Viscosity and Quantum Mechanics


Viscosity describes a fluid’s resistance to changing its shape. Like surface tension, it’s a fundamental property of a fluid that comes from the interactions between molecules. But viscosity is a slippery beast, and especially so for liquids. There is no generic way to calculate a liquid’s thermodynamic properties from quantum dynamical first principles. But that hasn’t stopped theoretical physicists from making progress on deducing the connections between quantum mechanics and liquids.

Although viscosity changes with temperature, all liquids have a minimum viscosity, and those minima are all fairly close to the same value as water’s (excluding any superfluids, which are their own brand of quantum weirdness). Why would liquids share a similar minimum viscosity? Because it turns out the minimum viscosity is quantum! Physicists found that the minimum viscosity is set by an equation depending on Planck’s constant and the mass of an electron — both fundamental constants.

Physicists sometimes like to conjecture about the habitability of the universe if fundamental quantities like Planck’s constant had a different value. This work shows that changing that value would alter water’s viscosity, completely changing the viability of microscopic life! (Image credit: A. Rozetsky; research credit: K. Trachenko and V. Brazhkin; via Physics Today)

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