Fine-tuning shear thickening suspensions through ultrasound

Some shear thickening suspensions have viscosities that increase with stress to form solid structures. Their uses range from impact-resistant batteries to soft, flexible body armor that solidifies on impact. But they are notoriously challenging materials as their thickening behaviors are hard to control. Research has focused on developing mechanisms to reduce thickening, primarily by modifying fluid contents, which is not practical for many applications.

Barth et al. proposed the use of acoustic perturbations to control shear thickening and analyzed how suspension viscosity varies with ultrasound field strength. Their study yielded the first quantitative description for acoustic de-thickening magnitude.

“To analyze the data, we started with an existing model for shear thickening suspension viscosity,” said author Anna Barth. “When a shear thickening suspension is forcefully pushed and rigidifies, that’s a continuous phase transition. Statistical mechanics has an extensive toolkit for describing how systems behave near continuous phase transitions, so we used some of those ideas to describe the viscosity of a shear thickening suspension.”

Previous studies have used this model to show how viscosity depends on the suspension’s concentration of solid particles and the applied shear stress. Here, the researchers wanted to see how ultrasound could further affect this phase transition model.

“We found that ultrasound increases the repulsive force between particles in the suspension, and that the ultrasound effect can be completely incorporated into the model,” said Barth. “As a result, we can now predict how much the viscosity will decrease when you apply ultrasound, which lets you use ultrasound to dial in whatever viscosity you want.”

Source: “Acoustic tuning of shear thickening suspensions: A universal scaling analysis,” by Anna R. Barth, Navneet Singh, Stephen J. Thornton, Pranav Kakhandiki, Edward Y. X. Ong, Meera Ramaswamy, Abhishek M. Shetty, Bulbul Chakraborty, James P. Sethna, and Itai Cohen, Journal of Rheology (2026). The article can be accessed at https://doi.org/10.1122/8.0001140 .