Reversing our understanding of propellers in non-Newtonian fluids

Microrobots are a compelling technology for biomedical applications from delivering drugs to taking precise diagnostics. But while robots’ behavior in Newtonian fluids of constant viscosity is well-studied, the human body contains non-Newtonian fluids that can influence microrobots’ movement and functionality.

Koucheh et al. experimentally tested the swimming behavior of 3D-printed, magnetically operating swimmers in Newtonian and non-Newtonian fluids. They found that, near the container’s boundary, the structures rolled forward in Newtonian fluids and reversed direction in non-Newtonian shear-thinning fluids.

“Swimmers are prone to [doing] unexpected things when they’re near each other and when they’re near boundaries,” said author Ebru Demir. Microrobots may one day operate in enclosed, cylindrical environments like veins or the gastrointestinal tract.

“This study shows us that if you are in a shear-thinning fluid like our bodily fluids, and if you have something near a swimmer, its behavior is going to change,” Demir said.

The researchers recorded the motion of millimeter-scale swimmers in clear containers of either silicone oil (Newtonian) or aqueous xanthan-gum solutions (shear-thinning) at different viscosities. They then determined the trajectory and velocity of the swimmers at different rotational frequencies. The team found that non-Newtonian fluids prompted the propellers to slow their rolling motion, then reverse direction with increasing actuation frequency. The phenomenon was more pronounced as the fluid became more shear-thinning and when the structures had a helical tail.

The results suggest that guiding microrobots’ movements in the body will depend on the operating fluid, including non-Newtonian options such as blood and mucus.

“I would like to use engineering and fluid dynamics in a way that will help solve a big problem for humanity,” said Demir.

Source: “Shear-thinning rheology reverses wall-induced motion of low-Reynolds-number propellers,” by Amin Balazadeh Koucheh, Benjamin C. Ratner, On Shun Pak, Roberto Zenit, and Ebru Demir, Applied Physics Letters (2026). The article can be accessed at https://doi.org/10.1063/5.0333605 .

This paper is part of the Non-Newtonian Fluids: From Rheology to Hydrodynamics to Modern Applications Collection, learn more here .