Why aren’t there any (normal) liquids much less viscous than water?
Minimal quantum viscosity from fundamental physical constants
Authors: K. Trachenko and V. V. Brazhkin
Science Advances, 6, eaba 3747 (2020); DOI: 10.1126/sciadv.aba3747
Recommended with a commentary by Alexander Y. Grosberg, New York University
|View Commentary (pdf)|
This commentary may be cited as:
DOI: 10.36471/JCCM_August_2022_01
https://doi.org/10.36471/JCCM_August_2022_01
Dear Prof Grosberg
Thank you for your interest in our paper and for writing the commentary.
I thought to offer two quick comments in relation to your commentary:
– Some steps in our derivation indeed involve order-of-magnitude evaluation. Other steps are precise such as relation between the angular frequency and the period involving a numerical factor of 2pi which carries into the final result. We kept these factors where they are justified and specified where we made approximations.
– I don’t imagine that water is a special system in our theory. One point that comes from our analysis is that viscosity minima in all systems are close to each other by order of magnitude (our “fundamental quantum viscosity) because these minima are governed by fundamental constants.
Purcell does not say whether his viscosity tables included the values at high pressure, but I can imagine he was looking mostly at room pressure viscosities. At room pressure and temperature, water viscosity just happens to be close to the lower bound. Purcell may have chosen water as a benchmark because it is a very common fluid. For other fluids, the viscosity minimum can be lower (Fig. 1 in our Sci Adv paper) but this requires pressure and temperature that are different from our familiar ambient conditions.
I agree that it is interesting to discuss the implications for water and life: these two are indeed well attuned to the values of fundamental constants. We can relate this to the antropic principle operating at a higher level of biological and biochemical processes (earlier thinking was that the anthropic principle is mostly related to high-energy physics).
Kind regards
Kostya