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My textbook says the following on viscosity and pressure:

increase in pressure decreases the viscosity of water but for other liquids it increases. Viscosity of gases does not change much

No further explanation is provided.

I have two doubts regarding this:

  1. How does the viscosity of water decrease? Does it have something to do with the anomalous expansion of water?
  2. How does the viscosity of gases not change much on increasing the pressure? The gas molecules should have an increase in kinetic energy, contributing to more collisions and hence more viscosity
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  • $\begingroup$ These two issues are quite different, shouldn't be in one post, please have this in mind in the future. $\endgroup$ – Mithoron Feb 5 '20 at 1:25
  • $\begingroup$ Ok will do in the future. I can't post a new question (M. Farooq has already written an excellent answer) so for now I'll tweak the title a bit. $\endgroup$ – Aniruddha Deb Feb 5 '20 at 7:31
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One of the best websites for all advanced water properties is a website maintained by Dr. Martin Chaplin http://www1.lsbu.ac.uk/water/physical_anomalies.html#Pvisc

Keep in mind that viscosity is a really complicated subject and water is among the unique solvents. In general viscosity of liquids increases with pressure. At pressure extremes, let us say, 100,000 psi, many liquids become solids. Water also becomes a solid at really really high pressures. So when you talk about viscosity, you should talk about what pressure and what temperature.

From the website:

Water's pressure-viscosity behavior [534, 2890] can be explained by the increased pressure (up to about 100-200 MPa) causing deformation, so reducing the strength of the hydrogen-bonded network, which is also partially responsible for the viscosity. This reduction in cohesiveness more than compensates for the reduced void volume. It is thus a direct consequence of the balance between hydrogen-bonding effects and the van der Waals dispersion forces [558] in water; with hydrogen-bonding prevailing at lower temperatures and pressures. At higher pressures (and densities), the balance between hydrogen-bonding effects and the van der Waals dispersion forces is tipped in favor of the dispersion forces and the remaining hydrogen bonds are stronger due to the close proximity of the contributing oxygen atoms [655]. Viscosity, then, increases with pressure.

Water viscosity

Second question on the viscosity of gases

The textbooks seems to be talking about a limited range of P. Viscosity of gases is dependent on very low or very high pressures. However, for ideal gases and normal pressures, it is only dependent on the temperature. Yes, it is quite counterintuitive. Also viscosity of gases increases with temperature. This is another surprise. https://en.wikipedia.org/wiki/Viscosity#Pure_gases

You can try posting this second query in Physics forum. I don't know the derivation of that Wikipedia expression.

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  • $\begingroup$ Thanks for the answer. Could you also answer the second question? (Regarding the viscosity of gases) $\endgroup$ – Aniruddha Deb Feb 4 '20 at 9:09
  • $\begingroup$ I added the second part. You will have look up the mathematical derivation of the viscosity relationship. $\endgroup$ – M. Farooq Feb 4 '20 at 14:12

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