Fluids can be roughly divided into two categories: ordinary and strange. Ordinary ones, such as water and alcohol, work more or less as expected when pumped through pipes or mixed with a spoon. Among the strange ones, which include substances such as paint, honey, mucus, blood, ketchup and clothing, are a huge variety of behavioral mysteries that have troubled researchers over the centuries.
Such a long-standing puzzle, first formulated nearly 55 years ago, arises when certain liquids flow through cracks and holes in a porous landscape, such as porous soil. At first, the fluid will flow normally. But as its flow rate increases, it will cross a critical threshold at which it will suddenly seem to merge – its viscosity will rise like a martini, which turns into molasses.
A new study finds an effect on small molecules suspended in a fluid that rotate and stretch as the flow rate increases. At some point, molecular motion causes the fluid flow to become chaotic, rise, and pulsate in twisted vortices that return to themselves. The beginning of chaos is what impedes the movement of fluid. The discovery may have applications ranging from 3D printing to groundwater remediation and oil recovery.
“It’s a beautiful manuscript,” said Paulo Aratia, who studied complex fluids at the University of Pennsylvania and was not involved.
In the 1960s, rheologist Arthur Metzner and his student Ronald Marshall worked on oil fields, where engineers often injected water mixed with so-called propellant liquids into the ground to displace oil and help extract every drop of crude. oil. Scientists have noticed that when the propellant, which contains long-chain polymers, is pumped into the ground above a certain speed, it unexpectedly becomes much more viscous or sticky, an effect later found in many similar systems.
“Viscosity is one of the most important things you want to be able to predict, control and characterize,” said Sujit Data, a chemical engineer at Princeton University, who came across a 1967 article by Metzner and Marshall on the subject as a graduate student. . “I said to myself, ‘It’s kind of embarrassing that even after decades of in-depth research, we still have no idea why viscosity is this and how to explain the increase.’
Propellants and other viscoelastic fluids, as they are known, can contain long, complex molecules. Initially, scientists thought that perhaps these molecules accumulate in pores in the ground, throwing them like hairs in the sewer. But they soon realized that these were no ordinary clogs. As soon as the flow velocity fell below a critical threshold, the obstacle seemed to disappear completely.
A turning point came in 2015 when a group from the Schlumberger Gould Research Center in Cambridge, England, simplified the problem. The researchers built a two-dimensional analogue of sandy soil, with sub-millimeter-sized channels leading into a labyrinthine array of cross-shaped pieces. They then pump liquids containing different concentrations of molecules through the system. The team noticed that above a certain flow rate, the movement of the fluid became scattered and erratic in the spaces between the crosses, which significantly slowed down the overall movement of the fluid.
In theory, something like this should be almost impossible. Ordinary liquids are strongly influenced by inertia, their tendency to continue to flow. Water, for example, has great inertia. As the water moves faster and faster, small streams in the stream will begin to overtake other sections of the liquid, leading to chaotic vortices.
A complex liquid like honey, on the other hand, has very little momentum. It will stop flowing the moment you stop stirring it. Therefore, there are problems with the generation of “inertial turbulence” – the usual type of turbulence that occurs in rapid flow or under the wings of an aircraft.