Dr. Wilhelm Barthlott examined leaf surfaces, among other things, for clues to understanding how nature cleans itself. In the 1990s, he found the key: the roughness elements of the leaf were closely spaced. There were also no detergents or cleaning chemicals of any kind. What Dr. Barthlott discovered was the self-cleaning surface.

While most believe that “more” or “larger” is better, Barthlott saw that “smaller” explains how some of nature’s surfaces clean themselves—and found wondrous new capabilities, at the sub-micro level, in the details of structure at size levels previously thought to be too small to be of concern. Work at small levels of size is called nanotechnology. In the metric system, mil is 10^-3 meters (m), micro is 10^-6 m, and nano is 10^-9 —one trillionth of a meter. The dimensions of atoms are expressed in nanometers (nm); an atomic radius is about 1 Å (angstrom) or 0.1 nm. Proteins, which occur as large molecules, are about 5 Å in diameter or 0.5 nm. (In English units, 1 mil is 1/1000 of an inch, or 25,400 nm.)

For a plant leaf, self-cleaning works because the dirt particles to be removed are much larger than the spacing of the roughness on the leaf surface. Dr. Barthlott observed nano-sized roughness on the surface of a leaf of the white lotus flower. From that observation, he was able to determine a mechanism by which nature cleans the surfaces of some plants. A somewhat simplified explanation follows:

Figure 1 shows in concept that relatively large surface roughness elements trap water and dirt. The water moves the dirt along the surface. Figure 2 shows, in concept, the effect of the roughness spacing. The white lotus leaf turns out to have tiny points (nano-spaced surface roughness). When dirt falls on the leaf, it is supported on those points and then carried away when drops of water roll across the leaf.

The white lotus plant, in other words, has a self-cleaning leaf. The photograph¹ in Figure 2 shows the cause and the effect, which Dr. Barthlott described as the Lotus Effect. This is not the whole story, however. Plant leaves like those of the lotus are covered with a hydrophobic wax which reduces the adhesion of water so dirt is carried away whenever it rains. The plant regenerates the wax so that cleaning is insensitive to environmental conditions.

Why should contamination control professionals care about how plants clean themselves? Because the understanding behind these observations, called biomimicry, has made or will make available products such as a housepaint (Lotusan^®) that doesn’t need cleaning for 5 years; window glass that cleans itself; a self-cleaning paint for automobiles; toilets, sinks, and tubs that are practically self-cleaning; and a coating about 100 nm thick that makes tiles scratchproof. One would also care about the ability to make a metal surface which would clean itself of 0.3 µ particles (300 nm).

Another reason to care is that the Lotus Effect is one of a large number of instances where macro events we value can be created by a better understanding and control of structure at the nano level. This means control of atoms and molecules, the building blocks of our natural world…think about it.

1 W. Barthlott, “Scanning Electron Microscopy of the Epidermal Surface in Plants,” Scanning Electron Microscopy in Taxonomy and Functional Morphology, D. Claugher, Ed., Clarendon Press, Oxford, 1990.