In a study published in PNAS, a research team led by Prof. XU Ning from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences found that active matter and shear flows share similar thinning behaviors in self-propulsion and shear forces.

Active substances can spontaneously move by consuming their own or environmental energy with biological organisms being its typical representatives. Since active substances are non-equilibrium systems, a large number of active substance individuals gathering together will exhibit very rich and peculiar collective motion behaviors.

In this study, the team proposed that the collective motion behavior of active matter was similar to that of shear systems. It was among the first internationally to compare the two systems, despite significant differences in energy input methods between them, and found direct evidence of potential correlations.

The team showed that the viscosity changes in active matter with active force resembled those in sheared systems with shear stress, both stemming from the micro-mechanism of breaking up percolating particle clusters. It also showed that the faster the clusters broke, the more dramatically the viscosity dropped.

Besides, the team found that for ordinary Newtonian fluids, shear action only caused molecules to tend to align along the shear flow direction, forming percolating clusters that remained stable, thus viscosity remained unaffected by shear stress. However, the addition of active matter changed this scenario. Since the direction of active force could randomly deviate, it endowed active matter with the ability to break percolating clusters, leading to a decrease in solution viscosity.

This work offers a possible explanation for the superfluid-like phenomena caused by E. coli, and further substantiates the evidence of potential links between active matter and sheared systems.

Schematic of the correlation between active matter and sheared systems. (Image by Prof. XU et al.)