After heavy rain, water turns mercurial on nasturtium leaves. The water balls into a skittering drop, seeming to float just over the leaf’s surface. I was reminded of chasing liquid metal over chemistry lab benches in the days before kids were protected from such amusements. But similar delights, minus the metabolic cost, await in the garden.
The drops were dancing. Water on the leaves of other plants was sluggish, gathering in flat pools or damp stains. These plants were wet, soggy, but the nasturtium leaves were perfectly dry, even where silvery drops had sat a few seconds before.
A recent paper by James Bird and colleagues in Nature reported that nasturtium leaves are covered with “superhydrophobic ridges” (literally, “super-water-fearing ridges”). These minute structures on the leaf surface exert a strong repulsive force on water. When a water drop hits the surface, the repulsion is so strong that the drop recoils, shatters into minute droplets, and jumps back into the air. The Nature paper does not mention this, but my observations suggest that nasturtium leaves only shed large drops. On dewy mornings, smaller drops manage to cling, although they still sit as silvery globes.
Leaves of almost all land plants have a waxy covering that keeps water away from the core of the leaf and eventually causes water to run off. But on nasturtium leaves, water doesn’t just run, it springs and sprints. Nasturtium beats even the former superchampion of “hydrophobicity,” the water-shedding upper surface of lotus leaves. What function might this serve? We do not know, but shedding water must surely combat fungal infections by depriving spores of damp places in which to germinate.
All this makes for ephemeral beauty in the garden. It may also be of practical importance. Surfaces that vigorously repel water not only stay remarkably dry, but they self-clean and resist icing. Engineers would love to incorporate these features into all kinds of surfaces, especially cloth, windows, painted walls, airplane wings, and the insides of ketchup bottles (the BBC has a nice overview).
I look forward to venturing into the woods clad in a coat of nasturtium, a fig leaf for rainy climes.