Why do some fluids strangely slow down under pressure?


When pressurized, polymer solutions become less viscous and flow faster. But when traversing materials with many tiny holes and channels, solutions tend to get more viscous and grimy, reducing their flow rates.

But when traversing materials with many tiny holes and channels, solutions tend to get more viscous and grimy, reducing their flow rates.

Why some fluids strangely slow down under pressure as they flow through porosities has been a puzzle for 54 years.

In a new experiment, Princeton scientists solved this puzzle using a transparent porous medium made up of tiny glass beads, a transparent man-made rock. Thanks to this lucid support, scientists were able to visualize the movement of a polymer solution.

The experiment revealed that the long and disconcerting increase in viscosity in porous media occurs because the flow of the polymer solution becomes chaotic, much like the turbulent air of an airplane flight, swirling over it. – even and erasing the works.

Sujit Datta, assistant professor of chemical and biological engineering at Princeton and lead author of the study, said: “Surprisingly, until now, it was not possible to predict the viscosity of polymer solutions flowing in porous media. But in this article, we’ve finally shown that these predictions can be made, so we’ve found an answer to a problem that has eluded researchers for more than half a century.

Christopher Browne, a Ph.D. student in Datta’s lab and lead author of the article, said: “With this study, we have finally made it possible to see exactly what happens underground or in other opaque and porous media when polymer solutions are pumped. “

Scientists built the experimental apparatus, a small rectangular chamber filled with random tiny beads of borosilicate glass. The installation, similar to an artificial sedimentary rock, covered only half the length of a little finger. Scientists pumped a common polymer solution mixed with fluorescent latex microparticles to help see the solution flow around the beads in this fake rock.

The scientists formulated the polymer solution, so that the refractive index of the material compensates for the light distortion of the beads and makes the entire configuration transparent when saturated.

Scientists looked closely at the pores or holes between the beads to examine the flow of fluid through each pore. These pores occur at the scale of 100 micrometers (millionths of a meter) in size or similar to the width of a human hair.

As the polymer solution made its way through the porous medium, the fluid flow became chaotic, with the fluid crashing into itself and generating turbulence. These pores occur at the scale of 100 micrometers (millionths of a meter) in size, or similar to the width of a human hair, to examine the flow of fluid through each pore.

What is surprising is that, typically, the fluid flows at these speeds and in such tight pores is not turbulent, but “laminar”: the fluid moves smoothly and steadily. However, as the polymers moved through the pore space, they stretched, generating forces that built up and generated turbulent flow in different pores. This effect became more pronounced when pushing the solution to higher pressures.

Browne said, “I was able to see and record all of these uneven regions of instability, and these regions impact the transport of the solution through the medium.”

By collecting the data from the experiment, the scientists formulated a way to predict the behavior of polymer solutions in real situations.

Gareth McKinley, a professor of mechanical engineering at the Massachusetts Institute of Technology who was not involved in the study, commented on its importance.

“This study definitely shows that the large increase in pressure drop macroscopically observable through a porous medium has its microscopic physical origins in the viscoelastic flow instabilities that occur at the pore scale of the porous medium.”

Since viscosity is one of the most fundamental descriptors of fluid flow, the results help deepen the understanding of polymer solution flows and chaotic flows in general and provide quantitative guidelines to inform their applications to large scale on the ground.

Datta said, “The new information we generated could help practitioners in a variety of settings determine how to formulate the right polymer solution and use the right pressures needed to perform the task at hand. We are particularly excited about the application of the results in groundwater remediation.

Because polymer solutions are inherently slimy, environmental engineers inject the solutions into the soil at highly contaminated sites such as chemical plants and abandoned industrial plants.

Viscous solutions help remove traces of contaminants from affected soils. Polymer solutions also aid in oil recovery by pushing oil out of the pores of underground rocks. On the sanitation side, polymer solutions make it possible to “pump and treat”, a common method of cleaning groundwater polluted by industrial chemicals and metals which consists of bringing water to a surface treatment station.

Datta noted, “All of these polymer solution applications, and more, such as separations and manufacturing processes, should benefit from our findings. “

Overall, the new findings on polymer solution flow rates in porous media brought together ideas from several fields of scientific research, ultimately unraveling what had started as a long-standing, complex and frustrating problem.

Datta said, “This work establishes links between studies of polymer physics, turbulence and geosciences, following the flow of fluids in subterranean rocks as well as in aquifers. It’s great fun to be at the interface between all these different disciplines.

Journal reference:

  1. Christopher Browne et al. Elastic turbulence generates abnormal resistance to flow in porous media. DO I: 10.1126 / sciadv.abj2619

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