Exploring microbial motion in non-Newtonian environments

As electronics grow increasingly miniaturized, many researchers are exploring the idea of building microscopic robots to do what larger ones cannot. These microrobots could scout otherwise inaccessible areas, transport drugs or other cargo to targeted locations, or remove harmful substances. Inspiration for robot designs often comes from studying microbes, which have developed efficient movements for their environments.

These environments, however, are frequently non-Newtonian fluids that present additional challenges for modeling. For instance, blood and other body fluids exhibit shear-thinning rheology, while mixtures involving sediment are shear-thickened. These dynamics can affect the performance and optimal strategies for robots and other microswimmers.

Ouyang et al. modeled microswimmers moving through both shear thinned and shear thickened fluids while carrying cargo. They observed how the presence of the cargo affects the swimmers’ movements and determined the best strategies in different environments.

“What is the difference between the swimming speed of micro-swimming creatures when they are carrying goods or catching food and the swimming speed when they are moving alone? What is the physical mechanism behind it?” said author Zhenyu Ouyang. “These two problems are the starting point of our research.”

The authors examined two distinct methods of propulsion and compared them when either pushing or pulling cargo in both high and low shear environments. They found that microswimmers move faster in shear thickened fluids but have a lower hydrodynamic efficiency than shear thinned fluids. The best arrangement overall comes from a swimmer propelled by a rear flagellum pushing cargo in front of it.

The team plans to expand their results to incorporate three-dimensional models and viscoelastic fluids.

Source: “Locomotion of a micro-swimmer towing load through shear-dependent non-Newtonian fluids,” by Zhenyu Ouyang, Chen Liu, Tingting Qi, Jianzhong Lin, and Xiaoke Ku, Physics of Fluids (2023). The article can be accessed at https://doi.org/10.1063/5.0132452 .