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This experimental work builds on our previous studies on the post-impact characteristics of drops striking three-dimensional-printed fiber arrays by investigating the highly transient characteristics of impact. We measure temporal changes in drop penetration depth, lateral spreading, and drop dome height above the fiber array as the drop impacts. Liquid penetration of vertical fibers may be divided into three sequential periods with linearly approximated rates of penetration: (i) an inertial regime, where penetration dynamics are governed by inertia; (ii) a transitional regime exhibiting inertial and capillary action; and (iii) a capillary regime characterized purely by downward wicking. Horizontal fibers exhibit only the inertial and transitional stages, with wicking only observed horizontally along the direction of fibers. In horizontal hydrophilic fiber arrays, the time duration to reach the maximum lateral deformation of the drop is proportional to We1/4, as observed in drops impacting solid surfaces. There exists a critical Weber number below which the drop shows no radial deformation, and the critical value increases with decreasing fiber density. At large Weber numbers, drops splash. In contrast, vertical fibers restrict the lateral spreading of the drop, thereby suppressing a splash for all tested drop velocities, even those exceeding 5 m/s. 

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 ≈ 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 develop 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.