Competition between local activity, gravity, and boundary shear in active-passive granular mixtures

Organisms living within granular materials disturb their local environment as they move, sometimes creating bulk flows. To quantitatively examine these effects, we mix flour beetle larvae with natural grains of various sizes and examine the resulting flows. We characterize the rate of particle-scale rearrangements due to fluctuations by diffusing wave spectroscopy, the macroscopic flow rate via observing the angle of a relaxing sandpile, and the effective shear viscosity via a rheometer. We find that both the rates of rearrangements and the macroscopic flows depend on both the percentage of larvae, and the grain type of the mixture. Increasing the percentage of larvae increases the rearrangement rate and the flow rate; as such, we make an analogy to a fluid reducing its viscosity at higher temperatures. Additionally, we find that the rheology of the mixtures becomes insensitive to the fluctuations of the larvae when the mixtures are subjected to the shear force of the rheometer. By comparing the results across the experiments, we conclude that the rheology of the active-passive mixtures depends on four key factors: the fluctuation rate due to the active matter, the grain inertia, the cohesive force between the grains, and the driving shear.