Cross-sectional circularity governs drop penetration of horizontal fiber arrays

In this experimental work, we compare the drop impact behavior on horizontal fiber arrays with circular and wedge-like fiber cross sections. Non-circular fibers are commonplace in nature, appearing on rain-interfacing structures from animal fur to pine needles. Our arrays have various packing densities and are impacted by drops falling 0.2 - 1.6 m/s. Our previous study indicates that hydrophilic fibers more effectively arrest drop motion to prevent penetration. Here, despite being more hydrophilic than their non-circular counterparts, our circular fibers promote drop penetration by 26% more than their wedge-like counterparts through the suppression of lateral spreading and promotion of drop fragmentation within the array. We therefore introduce fiber circularity as a variable that can greatly influence the ability of a structure to resist water. Liquid blob shape within fiber array after impact is likewise influenced by the cross-sectional profile. Using conservation of energy, we develop a model to predict the penetration depth from the fiber properties, and drop Weber number, and generalize our model to accommodate a wide range of fiber cross-sectional geometry.