Lotus plants (Nelumbo nucifera) stay dirt-free, an obvious advantage for an aquatic plant living in typically muddy habitats, and they do so without using detergent or expending energy. The plant’s cuticle, like that of other plants, is made up of soluble lipids embedded in a polyester matrix – wax – but the degree of its water repellency is extreme (superhydrophobic). This is accomplished through the micro-topography of their leaf surfaces, which while showing a variety of structures, all share a similar mathematical set of proportions associated with superhydrophobicity.
Lotus leaves, for example, exhibit extensive folding (i.e., papillose epidermal cells) and epicuticular wax crystals jutting out from the plant’s surface, resulting in a roughened microscale surface. As water and air adhere less well than water and solids, roughened surfaces tend to reduce adhesive force on water droplets, as trapped air in the interstitial spaces of the roughened surface result in a reduced liquid-to-solid contact area. This allows the self-attraction of the polar molecule of water to express more fully, causing it to form spheres. Dirt particles on the leaf’s surface stick to these droplets, both due to natural adhesion between water and solids and because contact with the leaf surface is reduced by over 95% from the leaf’s micro-topography. The slightest angle in the surface of the leaf (e.g., caused by a passing breeze) then causes the balls of water to roll off due to gravity, taking the attached dirt particles with them and cleaning the leaf without using detergent or expending energy.
Surface finishes inspired by the self-cleaning mechanism of lotus plants and other organisms (e.g., many large-winged insects) have now been applied to paints, glass, textiles, and more, reducing the need for chemical detergents and costly labor.
This summary was contributed by Ashley Meyers.
This video gives you a closer look at the surface of the lotus leaf.
Learn more about the lotus’ strategy in Tom McKeag’s case study, “Return of the Swamp Thing” on pages 12-15 of Zygote Quarterly:
“The microrelief of plant surfaces, mainly caused by epicuticular wax crystalloids, serves different purposes and often causes effective water repellency. Furthermore, the adhesion of contaminating particles is reduced. Based on experimental data…it is shown here for the first time that the interdependence between surface roughness, reduced particle adhesion and water repellency is the keystone in the self-cleaning mechanism of many biological surfaces. The plants were artificially contaminated with various particles and subsequently subjected to artificial rinsing by sprinkler or fog generator. In the case of water-repellent leaves, the particles were removed completely by water droplets that rolled off the surfaces independent of their chemical nature or size. The leaves of N. nucifera afford an impressive demonstration of this effect, which is, therefore, called the ‘Lotus-Effect’ and which may be of great biological and technological importance.” (Barthlott and Neinhuis 1997:1)