Fluids can be Roughly divided into two categories: ordinary and exotic. Regular types, like water and alcohol, work more or less as expected when pumped through tubes or stirred with a spoon. Among the strange things—which include substances such as paint, honey, mucus, blood, ketchup, and ooplak— lie a variety of behavioral mysteries that have puzzled researchers over the centuries.
One such long-standing puzzle, first articulated nearly 55 years ago, emerged when certain fluids flow through cracks and holes in porous landscapes like spongy soil. At first the fluid flows normally. But as its flow rate increases, it will cross a critical threshold where it suddenly looks like it’s congealing—its viscous as a martini turns into molasses.
A new study proves the effect on small particles suspended in a liquid that circulate and expand with a higher flow rate. At some point, the molecular motion causes the fluid flow to become chaotic, rushing and undulating in twisting vortices that twist back on themselves. The onset of chaos is what hinders the movement of the fluid. This discovery could have applications ranging from 3D printing to groundwater treatment and oil recovery.
“This is a beautiful manuscript,” said Paulo Aratia, who studies complex fluids at the University of Pennsylvania and was not involved in the work.
In the 1960s, Realist scientist Arthur Metzner and his undergraduate student Ronald Marshall were working in oil fields, where engineers often pumped water mixed with so-called propellant fluids into the ground to replace the oil and help extract every drop of crude. The scientists noted that when the propellant fluid, which contains long-chain polymers, was pumped into the ground above a certain rate, it unexpectedly appeared to become more viscous or viscous, 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 Datta, a chemical engineer at Princeton University who came across Metzner and Marshall’s 1967 paper on the topic as a graduate student. “I was like, ‘This is kind of embarrassing even after decades of deep research, we still have no idea why the viscosity is what it is, and how to explain the increase.'” “
Propulsion fluids and other viscoelastic fluids, as they are known, can contain long and complex particles. At first, scientists thought these particles might have been accumulating in pores in the ground, releasing them like hairs down a drain. But they soon realized that these weren’t just clogs. Once the flow rate fell below a critical threshold, the obstruction seemed to disappear completely.
A turning point occurred in 2015 when a group at the Schlumberger Gold Research Center in Cambridge, England, simplified the problem. The researchers built a two-dimensional analogue of sandy soil, with sub-millimeter-sized channels that lead to a maze of intersecting pieces. Then they pumped liquids containing different concentrations of particles through the system. The team noted that above a certain flow rate, the fluid’s motion became chaotic and turbulent in the spaces between junctions, significantly slowing the fluid’s overall motion.
In theory, something like this should be nearly impossible. Ordinary fluids are strongly affected by their inertia, their tendency to keep flowing. Water, for example, has a lot of inertia. As the water moves faster and faster, small currents within the flow will begin to outpace other parts of the liquid, creating chaotic vortices.
By contrast, a complex liquid like honey has very little inertia. It will stop flowing the moment you stop stirring. Because of this, they have trouble generating “inertial turbulence” – the normal type of turbulence that occurs in a flowing stream or under the wings of an airplane.