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For most fluids, an increase in pressure should lead to a burst of speed, like squeezing ketchup from a tube. But when flowing through porous materials such as soil or sedimentary rock, certain liquids slow down under pressure. Pinpointing the cause of this slowing would benefit industries such as environmental clean-up and oil extraction, where pumping one liquid into the ground forces another out; however, such movement is challenging to observe directly.

Princeton University chemical engineer Christopher Browne and physicist Sujit Datta offer a solution to this puzzle. By tweaking a special liquid to be transparent and pumping it through the pores of an equally transparent artificial rock, they documented how the liquid’s movement becomes chaotic, causing swirling eddies that gum up the pores and slow the flow.

The fluids of interest, called polymer solutions, are dissolved versions of large stretchy molecule chains common in biology as well as the cosmetics and energy industries. Theoretical studies have suggested that when the chains stretch through a nearly flat channel and then recoil, they generate forces that stir up eddies. But whether that turbulence “arises in realistic 3-D soils, sediments and porous rocks has been hotly debated,” Datta says.

To resolve the controversy, the researchers pumped a synthetic polymer solution into a simulated “sedimentary rock” built from a box filled with tiny glass beads. They tweaked the polymer solution’s precise chemistry by diluting it slightly to change how light refracts, rendering the “rock” fully transparent even when saturated.

The scientists laced the polymer with fluorescent chips and tracked its movement through the pores under a microscope, recording patchy regions of eddies and measuring how the solution flowed under differing pressure. This confirmed that the macroscale slowing had its microscopic origins where researchers had suspected: polymer chains stretching out and then coiling back as they passed through pores. The findings appeared in Science Advances.

“Visualizing flow inside a 3-D porous media literally gives a window into something that was impossible to see,” says University of Pennsylvania biochemical engineer Paulo Arratia, who was not involved in the study. As a next step, “if you could actually see the molecules stretching and recoiling, that would be wonderful [to] connect the molecular point of view to the microscopic.”

Industrial applications require knowing which specific pressures are needed to push a polymer solution through a porous material at a given flow rate. The study provides a physical model describing that relation and could predict, for example, how much contaminant can be retrieved from a chemical site by injecting a solution. “Without predictability,” Datta says, “injection operations are trial and error.”

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