Stirring an impossible liquid

Imagine a swirl of milk in a cup of tea, or a drop of food colouring in a glass of water. The way liquids move and flow is intricate and almost impossibly complex. But that doesn’t stop scientists from trying to understand this movement.

Seventy years ago, Lars Onsager worked out equations to describe how fluids flowed and mixed. However, his model was very simple. It only worked in two dimensions, so the fluid couldn’t have any depth. Even worse, it required a perfectly runny fluid – something with zero viscosity. Lars made some interesting predictions for this impossible situation that no one could test.

An impossible stir

Fast forward 70 years, and scientists at the University of Otago and University of Queensland realised that they could actually run Lars’ impossible experiment. They created a very thin oval of a perfectly runny substance known as ⁸⁷rubidium Bose–Einstein condensate. Then they used lasers to stir the condensate and watched what happened.

Stirring caused little twists, known as vortexes, to appear. When they stirred gently, the vortexes were all mixed together, spinning clockwise or anticlockwise. But when they stirred harder the vortexes organised into clusters, all spinning the same direction. This self-sorting behaviour was exactly what Lars predicted, 70 years ago!

The scientists weren’t too surprised at the result. Vortex clusters have been spotted in other fluids, including the Great Red Spot on Jupiter. But it’s not every day you get to run an impossible experiment!

Another state of matter?

You’ve heard of solids, liquids and gases, but there are other, more extreme forms of matter out there. Bose–Einstein condensate is a state of matter that exists near absolute zero (the lowest possible temperature).

A Bose–Einstein condensate acts something like a giant ‘super atom’. Quantum effects typically happen to small things, such as atoms. But in a Bose–Einstein condensate, quantum behaviour starts to happen on bigger and bigger scales!

A million-dollar question

Scientists are still trying to come to grips with how fluids move. There’s a million-dollar prize waiting for someone to solve just one very famous question about fluids!

The prize will go to someone who can solve the ‘Navier–Stokes existence and smoothness problem’. The problem looks at the most common equations used to predict fluid flow, and asks the question: do these equations always give a sensible answer? Many mathematicians think they might not. It’s possible that sometimes the answer could blow up and equal infinity!

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