Sequential drop impacts onto horizontal fiber arrays*

We experimentally investigate drop infiltration into horizontally oriented fiber arrays imposed by sequential drop impacts. Two successive drop impacts are filmed striking 3D-printed fiber arrays with varying densities, surface wettability, and a fixed fiber diameter. The penetration depth and the lateral width of drop spreading within fiber layers is quantified. Hydrophobic fibers more effectively prevent an increase in penetration depth by the second impacting drop at low impact Weber numbers, whereas hydrophilic fibers prevent a change in penetration depth as Weber number increases. Impact outcomes, such as penetration depth and lateral spreading, are insensitive to impact eccentricity between the first and second drops. As expected, denser, staggered fibers reduce infiltration, preventing the entire drop mass from entering the array. Drop fragmentation, which is promoted by hydrophobicity, larger inter-fiber spacing, and higher drop impact velocity, limits increases in spread from a subsequent drop. Our experimental system is inspired by mammalian fur coats, and our results provide insight to how we expect natural fibers respond to falling drops and the structure innate to this multiscale covering.