Paddles and rakes: Fluid flow through bristled appendages of small organisms*

Many small organisms capture food particles, locomote through water or air, or create currents using appendages bearing arrays of long bristles. The performance of these arrays of hairs depends on the movement of fluid relative to them. We have modeled the fluid flow around such hairs, and have used the model to predict, for a range of biologically-relevant circumstances: (1) the steepness of the shear gradients adjacent to the hairs, (2) the leakiness of the gaps between pairs of hairs, and (3) the drag force on hairs with neighbors. We point out the circumstances under which bristled appendages function as rakes versus those under which they operate as paddles. Our results suggest that a simple change in size or speed of a bristled appendage can lead to a novel mode of functioning under some circumstances, whereas in other situations differences in morphology or behavior have little effect on performance.