Drop impact and infiltration of vertical fiber arrays

In this experimental study, we investigate drops impacting 3D-printed vertical fiber arrays, inspired by mammalian fur coats. Drops impact fiber strands fixed to a flat base at one end, inertially penetrating and spreading laterally by capillarity. We vary the packing density, fiber alignment, wettability of the arrays, and drop velocity. We identify a trans-fiber oscillatory motion due to fiber irregularity apart from the expected spreading and penetration dynamics on isotropic fibrous arrays with radial symmetry. Using image analysis, we measure the degree of drop penetration and spread into the fiber array and qualitatively categorize impact types. Impact Weber number and a modified porosity define penetration, retraction, and drop rebound regimes. Denser fiber packing reduces liquid penetration. In vertically oriented fibers, the arrangement of fiber strands is found to be irrelevant. By assuming a constant deceleration of the drops upon impact, we find that impact force is independent of fiber arrangement, density, or strand wettability. Furthermore, our results demonstrate that hydrophilic arrays have greater penetration than their hydrophobic counterparts due to capillary effects, a result that contrasts drop impact infiltration of horizontally aligned fibers.