In a recent study, chemical and biological engineering professor Sujit Datta and fifth-year graduate student Christopher Browne discovered why certain fluids increase in flow resistance under pressure when flowing through porous media — a question that has puzzled researchers for more than half a century.
The fluids in question are polymer solutions: essentially, any solutions that have polymers, substances consisting of very large molecules, dissolved within them. As Browne explained in an interview with The Daily Princetonian, these substances often behave very differently under different conditions. Common examples include ketchup, shampoo, and even the mucus that lines our lungs.
Polymers are popular additives in a variety of processes, but the properties of the resulting polymer solutions “are not very well understood at all,” Browne said. “People haven’t been able to really study this for a long time, because these systems are in porous rocks that are opaque.”
Over the past few years, Browne and Datta have been developing a method to visualize the movement of polymer solutions through a porous medium.
Packing glass beads into a narrow tube, the researchers created a medium that modeled rocks. The pair also formulated a polymer solution that would be completely transparent when saturated. To visualize the flow, they put fluorescent dye in the polymer solution and tracked it using an excitation laser and sensor.
Designing this method, however, was only half the challenge. Browne and Datta also sought to understand what the root cause of the solution's turbulent flow was.
“Understanding an unstable flow is a difficult thing to do,” Browne said. “It took a lot of time reading through literature on turbulence and going back to what people did decades ago in this very different field to come up with ways we can analyze it, draw similarities, and quantify these things.”
The extra flow resistance in polymer solutions used to be attributed to the stretching of individual dissolved molecules as they move. This study, however, suggests that the increased resistance is due to elastic instability, or how the flow of a polymer solution becomes turbulent as it moves through a porous medium.
According to Datta, the discovery is significant for many energy, environmental, and industrial processes. He explained that polymer solutions have “long been used” in tasks like oil extraction and removing contaminants from groundwater.
“The new understanding of how and when these chaotic flows arise in porous media provided by our work can thus help to inform the design of fluids and selection of operating conditions for groundwater remediation processes,” he told the ‘Prince.’
In the future, Browne said he hopes to develop a more complex and representative model that accounts for the differences found in the natural rock formations that surround reservoirs and aquifers.
Berkley Yiu is a news contributor for the ‘Prince’. He can be reached at by3040@princeton.edu.
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