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This experimental work investigates the impact dynamics of drops on vertically oriented, three-dimensional-printed (3D-printed) fiber arrays with variations in packing density, fiber arrangement, and wettability. These fiber arrays are inspired by mammalian fur, and while not wholly representative of the entire morphological range of fur, they do reside within its spectrum. We define an aspect ratio, a modified fiber porosity relative to the drop size, that characterizes various impact regimes. Using energy conservation, we derive a model relating drop penetration depth in vertical fibers to the Weber number. In sparse fibers where the Ohnesorge number is less than 4×10−3, penetration depth scales linearly with the impact Weber number. In hydrophobic fibers, density reduces penetration depth when the contact angle is sufficiently high. Hydrophilic arrays have greater penetration than their hydrophobic counterparts due to capillarity, a result that contrasts the drop impact-initiated infiltration of horizontal fibers. Vertical capillary infiltration of the penetrated liquid is observed whenever the Bond number is less than 0.11. For hydrophilic fibers, we predict that higher density will promote drop penetration when the contact angle is sufficiently low. Complete infiltration by the drop is achieved at sufficient times regardless of drop impact velocity.
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This content will become publicly available on December 1, 2026
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 ≈ 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.
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- Award ID(s):
- 1825801
- PAR ID:
- 10653639
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Physics of Fluids
- Volume:
- 37
- Issue:
- 12
- ISSN:
- 1070-6631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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