There’s A Limit To How Runny A Liquid Can Be

The newly discovered equation could even have implications in black holes. Mirko Sobotta/Shutterstock

Katy Pallister 27 Apr 2020, 15:23

Imagine a world where the water coming out of the shower was the same consistency as your toothpaste, or your morning coffee flowed like cooking oil. To be honest, it may not seem as crazy as some of the things we’re hearing right now, but the runniness, or viscosity, of fluids governs much of our everyday lives.

Whilst it is well known that you can decrease a liquid's viscosity (resistance to flow) by heating it, the point at which the liquid reaches its minimum level before it boils and becomes a gas hasn’t been formally identified – until now. Scientists from the UK and Russia have formulated an equation to quantify this limit, using fundamental physical constants that define our universe.

“This result is startling,” Professor Kostya Trachenko, lead author of the paper from Queen Mary University of London, said in a statement. “Viscosity is a complicated property varying strongly for different liquids and external conditions. Yet our results show that the minimal viscosity of all liquids turns out to be simple and universal.”

The lower bound of the new property "elementary" viscosity (ιm) is dependent on two fundamental concepts: the (reduced) Planck constant (ℏ) and the proton-to-electron mass ratio (mp / me ). thehackneycollective.com

Calculating viscosity from theoretical concepts had been considered impossible due its strong and convoluted dependence on system variables such as liquid structure, composition, and interactions, as well as external conditions. Achieving the impossible, Trachenko and co-lead author Vadim Brazhkin, from the Russian Academy of Sciences, managed to develop an equation showing that just two fundamental physical constants govern how runny a liquid can be; the Planck constant (generally used in formulations of quantum mechanics) and the proton-to-electron mass ratio (one of few dimensionless quantities of importance in many areas, such as star formation and life-supporting environment).

“The lower fundamental limit reminds us how fundamental constants of nature affect us daily, starting from making a morning cup of tea by extending their overarching rule to specific, yet complex, properties such as liquid viscosity,” Trachenko explained.

Published in Science Advances, the discovery of this theoretical limit on a liquid’s runniness has practical applications as well. The equation could act as a guide in the production of new low-viscosity fluids for industry and also prevent the waste of resources in beating this limit, as nature has already dictated it. However, it’s not just on Earth that this equation could be employed.

“There are indications that the fundamental lower limit of liquid viscosity may be related to very different areas of physics: black holes as well as the new state of matter, quark-gluon plasma, which appears at very high temperature and pressure,” Brazhkin added. “Exploring and appreciating these and other connections is what makes science ever so exciting."

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