Stretching and smashing fluids to understand the physics of flow

LONDON - Researchers have used new methods to try to understand the flow properties of concentrated solutions of particles in fluids of everyday use products like toothpaste and cosmetics.

Dr Michael Smith from the School of Physics and Astronomy, along with collaborators at University of Edinburgh and Politecnico di Torino tested various complex fluids in a different way using an ‘extensional rheometer’.

This device stretches it out between two plates at varying speeds to measure the flow properties. The method and the results gathered have revealed new physics which will have much better applications in manufacturing, for example, in the packaging and dispensing designs of many household products.

“Our observation of the fluid with a high speed camera revealed some intriguing effects depending on the concentration of particles and the speed at which the plates were moved. At low velocities the fluid is observed to behave like a liquid but at higher velocities and concentrations of particles the fluid can actually fracture like a solid,” said Smith.

“This happens if you dissolve a large amount of cornflour in some water, for example. The high concentration of tiny particles inside the fluid jam into one another forming clusters which lock solid if disturbed at a high enough speed,” he said.

“It is a bit like trying to move through a street crowded with an enormous number of people. If you move slowly enough you can make progress and the crowd and you ‘flow’. However, if you try and sprint down the street you will just knock into so many people that you’ll never be able to move at the speed you want to and hence everything becomes grid locked,” he added.

In particular an effect known as ‘dilatancy’ in which some of the particles poke through the surface of the liquid was found to play a crucial role in the jamming of the particles.

“The most incredible results were observed when the fluid was stretched at a velocity just below that required to form a jammed fluid. The fluid was found to form a thin filament which narrowed until it was about hundred particles in diameter. At this point the fluid was observed to recoil elastically, like a rubber band!” Smith said.

“”This is particularly fascinating since the particles are specifically designed to behave like hard spheres with no attractive forces. Where does the elasticity come from? The liquid drains from the filament faster than the particles causing them to poke through the surface as before. The liquid surface forms a meniscus around the particles. It is this curved surface of the fluid which the researchers believe stores the energy and results in the unusual behaviour,” he added.

“We hope this research provides an important initial step in understanding how the physics in common industrial flows may differ from the carefully controlled set up found in conventional academic studies.”

The research is published in Nature Communications. (ANI)