Liquids can roughly divided into two categories: the regular type and the exotic type. Common substances, like water and alcohol, work more or less as expected when pumped through a pipe or stirred with a spoon. Nestled among the odd—including substances like paint, honey, mucus, blood, ketchup, and oobleck—are countless behavioral mysteries that have baffled researchers for centuries. .
One such age-old puzzle, first explained nearly 55 years ago, arises when certain liquids flow through cracks and holes in a porous landscape like porous soil. At first the liquid will flow normally. But as its flow rate increases, it crosses a critical threshold where it will suddenly clump – its viscosity increases like a martini turns to molasses.
ONE new research The pin acts on small molecules suspended in the liquid that rotate and elongate as the flow rate increases. At some point, the molecular motion causes the fluid to become turbulent, surging and rippling in complex, self-revolving eddies. The onset of turbulence is what impedes fluid movement. The detection could have applications ranging from 3D printing to groundwater treatment and oil recovery.
“This is a beautiful manuscript,” said Paulo Arratia, who studies complex fluids at the University of Pennsylvania and was not involved in this work.
In the 1960s, rheologist Arthur Metzner and his college student Ronald Marshall were working on oil fields, where engineers used to pump water mixed with so-called propellants into the ground to move oil and helps to extract every drop of crude oil. The scientists found that when a propellant fluid, which contains long-chain polymers, was pumped into the ground above a certain ratio, it seemed to suddenly become much more viscous or sticky, an effect that was later found found in many similar systems.
“Viscosity is one of the most important things that you want to be able to predict and control and characterize,” says Sujit Datta, a chemical engineer at Princeton University, who came across a 1967 paper by Metzner and Marshall on the subject as a graduate student. “I said, ‘It’s a shame that even after decades of intensive research, we still don’t know why viscosity is, and how to explain this increase. ”
Thrust fluids and other viscous liquids, as they are known, can contain long and complex molecules. At first, scientists thought that perhaps these molecules were piling up in pores in the ground, causing them to rise like hairs in a sewer. But they soon realized that these were not simple clogs. As soon as the flow rate drops below the critical threshold, the obstruction seems to disappear completely.
A turning point came in 2015 when a group at the Schlumberger Gould Research Center in Cambridge, England, simplified the problem. The researchers built a two-dimensional analog of sandy soil, with sub-millimeter-sized channels leading to a labyrinthine array of cross-shaped fragments. They then pumped liquids containing different concentrations of molecules through the system. The team found that above a certain flow rate, the fluid’s motion became messy and disordered in the space between the crosses, slowing the overall motion of the fluid.
In theory, something like this is almost impossible. Ordinary fluids are greatly affected by their inertia, their tendency to continue to flow. For example, water has a lot of inertia. As the water moves faster and faster, small currents in the flow will begin to extend beyond other parts of the liquid, resulting in turbulent eddies.
In contrast, a complex liquid like honey has very little inertia. It will stop flowing as soon as you stop stirring it. As a result, it has trouble generating “inertial turbulence” – the kind of turbulence that normally occurs in fast currents or underneath aircraft wings.
