The dream of deploying tiny robots to swim through the human body and deliver drugs directly to diseased cells has long captivated engineers. Imagine a swarm of microscopic devices carrying chemotherapy exclusively to a tumor, sparing healthy tissue from debilitating side effects. That vision, however, faces a fundamental obstacle: bodily fluids like blood and mucus are not simple liquids. They are non-Newtonian fluids whose viscosity changes dramatically under the shear forces generated by a swimming robot. Now, a multi-institution team has uncovered a startling behavior that could turn this complexity into a control strategy. When propelled through a synthetic mucus-like fluid, their millimeter-scale swimmers didn’t just speed up—they reversed their sideways motion entirely, a phenomenon predicted by simulations but never before seen in the lab.
The research team, led by Ebru Demir, an assistant professor of mechanical engineering and mechanics at Lehigh University, along with collaborators On Shun Pak of Santa Clara University and Roberto Zenit of Brown University, set out to understand how low-Reynolds-number propulsion—the physics regime where viscous forces dominate—translates into non-Newtonian environments. Their work appears in the journal Applied Physics Letters. The challenge is that at the scales relevant to micro-robots, inertia is negligible, and the fluid’s response to being sheared becomes the dominant factor. Blood and mucus are shear-thinning: they become less viscous when stirred or deformed. That property can completely alter the flow field around a swimmer, and the team wanted to map those alterations experimentally.
To isolate the effects, the researchers built two types of magnetically driven swimmers: a sphere and a helix that mimics the corkscrew shape of certain bacteria. Though the robots were millimeter-sized, the team matched the physics of true microswimmers by dramatically increasing the viscosity of their test fluids, preserving the correct Reynolds number. Each swimmer was embedded with a magnet and actuated by a rotating magnetic field, causing it to spin and translate forward. The experiments first ran in a Newtonian fluid of constant viscosity, where the swimmers behaved as expected—increasing the rotation frequency yielded faster forward motion, and a predictable wall-induced drift sent them on a diagonal trajectory.
Then the team replaced the fluid with a shear-thinning analog designed to mimic biological mucus. Under the same magnetic actuation and increasing frequency, the forward speed still increased, but the sideways motion flipped. Rather than drifting in the direction they had in the Newtonian case, the swimmers began to slide backwards relative to the wall. “This phenomenon has been shown numerically by our research group, but this was the first time it’s been shown experimentally,” said Amin Balazadeh Koucheh, a second-year PhD student in Demir’s lab and the paper’s lead author. The observation was robust: both the sphere and the helix exhibited the same reversal, confirming that the behavior stems from the fluid’s rheology rather than the shape of the swimmer.
The physics behind the reversal lies in how shear-thinning modifies the force distribution around the rotating body. As the swimmer spins faster, the shear rate near its surface increases, causing the local viscosity to drop. This alters the pressure and viscous stress fields between the swimmer and the nearby wall, effectively reversing the direction of the wall-induced lift force. The result is a counterintuitive locomotion strategy where the same spin that propels the robot forward can also push it laterally in the opposite direction depending on the fluid’s composition. “We used to think of the fluid as just the medium,” Demir said. “Now we’re starting to see it as part of the machine.”
For the future of targeted drug delivery, such an effect is a powerful handle. Understanding that a swimmer’s trajectory can be fundamentally rerouted by the fluid’s shear-thinning properties opens the door to designing robots that exploit their environment. A device could, for example, use changes in local viscosity—such as those encountered when moving from blood plasma into a mucus layer—to steer itself without additional control inputs. The team’s next step is to replicate these experiments at the true microscale, with robots an order of magnitude smaller and in a wider variety of shapes, to see how the reversal scales and whether it can be tuned.
The findings also underscore a broader principle in micro-robotics: the environment is not a passive stage but an active participant in locomotion. Ben Ratnor, a master’s student and co-first author of the study, summarized it succinctly: “It’s almost like changing the fluid changes where the finish line is for each swimmer.” While clinical applications remain years away, incremental insights like this are essential for moving from theoretical models to real-world control. The work was supported by the U.S. National Science Foundation, and the paper’s title—“Shear-thinning rheology reverses wall-induced motion of low-Reynolds-number propellers”—captures the precise discovery that could one day guide surgical microrobots through the body’s most challenging terrains.
Subject of Research: Locomotion of low-Reynolds-number swimmers in non-Newtonian fluids
Article Title: Shear-thinning rheology reverses wall-induced motion of low-Reynolds-number propellers
News Publication Date: 15-Jun-2026
Web References:
- https://doi.org/10.1063/5.0333605
- Lehigh University faculty profile: Ebru Demir
- Santa Clara University profile: On Shun Pak
- Brown University profile: Roberto Zenit
References: Applied Physics Letters, DOI: 10.1063/5.0333605
Image Credits: Christa Neu/Lehigh University
Keywords
microrobots, drug delivery, non-Newtonian fluid, shear-thinning, rheology, low Reynolds number, magnetic actuation, fluid dynamics, targeted therapy, swimming robots, wall-induced motion, corkscrew propulsion
