Dynamic Flow Response of Fluid-immersed Hair Beds

Fluid immersed hair beds are a common occurrence in organisms. Bronchial cilia, the endothelial glycocalyx layer, and intestinal microvilli (among others) serve critical regulatory functions. To better understand these functions, and the role of passive hairs on fluid dynamics in biological settings, we investigate the transient characteristics of hair beds immersed in a low Reynolds number environment. Large amplitude oscillatory shear rheology experiments are conducted with the hair-beds and their response is characterized with the mechanical impedance, a quantity describing the system's effective viscosity. While the fluid and solid both have linear constitutive properties, we show that interactions between hair-bed deformations and fluid flows induce a nonlinear relationship between the forcing parameters and the system's impedance, contrasting the rigid limit. The nonlinearity of the system, and the ability of the hairs to respond with many degrees of freedom to shear makes them an interesting candidate to stir fluids.