Cross-sectional circularity promotes dynamic drop penetration of horizontal fiber arrays

In this experimental work, we compare the drop impact behavior on horizontal fiber arrays with circular and wedged fiber cross sections. Non-circular fibers are commonplace in nature, appearing on rain interfacing structures from animal fur to pine needles. Our arrays of packing densities of ≈50, 100, and 150 cm⁻² are impacted by drops falling at 0.2–1.6 m/s. A previous work has shown that hydrophilic, horizontal fiber arrays reduce dynamic drop penetration more than their hydrophobic counterparts. In this work, we show that circularity, like hydrophobicity, increases drop penetration. Despite being more hydrophilic than their non-circular counterparts, our hydrophilic circular fibers promote drop penetration by 26% more than their non-circular counterparts through suppression of lateral spreading and promotion of drop fragmentation within the array. Circular fiber cross sections induce a more circular liquid shape within the fiber array after infiltration. Using conservation of energy, we developed a model that predicts the penetration depth within the fiber array using only measurements from a single external camera above the array. We generalize our model to accommodate fibers of any convex cross-sectional geometry.