The rheology, i.e., the deformation and flow, of polymers is generally well-understood. However, this is not true for active polymer systems with self-driven, moving particles. Such active systems are usually far from equilibrium and challenging to study. There are only few model systems, which use a comparatively small number of simple moving particles.
Antoine Deblais, Sander Woutersen, and Daniel Bonn, University of Amsterdam, The Netherlands, have used living sludge worms (Tubifex Tubifex) as an experimental model for active polymers and studied their rheology. These worms are sold as food for aquarium fish and, thus, readily available in large quantities. The team performed some of the same rheology experiments that are usually performed on polymers on groups of the worms in water. The team used a custom-designed rheology cell made from a beaker with a turning plate inside. They studied shear thinning, a phenomenon in which polymer chains align during flow and the flow resistance is reduced. The activity of the worms was controlled by changing the temperature or by adding small amounts of alcohol to the water to inactive them.
Using the worm model, the team found that shear thinning in active polymer-like systems is strongly influenced by the level of activity. At low shearing rates, an active worm solution was 10–100 times less viscous than a passive worm solution. At very high shearing rates, in contrast, the viscosity of an active worm solution was higher that of a passivated one. The team attributes both effects to the movement of the worms: At low shear rates, the movement could help the worms to “untangle” and improve flow, while at high shear rates, the movement might undo some of the shear-induced alignment and impede flow. According to the researchers, their worm model could be useful for developing a theory of the rheology of active polymers.
- Rheology of Entangled Active Polymer-Like T. Tubifex Worms,
A. Deblais, S. Woutersen, D. Bonn,
Phys. Rev. Lett. 2020.