When you pour water into a nearly empty soap dispenser hoping to salvage the last sudsy drops, physics conspires against you. The water shoots straight through the thickened soap and out the nozzle with barely a bubble—a small domestic frustration that points to a profound challenge in everything from oil drilling to carbon storage.
This phenomenon, which physicists call "viscous fingering," occurs whenever a thin, runny fluid is forced against a thick one in a confined space. The boundary between them becomes unstable, and the less viscous liquid finds a path of least resistance, piercing through in branching, finger-like protrusions. Sidney Nagel, the Stein-Freiler Distinguished Service Professor of Physics at the University of Chicago, describes it as one of the most-studied examples of pattern formation, consistently revealing how branched structures emerge throughout nature—from rivers splitting into tributaries to fractals in minerals and stone.
The practical stakes are enormous. Oil companies inject carbon dioxide into underground reservoirs to push crude oil toward extraction wells, but when viscous fingering takes hold, the gas shoots straight through to the well instead, leaving the oil stranded below. Similarly, efforts to sequester carbon dioxide deep underground for climate change mitigation face the same problem: the gas's low viscosity makes it vulnerable to fingering through the denser fluids surrounding it, threatening the integrity of the seal itself.
For decades, physicists have understood the basic conditions that trigger viscous fingering. The instability emerges when fluids have sharply different viscosities, when the interface between them is sharp and abrupt, and when the thinner fluid moves fast enough that it doesn't have time to gradually seep into the thicker one. But Zhaoning Liu, a graduate student in Nagel's lab and lead author of a new study published in Science Advances, asked a different question: could you control finger formation by physically reshaping the interface itself, without changing the viscosity ratio?
To test this, the team used a classical apparatus—two flat, parallel plates separated by an extremely thin gap. They filled the gap with a viscous solution, then injected a low-viscosity solution through a small hole in the top plate. As the thinner liquid spread outward, pushing the thick liquid aside, the advancing edge formed a blunt, curved boundary with a sharp face. Fingers inevitably formed.
Then the researchers added a twist: they slid the bottom plate back and forth—a process called shearing—while varying both the distance and speed of the motion. The shearing action bulged the interface outward, forming a pointer curve that smoothed the sharp edge. The results were striking. The farther and faster they moved the plates, the longer it took for fingers to start forming, and once they did appear, they grew more slowly.
This discovery establishes a direct causal link between interface shape and stability—and more importantly, it offers a new lever for controlling a process that shapes countless industrial applications. "This study demonstrates a new way to control and delay the instability onset," Nagel said, "which plays a role in so many industrial processes involving fluids, from oil extraction from the Earth to carbon sequestration."
The implications ripple across climate technology, resource extraction, and fluid dynamics research. By simply reshaping where two fluids meet, engineers may be able to protect reservoirs, improve extraction efficiency, and safeguard carbon storage—all without altering the fundamental properties of the fluids themselves.