Oscillating rheological behavior of Turbatrix aceti nematodes

We present an experimental investigation of the rheological aspects of collective motion by the swimming Turbatrix aceti nematodes. We discover that these nematodes can significantly change the rheological properties of the suspension due to their body oscillations and form synchronized waves, which produce strong fluid flows. The strength of the collective state changes the shape of the interface where they swim in synchronization. We unravel that the effective viscosity of the nematode suspension at higher shear rates shows steady viscous behavior with time, where no significant effect of nematode activity is observed. For the first time, we have reported that at low shear rates, the activity effect is significant enough to generate oscillating viscous effects. In addition, we also measured the influence of the nematode concentration on suspension viscosity. This work opens a new way for understanding the rheological aspects of active matter under low and high shear rates. We illustrate these dynamics by showing that the force generated by these nematodes is sufficient to change the suspension rheology. The various aspects of nematodes, especially their large size and ease of culturing, make them a good model organism for experimental investigation as active fibers with oscillations. The oscillating behavior regulates the interfacial phenomenon and produces oscillatory rheological dynamics at low shear rates. The results of our work can be utilized to further study the novel metamaterials with negative viscosity, which have applications in healthcare and energy systems.