, which isn't made of molecules at all. Stronger forces between particles give higher surface tension, and usually also higher viscosity. So water's fairly high surface tension is usually credited to hydrogen bonding, and mercury's exceptionally high surface tension is put down to metallic bonding; while the increasing viscosity of aliphatic hydrocarbons with more carbons is said to be due to increasing London dispersion forces.

It's easy to see that the two are poorly correlated, though. Ethanol has almost identical surface tension to acetone, for example, but it's more than three times as viscous. Most strikingly, mercury has really extraordinarily high surface tension, but is no more viscous than cold water.

Is it completely wrong to expect at least some kind of loose correlation between surface tension and viscosity? Is viscosity just a far more complex phenomenon, a proper explanation of which lies way beyond the hand-wavy school science that chemistry teachers use to give a vague idea about things like trends in the alkanes? Would it be better to emphasise entanglement of long molecules when trying to explain the viscosity of longer alkanes, rather than just stronger forces?

Is mercury's low viscosity mainly down to its small particle size, or is there much more to it than that?

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  • $\begingroup$ I'd say it is due to the nature of metallic bond. See, it is undirected. $\endgroup$ – Ivan Neretin Feb 8 '16 at 6:10
  • $\begingroup$ How is that significantly different from any of the intermolecular forces behind viscosity and surface tension in other liquids, @Ivan? $\endgroup$ – Oolong Sep 7 '16 at 8:56
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    $\begingroup$ Sorry, I don't remember what I had in mind back then in February when writing my first comment. As it seems now, yes, it would be better to emphasize the entanglement of long molecules and all. $\endgroup$ – Ivan Neretin Sep 7 '16 at 9:00
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    $\begingroup$ Related discussion on physics SE: physics.stackexchange.com/questions/148792/… Also, this chemical engineering article developed an empirical relationship between viscosity and surface tension: google.com/… $\endgroup$ – Tyberius Sep 4 '17 at 16:41
  • $\begingroup$ One important difference is that surface tension is an elastic property, another that while surface deformation requires some viscous flow (which has a similar activation energy than bulk viscous flow), it also requires a net severing (or re-forming) of intermolecular bonds, which has a different activation energy and $\Delta H \neq 0$ $\endgroup$ – Karl Oct 5 '18 at 5:41

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