Drop penetration of fur-like fiber arrays*

In this experimental study we combine of drop impact onto porous media and impacts onto single fibers to study drop impact into fiber arrays inspired by mammalian fur coats. In our 3D-printed arrays, we can vary the packing density, fiber alignment and shape, and wettability. Drops impact fibers fixed at both ends, inertially penetrating and spreading across the upper surface. Using image analysis we measure penetration depth, wetted diameter, and volume of penetration into the array. Impact Weber number and a modified porosity define penetration, retraction, and rebound. As packing density increases and porosity decreases, penetration is replaced by inertial spreading, which is of course no surprise. However, our results indicate that as density grows past what we can manufacture, toward the density of lying hairs, penetration of fibrous arrays by falling drops is nearly zero. Thus we expect mammalian skin to remain dry in the most violent rainfall.