Effect of movable <i>Paramecium caudatum</i> on the effective viscosity of aqueous polymer solution

The effective viscosity of an active suspension depends on the mobility of microswimmers. In addition, there is an inverse effect of the rheological properties of the liquid on the motion of the microswimmers themselves, so the analysis of the effect of mobile microswimmers on the effective viscosity of suspensions based on non-Newtonian fluids is extremely complex. Here, we analyze the effect of the mobile unicellular micro-organism Paramecium caudatum on the effective viscosity of an aqueous solution of hydroxyethylcellulose. The results of the study show that the effective viscosity of the suspension has a maximum depending on the concentration of mobile microswimmers in the suspension. The ambiguous effect of microswimmers on the effective viscosity of the suspension is due to the competition between two competing factors. On the one hand, the motion of microswimmers contributes to the perturbation of the shear rate field in the liquid and thereby leads to additional energy dissipation and, as a consequence, to an increase in the effective viscosity. On the other hand, due to the fact that microswimmers move in a polymer solution, which is characterized by a shear-thinning property, their mobility contributes to a decrease in the effective viscosity of the suspension.