Dynamics of Drop Impact and Infiltration into Fur-Like Structures

This experimental work unifies insights from a series of investigations into the dynamics of drop impact and liquid infiltration in fur-like fiber structures, inspired by the multiscale architecture of mammalian pelage. Using both natural fur samples and 3D-printed fiber arrays with tunable geometries, densities, wettabilities, and orientations, we characterize the mechanisms governing drop penetration depth, lateral spreading, and splash suppression across a broad range of Weber numbers. Through sequential drop impacts, we observe that liquid infiltration into fiber networks saturates over time, establishing a dry insulating zone near the base—analogous to that in mammalian coats. This saturation behavior is quantitatively linked to macroscopic pelage parameters such as fiber density, length, contact angle, and cross-sectional geometry, as well as microscopic traits including scale roughness and aspect ratio.

Comparative experiments on horizontal and vertical arrays reveal directional asymmetries: horizontal fibers primarily exhibit inertial and transitional penetration regimes, while vertical arrays include a capillary-dominated phase marked by sustained wicking. Fiber orientation and cross-sectional shape significantly affect penetration dynamics: wedge-shaped fibers suppress fragmentation and promote lateral spreading more effectively than circular fibers, despite their greater hydrophilicity. Hydrophobicity delays initial infiltration, while higher fiber density and staggered arrangement reduce total penetration volume. The interplay between impact velocity, wettability, and geometry determines whether secondary drops increase infiltration depth or are arrested at the surface.

We introduce several dimensionless parameters—including a modified porosity-to-drop-size ratio and a fiber aspect ratio—to describe the onset of splash, critical Weber number for deformation, and the Bond number as a predictor of capillary infiltration. Energy-based models are developed to predict penetration depth from above-array imaging, applicable to fibers of arbitrary convex cross section. Collectively, our findings unravel how the multi-functional design of natural and synthetic fibrous surfaces can resist wetting through a synergy of geometry, material properties, and impact dynamics, providing design guidelines for engineered hydrophobic coatings, bioinspired textiles, and raindrop-resilient systems.