Heat-thickening in active particle suspensions

The rheology of active suspensions is rich, complex and counter-intuitive due to the presence of self-propelled particles. In our study, the shear viscosity of puller suspensions of several volume fractions has been experimentally measured at different temperatures and shear rates. Firstly, we report that puller suspensions show an order of magnitude increase in viscosity with temperature. Complementing the rheology measurements with microscopic observations, we find that the beating frequency of the propulsion apparatus of single micro swimmers increase with temperature. Since the motility of the microswimmer leads to the addition of stresses into the fluid, also called as 'active' stresses, we are able to use the acquired beating frequency data to explain the surprising effect of temperature on the rheology of the suspension. Secondly, we also observe that puller suspensions are shear-thinning. We find that shear-thinning emerges because the added 'active' stress is independent on the imposed shear rate. Further, a simple model relating the viscosity of the suspension to the motility of the microswimmer satisfactorily describes both features of our experimental data. This work highlights the cruical role of motility in active-suspension rheology.