Swimming through non-Newtonian fluids

Conventional rheometric devices, such as cone-and-plate rotational rheometers, need to extract a fluid from its source to measure it, a process that can alter some fluid properties or can be impossible. But what if researchers could bring a device to the fluid instead?

Chiu et al. simulated and engineered a tethered swimmer that uses propulsive forces to determine rheological parameters that quantify a fluid’s degree of elasticity of a non-Newtonian fluid.

“The tethered swimmer can measure the zero-shear first normal stress coefficient, which is not achievable on standard benchtop rheometers because of limited sensitivity,” author Selena Chiu said.

To develop this device, the researchers first established transfer functions — equations that relate the propulsion of the swimmer to the fluid’s properties.

The swimmer itself is made of two spheres magnetically coupled to a motor, all connected to a digital scale that measures propulsive force. From there, the researchers can use their transfer functions to determine the rheometric properties.

“Currently, the device is just made from off-the-shelf components,” Chiu said. “So anyone can make this if they know how to put together Arduino boards and use a 3D printer.”

While their device only measures a few rheometric properties of non-Newtonian fluids, the researchers hope to expand its capabilities for different fluids and properties. They would need to develop additional transfer functions, which would expand the possibilities for rheological measurements.

“If we can further develop the device, then it could be used in situ,” Chiu said. “If we could, say, install the device in a reactor and run it while the processes are running, then we would be able to have time-resolved measurements.”

Source: “Measuring the properties of a viscoelastic fluid using a tethered swimmer,” by Selena Chiu, Adil Jussupov, Cody Moose, Gerald G. Fuller, Manu Prakash, and Eric S.G. Shaqfeh, Journal of Rheology (2026). The article can be accessed at https://doi.org/10.1122/8.0001162 .