Fractured artificial rock helps crack a 54-year-old mystery

Princeton researchers developed a method to understand how polymers flow through tiny channels under pressure. Credit to David Kelly Crow
Princeton researchers solved a 54-year old mystery about why fluids seem to slow down when they pass through porous materials like soils or sedimentary rocks. These findings could improve many processes in the energy, environmental, and industrial sectors. They can be used to recover oil or remediate groundwater.

These fluids are known as polymer solutions. These solutions, which are common examples include cosmetic creams or the mucus in our noses, contain dissolved polymers. They are large molecules that have many repeating subunits. Polymer solutions tend to flow faster when put under pressure. However, solutions that are passed through materials with many tiny holes or channels tend to be more viscous, which reduces their flow rate.

Princeton researchers developed an innovative experiment that used a porous medium with transparent glass beads and artificial rock to get at the root cause of the problem. Researchers were able to see the movement of a polymer solution through this transparent medium. The long-awaited increase in viscosity of porous media was revealed to be due to the flow of the polymer solution becoming chaotic. This is similar to turbulent air on an airplane ride. It swirls into itself and eats up the work.

"It has been surprising that it has not been possible until now to predict the viscosity polymer solutions flowing through porous media," stated Sujit Datta (an assistant professor in chemical and biological engineering at Princeton) and senior author on the Nov. 5 Science Advances study. "But, in this paper we've finally shown that these predictions can actually be made. This is a solution to a problem that's eluded researchers over half a century."

Christopher Browne, a Ph.D. candidate in Datta's laboratory and the paper's main author, said, "With this study we finally made it possible for us to see exactly what's happening underground or within opaque, porous media when Polymer Solutions are being pumped through."

Browne conducted the experiments and constructed the experimental apparatus. It was a small rectangular chamber randomly filled with tiny borosilicate glasses beads. This setup was similar to artificial sedimentary rocks. It measured only half the length of a pinky. Browne pumped a common solution of polymer laced with fluorescent microparticles into the faux rock to see how the solution was flowing around the beads. Researchers formulated the polymer solution to offset light distortion caused by the beads. This made the entire setup transparent once it was saturated. Datta's laboratory has used this innovative technique to create transparent soil to study agricultural droughts and other research.

Browne zoomed in on the pores between beads using a microscope to inspect the fluid flow through them. The fluid flow became chaotic as the polymer solution made its way through porous media. This caused fluid to crash back into itself, causing turbulence. Surprisingly, fluid flows in tight pores at such speeds are usually not turbulent but "laminar". The fluid moves steadily and smoothly. However, as the polymers moved through the pores, they created forces that accumulated, which led to turbulent flow in various pores. This effect was more apparent when the solution was pushed through at higher pressures.

Browne stated, "I was able see and record all the patchy regions instabilities, and these regions really affect the transport of solution through the medium."

Princeton researchers developed a method to understand how polymers flow through tiny channels under pressure. Credit to David Kelly Crow

Princeton researchers used data from the experiment to develop a method to predict how polymer solutions would behave in real-life situations.

Gareth McKinley from the Massachusetts Institute of Technology, who was not part of the study, provided comments about its significance.

McKinley stated, "This study clearly shows that the large increase of the macroscopically observed pressure drop across porous media has its microscopic physical origins as viscoelastic flow instability that occurs on the porous medium's pore scale."

Because viscosity is a fundamental characteristic of fluid flow, these findings will not only deepen our understanding of chaotic and polymer solution flows, but also provide quantitative guidelines for their application at large scales.

Datta stated that the new insights they have provided could be used by practitioners in a variety of settings to determine the best polymer solution and the pressures required to complete the task. We are particularly excited about groundwater remediation, which will benefit from our findings.

Polymer solutions are inherently messy so environmental engineers inject them into ground at highly polluted sites like abandoned chemical factories or industrial plants. Viscous solutions can push out trace contaminants from soils. By pushing oil out of pores in underground rocks, polymer solutions also aid in oil recovery. Polymer solutions can be used to "pump and treat" groundwater that has been polluted by industrial chemicals or metals. This involves bringing the water up to a surface treatment station. Datta stated that polymer solutions can be used in many other applications, including manufacturing and separation processes.

The new findings regarding flow rates of polymer solutions in porous media gathered ideas from many fields of scientific inquiry to finally disentangle what was initially a complex, long-frustrating problem.

Datta stated that "this work draws connections between studies in polymer physics and turbulence as well as geoscience, following fluid flow through aquifers as well as underground rocks." It's great to be able to connect all these disciplines.

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More information: Christopher A. Browne and colleagues, Elastic Turbulence generates anomalous flows resistance in porous media Science Advances (2021). Journal information: Science Advances Christopher A. Browne et al, Elastic turbulence generates anomalous flow resistance in porous media,(2021). DOI: 10.1126/sciadv.abj2619