We are ‘hairy’ on the inside: beds of passive fibres anchored to a surface and immersed in fluids are prevalent in many biological systems, including intestines, tongues, and blood vessels. These hairs are soft enough to deform in response to stresses from fluid flows. Yet fluid stresses are in turn affected by hair deformation, leading to a coupled elastoviscous problem that is poorly understood. Here we investigate a biomimetic model system of elastomer hair beds subject to shear-driven Stokes flows. We characterize this system with a theoretical model that accounts for the large-deformation flow response of hair beds. Hair bending results in a drag-reducing nonlinearity because the hair tip lowers towards the base, widening the gap through which fluid flows. When hairs are cantilevered at an angle subnormal to the surface, flow against the grain bends hairs away from the base, narrowing the gap. The flow response of angled hair beds is axially asymmetric and amounts to a rectification nonlinearity. We identify an elastoviscous parameter that controls nonlinear behaviour. Our study raises the hypothesis that biological hairy surfaces function to reduce fluid drag. Furthermore, angled hairs may be incorporated in the design of integrated microfluidic components, such as diodes and pumps.

我们内部是“毛茸茸的”：固定在表面并浸没在液体中的无源纤维床在许多生物系统（包括肠，舌和血管）中很普遍。这些毛发足够柔软，能够响应来自流体流动的应力而变形。然而，流体压力又受毛发变形的影响，从而导致耦合弹性粘滞问题，人们对此知之甚少。在这里，我们研究弹性体毛床受剪切驱动的斯托克斯流的仿生模型系统。我们用一个理论模型来表征该系统，该模型考虑了毛床的大变形流动响应。头发弯曲会导致减少阻力的非线性，因为头发尖端朝着底部降低，从而扩大了流体流动的间隙。当头发悬垂成与表面垂直的角度时，逆着谷物的流动使毛发从底部弯曲，缩小了缝隙。倾斜毛发床的流动响应在轴向上是不对称的，相当于整流非线性。我们确定了弹性粘度参数，该参数可控制非线性行为。我们的研究提出了这样的假设，即生物毛状表面具有减少液体阻力的功能。此外，成角度的毛发可以结合到集成的微流控组件（例如二极管和泵）的设计中。