Search for: All records

This experimental work builds on our previous studies on the post-impact characteristics of drops striking three-dimensional-printed fiber arrays by investigating the highly transient characteristics of impact. We measure temporal changes in drop penetration depth, lateral spreading, and drop dome height above the fiber array as the drop impacts. Liquid penetration of vertical fibers may be divided into three sequential periods with linearly approximated rates of penetration: (i) an inertial regime, where penetration dynamics are governed by inertia; (ii) a transitional regime exhibiting inertial and capillary action; and (iii) a capillary regime characterized purely by downward wicking. Horizontal fibers exhibit only the inertial and transitional stages, with wicking only observed horizontally along the direction of fibers. In horizontal hydrophilic fiber arrays, the time duration to reach the maximum lateral deformation of the drop is proportional to We1/4, as observed in drops impacting solid surfaces. There exists a critical Weber number below which the drop shows no radial deformation, and the critical value increases with decreasing fiber density. At large Weber numbers, drops splash. In contrast, vertical fibers restrict the lateral spreading of the drop, thereby suppressing a splash for all tested drop velocities, even those exceeding 5 m/s. 

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

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.