Superhydrophobic New Zealand leaves: contact angle and drop impact experiments

Abstract

Dynamic wetting of the adaxial leaf surfaces of several New Zealand native plants has been studied using water drops, contact angle measurements and drop impact experiments. Three native plants (Arthropodium bifurcatum, Euphorbia glauca and Veronica albicans) are identified as superhydrophobic, having advancing contact angles !1608, with B128 hysteresis and B108 roll off angle. These data are comparable to leaves of the lotus (Nelumbo nucifera), an international superhydrophobicity benchmark. Scanning electron microscopy reveals a range of epidermal microstructures and submicron epicuticle wax textures on native leaves, including superhydrophobic microstructures that are similar to those found on exotic leaves. Drop impacts on N. nucifera, A. bifurcatum and E. glauca were filmed using a high speed camera at Weber number between 15 and 200. Drops impacting on the two native plants exhibited asymmetry due to vascular bundles on the leaf surfaces. Biomimicry and dynamic wetting are important for development of industrial superhydrophobic surfaces.

Plants that lead: do some surface features direct enemy traffic on leaves and stems?

“…Rapid drainage limits the establishment and accumulation of dust and epiphylls, which interfere with photosynthesis. Water repellence in many plants is enhanced by the presence of micropapillae and wax crystals, which create small-scale roughness on the leaf surface and enable droplets to roll off, often aided by wind and gravity (Neinhuis & Barthlott, 1997;Holder, 2007;Hsu, Woan & Sigmund, 2011;Bixler & Bhushan, 2013;Fritsch, Willmott & Taylor, 2013;Watson, Gellender & Watson, 2014). The crystals and other microtopographic features promoting drainage are sometimes anisotropic (Hsu et al, 2011;Bixler & Bhushan, 2013;Fritsch et al, 2013) and some may be situated on hinged trichomes, although the ability of leaves to shed water and contaminants is not obviously linked to the presence of adapically oriented or hooked hairs.…”

Uptake of phosphorus from surfactant solutions by wheat leaves: spreading kinetics, wetted area, and drying time

“…Conventional wetting theories, such as the Cassie 19 and Wenzel 20 equations, are often inadequate for describing the wettability of leaves 18 on which contact line pinning (and thus contact angle hysteresis) can dominate behaviour. 16,[27][28][29][30] Given the complexity of the mechanisms involved, it is a tempting convenience to assume that improved spreading (faster spreading to lower contact angles with increased leaf area in contact with the foliar spray) will increase the uptake of the fertilizer. [21][22][23][24][25][26] The physical picture is further complicated by wetting dynamics, which can play an important role during droplet-leaf impact with respect to splashing and run-off.…”

High‐speed photography of water drop impacts on sand and soil

“…The major difference in those experiments was that the granular targets had relatively narrow size probability distribution functions, and were typically ∼ 100 μ m in size or smaller. More broadly, Equation (2) does not account for the nature of the solid surface, and has been most successful elsewhere for solid hydrophobic and partially wet surfaces (Marengo et al ., ; Josserand & Thoroddsen, ), including superhydrophobic leaves (Fritsch et al ., ) and hydrophobic polymer pillar arrays (Robson & Willmott, ). Likewise, the other capillary‐driven models tested predicted greater spreading than in our experiments, and we note that these scaling relations have usually been developed under specific experimental conditions, limiting their universality (Marengo et al ., ; Josserand & Thoroddsen, ; Wildeman et al ., ).…”