BETHLEHEM — A research team led by Ebru Demir from Lehigh University published a paper in the journal Applied Physics Letters in July 2026. The paper details insights into how tiny robots reverse direction within non-Newtonian fluids due to fluid rheology.

"We found that the rheology, or properties of the fluid, affected the locomotion of the swimmers," said Amin Balazadeh Koucheh, lead author of the paper. "Specifically, we found that the swimmer reversed its direction of motion when we increased the actuation frequency in a non-Newtonian fluid."

Koucheh, a second-year PhD student in Demir's research group, stated that this phenomenon had previously been shown numerically by their group, but this study marked its first experimental demonstration. "This phenomenon has been shown numerically by our research group, but this was the first time it's been shown experimentally," Koucheh said. The team used two types of tiny swimmers for their research: a sphere and a helix. The helix swimmer was designed to resemble the corkscrew shape of some bacteria.

Researchers embedded a magnet within each swimmer and magnetically actuated them, causing them to rotate and move forward. They first tested the swimmers in a Newtonian fluid. In this fluid, increasing the frequency of rotation caused the swimmers to move forward at a higher speed. The swimmers also exhibited a diagonal trajectory, moving sideways while progressing forward.

The team then placed the swimmers in a synthetic non-Newtonian fluid, which was designed to simulate substances like mucus or blood. In this fluid, the swimmers moved sideways in the opposite direction compared to their movement in Newtonian fluid under identical magnetic actuation. "Both types of swimmers showed this backward sliding motion," Koucheh said. "So we now understand that rheology alone is enough to affect this phenomenon rather than shape," Koucheh said.

Demir, an assistant professor of mechanical engineering and mechanics in Lehigh University's P.C. Rossin College of Engineering and Applied Science, commented on the implications. "The same swimmer behaves completely differently depending on what it's swimming through and that's a powerful handle for control," she said.

Ben Ratnor was a co-first author on the study. On Shun Pak from Santa Clara University and Roberto Zenit from Brown University collaborated with Demir on the research. The paper, titled "Shear-thinning rheology reverses wall-induced motion of low-Reynolds-number propellers," has a DOI of 10.1063/5.0333605.

Why It Matters

The research provides experimental evidence that the properties of a fluid, specifically its rheology, can cause tiny robots to change their direction of movement. This finding can influence the development and control of micro-swimmers designed for applications in bodily fluids such as mucus and blood. The ability to control these swimmers is relevant for potential medical applications requiring targeted movement within the human body.