Leaf Ridges Reduce Contact Time With Water Drops

macro photo of water droplets on dark green leaf

Biological Strategy

Nasturtium

Jeanette Lim

Image: Dan Carlson / Unsplash / Free non-commercial use

Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Modify Speed

Modifying speed or magnitude of velocity is important for some living systems because it enables them to control their movement to access resources, escape predators, and more. Modifying speed requires not only overcoming inertia, but also minimizing the energy needed to make the change. Therefore, living systems have strategies to safely shift from fast to slow or slow to fast. An example is a bird called the kingfisher, which streamlines its body and feathers to quickly move from hovering over water to diving through the air and into the water. Once in the water, the kingfisher slows down by spreading its wings to avoid diving too deep.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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Ridges on garden nasturtium leaves reduce contact time with water drops by enabling faster drop recoil.

Many plants have hydrophobic (water repellent) leaves. When raindrops fall on these surfaces, they tend to bounce off, and the less time a liquid drop spends in contact with a surface, the lower the likelihood that it will stick and leave some moisture behind.

Garden nasturtium (Tropaeolum majus), a common backyard garden plant, has leaves that appear to reduce the contact time of incoming liquid drops by altering drop hydrodynamics with a “macrotextured” surface of relatively large veins and ridges.

On most smooth or microtextured surfaces, incoming drops hit the surface and then experience “axisymmetric recoil”: the drop spreads out into a pancake shape with uniform thickness, and then bounces back up, recoiling with a roughly circular shape and uniform speed around its edges. The center of the spread out drop is static, and has little role in the recoil. On surfaces with macrotexture (up to 150 microns high) ridges and veins, like nasturtium leaves, if a drop hits a ridge, it experiences “center-assisted recoil” instead. The ridge alters the behavior of the drop so that it recoils in an asymmetrical shape, and the liquid in the center of each spread out region contributes to recoil. This is possible because the liquid layer above the ridge is thinner than elsewhere on the surface, so it has less liquid to accelerate back up and recoils faster than other parts of the drop. This non-uniform recoil speed within the drop tends to make it fragment, resulting in small, connected fractions that recoil more quickly. In addition, more of the drop contributes to recoil than would be the case with one big mass of water.

Whether or not reduced drop contact time gives the nasturtium plant any functional benefit has yet to be determined, but researchers who have developed and tested synthetic surfaces with macrotextures have demonstrated similar effects. This finding may be of use for surfaces where water contact time is an issue.

Image: Leon Brooks / Public Domain - No restrictions

Image: George Chernilevsky / Public Domain - No restrictions

Last Updated February 3, 2018

References

“Surfaces designed so that drops do not adhere to them but instead bounce off have received substantial attention because of their ability to stay dry, self-clean^5,6,7 and resist icing^8,9,10. A drop striking a non-wetting surface of this type will spread out to a maximum diameter^11,12,13,14 and then recoil to such an extent that it completely rebounds and leaves the solid material^15,16,17,18. The amount of time that the drop is in contact with the solid—the ‘contact time’—depends on the inertia and capillarity of the drop¹, internal dissipation¹⁹ and surface–liquid interactions^20,21,22. And because contact time controls the extent to which mass, momentum and energy are exchanged between drop and surface²³, it is often advantageous to minimize it. The conventional approach has been to minimize surface–liquid interactions that can lead to contact line pinning^20,21,22; but even in the absence of any surface interactions, drop hydrodynamics imposes a minimum contact time that was conventionally assumed to be attained with axisymmetrically spreading and recoiling drops^21,24. Here we demonstrate that it is possible to reduce the contact time below this theoretical limit by using superhydrophobic surfaces with a morphology that redistributes the liquid mass and thereby alters the drop hydrodynamics.” (Bird et al. 2013:385)

Journal article

Reducing the contact time of a bouncing drop

Nature, 503: 385-388 | 21/11/2013 | Bird JC; Dhiman R; Kwon H-M; Varanasi K

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