Drop impact onto pine needle fibers with non-circular cross section

Abstract

Pine needles have evolved alongside rainfall for millennia. Needles are robust to raindrop impacts and the portions of mass impacting raindrops leave behind. Pine needles also provide a unique fiber profile by which to study liquid drop collisions on slender structures. In this experimental study, we use high-speed videography to film the impact of water drops onto green, rigidly supported Pinus palustris needles, which are of non-circular cross section. Impacts result in drop division into thin, ephemeral lobes, which collapse within milliseconds, splitting the parent drop. We measure orientation-dependent lobe shapes and separation angles, impact force, drop spreading on fibers, and mass retention. Lobes formed by impact with the rounded side of needles are parabolic in shape, while those with the wedged side are irregular. For a fixed drop size, impact forces on rounded surfaces are consistently less than those experienced by wedged surfaces, and the disparity in the impact force between the two orientations grows with the Weber number. The smaller of the two drop sizes on test is more effective at generating impact forces per drop weight than the larger drops. Post-impact, fibers in both orientations retain residual mass from the impacting drop. The wedged surface is more effective at splitting drops and shedding drop remnants.