Nonlinear flow response of soft hair beds

We are ‘hairy’ on the inside: surfaces such as intestines, tongues, and blood vessels comprise beds of fibers immersed in fluids. Fluid flows deform hairs— but hair deformation affects fluid flows. We investigate these mechanics by measuring the flow response of artificial hair beds and developing a theoretical model. We identify two nonlinearities. First, a drag-reducing nonlinearity occurs when hairs bend down and widen the gap through which fluid flows. We hypothesize biological hairs reduce drag in-vivo via this nonlinearity. Second, a rectification nonlinearity occurs when fluid flows against the grain of angled hairs. Rectification can be used to build microfluidic diodes and pumps for various applications, including miniature hydraulic robots.