An artificial porous ‘rock’ helps unravel a mystery from half a century ago



Polymeric solutions, including cosmetic creams and mucus from our nose, contain dissolved polymers, that is, materials made up of large molecules with many repeating subunits. Normally, when put under pressure, they become less viscous and flow more quickly.

But when they pass through porous materials, such as sedimentary rocks where there are many tiny holes and channels, these solutions become more viscous and sticky, and they slow down, a phenomenon they described. in 1967 American researchers.

However, the flow of polymer solutions in porous media is essential for many industrial and energy processes, where it is a question of operating, through trial and error, in the face of the unpredictable resistance experienced by these fluids.

The explanation of this phenomenon has been a puzzle for science for more than 50 years, but now researchers at Princeton University (USA) have solved the mystery by combining novel techniques for manufacturing an artificial porous material, obtaining images and analysis. The results are published in ‘Science Advances’.

“In this work we show for the first time that this anomalous resistance is caused directly by an instability of the flow, called elastic turbulence, in which the flow becomes chaotic, similar to a typical turbulence (like those of the air when traveling by plane), but in this case driven by the elasticity of the polymer “, Christopher Browne, the main author, explains to SINC.

Transparent rock and fluorescent solution

To get to the root of the problem, the team used a transparent porous medium made of tiny borosilicate glass beads: a small translucent artificial rock that allowed them to visualize the movement of the polymer solution, to which fluorescent latex microparticles were added to facilitate the task.

The experiment revealed that the prolonged increase in viscosity in porous media is due to the flow becoming chaotic, turbulent, swirling in on itself and causing blockages.

“Most fluids produce a constant laminar flow in porous media, but the addition of a dilute concentration of polymers produces this unstable and chaotic flow, generating an excess of resistance that hinders the flow of the liquid”, explains Browne, who emphasizes: “We have finally been able to see exactly what happens underground or inside other opaque and porous media when polymer solutions are pumped.”

“Surprisingly, the viscosity of polymer solutions flowing in porous media could not be predicted until now, but we showed that it can be done,” says another author, Sujit Datta, “and this may help practitioners in various fields to determine how to formulate the proper polymer solution and use the correct pressures needed to carry out certain tasks. ”

The researchers used the data collected in the experiment to formulate a way to predict the behavior of polymer solutions that could be applied in real situations in areas such as industry, energy and the environment.

Because polymer solutions are inherently viscous, environmental engineers inject them into heavily polluted soils, such as chemical factories and abandoned industrial plants, to facilitate the removal of pollutants.

Similarly, oil can also be recovered from the pores of underground rocks, “and we are especially excited about applying the findings to cleaning groundwater (making it easier to pump and treat it on the surface),” says Datta.

“This work establishes connections between the studies of polymer physics, turbulence and geoscience, following the flow of fluids in subsoil rocks, as well as through aquifers”, concludes the researcher.

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