Drop Impact on Bio-inspired Fiber Arrays and Mammalian Fur

How fiber geometry, packing density, and pelt structure govern raindrop penetration

Gene Patrick S. Rible*, Andrew K. Dickerson
Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville
*Contact researchers: grible@vols.utk.edu
The story All six papers
On the cover of Langmuir 40(26), 2024

On the cover of Langmuir 40(26), 2024

About this research program

The first systematic experimental account of how mammalian fur sheds rain. This seminal six-paper program from the Fluids and Structures Lab (FaST Lab) at the University of Tennessee, Knoxville establishes how mammalian fur, and 3D-printed fiber arrays inspired by it, govern penetration by impacting drops. The papers progress from single-orientation arrays (horizontal, then vertical) through sequential impacts and cross-sectional geometry to the natural mammalian pelts that motivated the work, laying the experimental foundation for the mechanics of fur wetting.

The story, in six papers

One result from each study, in the order the science unfolded.

Simplified models, one orientation at a time

Mammals cannot carry umbrellas, yet their fur keeps them warm and dry in a downpour. To uncover how, we 3D-print fur-like fiber arrays we can tune at will and drop water onto them. The first two studies isolate a single variable: the orientation of the fibers.

Front Cover

Langmuir 40, 13339 (2024)

Hydrophilic fur wins in the rain

Counter to intuition, hydrophilic horizontal fiber arrays resist dynamic drop penetration better than hydrophobic ones at the tested densities. Hydrophobic fibers only win at low Weber numbers; a critical Weber number Wec marks the crossover.

Go to page →
Featured Article

Physics of Fluids 37, 022108 (2025)

Standing fibers flip the rule

An energy-conservation model in which penetration scales linearly with We for sparse vertical fibers; hydrophilic vertical arrays penetrate more than hydrophobic ones (opposite to horizontal).

Go to page →

Toward realism: repeated drops, splashing, and fiber shape

Real rain is never a single clean drop on a pristine array. These three studies add the complications that matter: a second drop landing where the first already wet the array, splashing at raindrop speed, and the non-circular cross-section of real hairs.

Physics of Fluids 37, 072128 (2025)

The first drop shields the second

Fragmentation of the first drop, promoted by hydrophobicity and high impact velocity, limits the second drop's lateral spreading and penetration depth.

Go to page →

Physics of Fluids 37, 092106 (2025)

Vertical arrays kill the splash

Three sequential penetration regimes (inertial, transitional, capillary) on vertical fibers; vertical arrays suppress splashing for all tested drop velocities, even beyond 5 m/s.

Go to page →

Physics of Fluids 37, 122110 (2025)

Round fibers let water in

Circular fiber cross-sections increase drop penetration by 26% over wedged ones, despite being more hydrophilic. Geometry trumps wettability.

Go to page →

Back to real mammalian fur

Finally, we return to the animals themselves, dropping water onto real pelts from six species to confirm the mechanisms our engineered arrays revealed.

Bioinspiration & Biomimetics 21, 036008 (2026)

Real fur carves a dry zone

Six mammalian pelts: a dual-layer hydrophilic/hydrophobic guard-hair-vs-underfur structure produces a saturating dry zone under repeated rainfall.

Go to page →

GitHub Pages

Click each card to go to individual GitHub page.

Dynamic Drop Penetration of Horizontally Oriented Fiber Arrays

Gene Patrick S. Rible, Michael A. Spinazzola, III, Robert E. Jones, III, …, Andrew K. Dickerson

Langmuir 40, 13339 (2024)

Won the front cover of the journal

Counter to intuition, hydrophilic horizontal fiber arrays resist dynamic drop penetration better than hydrophobic ones at the tested densities. Hydrophobic fibers only win at low Weber numbers; a critical Weber number Wec marks the crossover.

Dynamic drop penetration of vertically oriented fiber arrays

Gene Patrick S. Rible, Visalsaya Chakpuang, Aidan D. Holihan, …, Andrew K. Dickerson

Physics of Fluids 37, 022108 (2025)

Selected as the journal's Featured Article

An energy-conservation model in which penetration scales linearly with We for sparse vertical fibers; hydrophilic vertical arrays penetrate more than hydrophobic ones (opposite to horizontal).

Sequential drop impacts onto horizontal fiber arrays

Gene Patrick S. Rible, Agustin Soto, Regina C. Shome, Andrew K. Dickerson

Physics of Fluids 37, 072128 (2025)

Fragmentation of the first drop, promoted by hydrophobicity and high impact velocity, limits the second drop's lateral spreading and penetration depth.

Vertically oriented fiber arrays suppress splashing by restricting spreading of impacting drops

Gene Patrick S. Rible, Syed Jaffar Raza, Joshua T. Watkins, …, Andrew K. Dickerson

Physics of Fluids 37, 092106 (2025)

Three sequential penetration regimes (inertial, transitional, capillary) on vertical fibers; vertical arrays suppress splashing for all tested drop velocities, even beyond 5 m/s.

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

Gene Patrick S. Rible, Syed Jaffar Raza, Jackson H. Boger, …, Andrew K. Dickerson

Physics of Fluids 37, 122110 (2025)

Circular fiber cross-sections increase drop penetration by 26% over wedged ones, despite being more hydrophilic. Geometry trumps wettability.

Fur roughness, density, and length reduce raindrop penetration of mammalian pelts

Gene Patrick S. Rible, John M. Wylie, Braeden K. Elbers, …, Andrew K. Dickerson

Bioinspiration & Biomimetics 21, 036008 (2026)

Six mammalian pelts: a dual-layer hydrophilic/hydrophobic guard-hair-vs-underfur structure produces a saturating dry zone under repeated rainfall.

Citations

@article{rible2024horizontal,
  author  = {Rible, Gene Patrick S. and Spinazzola, Michael A. and Jones, Robert E. and Constantin, Rachel U. and Wang, Wei and Dickerson, Andrew K.},
  title   = {Dynamic Drop Penetration of Horizontally Oriented Fiber Arrays},
  journal = {Langmuir},
  volume  = {40},
  number  = {26},
  pages   = {13339--13354},
  year    = {2024},
  doi     = {10.1021/acs.langmuir.4c00371}
}

@article{rible2025vertical,
  author  = {Rible, Gene Patrick S. and Chakpuang, Visalsaya and Holihan, Aidan D. and Sebek, Hannah P. and Osman, Hannah H. and Brown, Kyle R. and Wang, Wei and Dickerson, Andrew K.},
  title   = {Dynamic drop penetration of vertically oriented fiber arrays},
  journal = {Physics of Fluids},
  volume  = {37},
  number  = {2},
  pages   = {022108},
  year    = {2025},
  doi     = {10.1063/5.0246986}
}

@article{rible2025sequential,
  author  = {Rible, Gene Patrick S. and Soto, Agustin and Shome, Regina C. and Dickerson, Andrew K.},
  title   = {Sequential drop impacts onto horizontal fiber arrays},
  journal = {Physics of Fluids},
  volume  = {37},
  number  = {7},
  pages   = {072128},
  year    = {2025},
  doi     = {10.1063/5.0281512}
}

@article{rible2025splash,
  author  = {Rible, Gene Patrick S. and Raza, Syed Jaffar and Watkins, Joshua T. and Lin, Abbey and Chakpuang, Visalsaya and Dickerson, Andrew K.},
  title   = {Vertically oriented fiber arrays suppress splashing by restricting spreading of impacting drops},
  journal = {Physics of Fluids},
  volume  = {37},
  number  = {9},
  pages   = {092106},
  year    = {2025},
  doi     = {10.1063/5.0286271}
}

@article{rible2025crosssection,
  author  = {Rible, Gene Patrick S. and Raza, Syed Jaffar and Boger, Jackson H. and Osman, Hannah H. and Holihan, Aidan D. and Elbers, Braeden K. and Brown, Kyle R. and Schenck, Christopher M. and Reed, Benjamin J. and Dickerson, Andrew K.},
  title   = {Cross-sectional circularity promotes dynamic drop penetration of horizontal fiber arrays},
  journal = {Physics of Fluids},
  volume  = {37},
  number  = {12},
  pages   = {122110},
  year    = {2025},
  doi     = {10.1063/5.0287633}
}

@article{rible2026fur,
  author  = {Rible, Gene Patrick S. and Wylie, John M. and Elbers, Braeden K. and Dooley, David Job and Thomas, Cora L. and Dickerson, Andrew K.},
  title   = {Fur roughness, density, and length reduce raindrop penetration of mammalian pelts},
  journal = {Bioinspiration & Biomimetics},
  volume  = {21},
  number  = {3},
  pages   = {036008},
  year    = {2026},
  doi     = {10.1088/1748-3190/ae66c1}
}

Discussion

Questions, comments, and follow-up work. Sign in with GitHub to post.