The
lotus effect refers to the very high water repellence (superhydrophobicity) exhibited by the leaves of the lotus flower (
NelumboNelumbo is a genus of aquatic plants with large, showy flowers resembling water lilies, commonly known as lotus. The generic name is derived from the Sinhalese word Nelum. There are only two known living species in the genus. The sacred lotus is native to Asia, and is the better known of the two...
).
Dirt particles are picked up by water droplets due to a complex micro- and nanoscopic architecture of the surface, which minimizes adhesion.
This effect can easily be demonstrated in many other plants, for example
Tropaeolum (nasturtium),
OpuntiaOpuntia, also known as nopales or paddle cactus , is a genus in the cactus family, Cactaceae.Currently, only prickly pears are included in this genus of about 200 species distributed throughout most of the Americas. Chollas are now separated into the genus Cylindropuntia, which some still consider...
(prickly pear),
AquilegiaAquilegia is a genus of about 60-70 species of perennial plants that are found in meadows, woodlands, and at higher altitudes throughout the Northern Hemisphere, known for the spurred petals of their flowers.-Etymology:The genus name Aquilegia is derived from the Latin word for eagle , because...
(columbine), cane, and on the wings of certain insects.
The phenomenon was first studied by Dettre and Johnson in 1964 using rough hydrophobic surfaces. Their work developed a theoretical model based on experiments with glass beads coated with paraffin or PTFE telomer. The self-cleaning property of superhydrophobic micro-
nanostructuredNanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
surfaces was studied by Barthlott and Ehler in 1977, and perfluoroalkyl and perfluoropolyether superhydrophobic materials were developed by Brown in 1986 for handling chemical and biological fluids. Other biotechnical applications have emerged since the 1990s.
Functional principle
Due to their high surface tension, water droplets tend to minimize their surface trying to achieve a spherical shape. On contact with a surface, adhesion forces result in
wettingWetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...
of the surface. Either complete or incomplete wetting may occur depending on the structure of the surface and the fluid tension of the droplet.
The cause of self-cleaning properties is the hydrophobic water-repellent double structure of the surface. This enables the contact area and the adhesion force between surface and droplet to be significantly reduced resulting in a self-cleaning process.
This hierarchical double structure is formed out of a characteristic epidermis (its outermost layer called the cuticle) and the covering waxes. The epidermis of the lotus plant possesses papillae with 10 to 20 µm in height and 10 to 15 µm in width on which the so-called epicuticular waxes are imposed. These superimposed waxes are hydrophobic and form the second layer of the double structure.
The hydrophobicity of a surface is related to its
contact angleThe contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...
. The higher the contact angle the higher the hydrophobicity of a surface. Surfaces with a contact angle < 90° are referred to as hydrophilic and those with an angle >90° as hydrophobic. Some plants show contact angles up to 160° and are called super-hydrophobic meaning that only 2–3% of a drop's surface is in contact. Plants with a double structured surface like the lotus can reach a contact angle of 170° whereas a droplet’s actual contact area is only 0.6%. All this leads to a self-cleaning effect.
Dirt particles with an extremely reduced contact area are picked up by water droplets and are thus easily cleaned off the surface. If a water droplet rolls across such a contaminated surface the adhesion between the dirt particle, irrespective of its chemistry, and the droplet is higher than between the particle and the surface.
As this self-cleaning effect is based on the high surface tension of water it does not work with organic solvents. Therefore, the lotus-effect is no protection against graffiti.
This effect is of a great importance for plants as a protection against
pathogenA pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s like fungi or
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
growth, and also for animals like butterflies, dragonflies and other insects not able to cleanse all their body parts.
Another positive effect of self-cleaning is the prevention of contamination of the area of a plant surface exposed to light resulting in a reduced photosynthesis.
Technical application
Some
nanotechnologistsNanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
have developed treatments, coatings, paints, roof tiles, fabrics and other surfaces that can stay dry and clean themselves in the same way as the lotus leaf. This can usually be achieved using special fluorochemical or silicone treatments on structured surfaces or with compositions containing micro-scale particulates. Super-hydrophobic coatings comprising Teflon microparticles have been used on medical diagnostic slides for over 30 years. It is possible to achieve such effects by using combinations of
polyethylene glycolPolyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...
with
glucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and
sucroseSucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
(or any insoluble particulate) in conjunction with a hydrophobic substance.
As self cleaning of superhydrophobic microscopic to nanoscopic surfaces is based on a purely physio-chemical effect it can be transferred onto technical surfaces on a biomimetic basis. One example of the products with superhydrophobic self-cleaning properties is the facade
paintPaint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
Lotusan.
Further applications have been marketed, such as self-cleaning glasses installed in the sensors of traffic control units on German autobahns developed by a cooperation partner (Ferro GmbH). Evonik AG has developed a spray for generating self-cleaning films on various substrata.
Lotus effect superhydrophobic coatings applied to microwave antennas can significantly reduce
rain fadeRain fade refers primarily to the absorption of a microwave radio frequency signal by atmospheric rain, snow or ice, and losses are especially prevalent at frequencies above 11 GHz. It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a...
and the buildup of ice and snow.
“Easy to clean” products in ads are often mistaken in the name of the self-cleaning process of the lotus-effect. Patterned superhydrophobic surfaces also show promise for "lab-on-a-chip" microfluidic devices and can greatly improve surface-based bioanalysis.
Research history
Although the self-cleaning phenomenon of the lotus was known in Asia for more than 2000 years (reference to the lotus effect,
padmapatramivambhasa, is found in
Bhagavad GitaThe ' , also more simply known as Gita, is a 700-verse Hindu scripture that is part of the ancient Sanskrit epic, the Mahabharata, but is frequently treated as a freestanding text, and in particular, as an Upanishad in its own right, one of the several books that constitute general Vedic tradition...
), its mechanism was explained only in the early 1970s after the introduction of the
scanning electron microscopeA scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
. Studies were performed with leaves of
Tropaeolum and lotus (
NelumboNelumbo is a genus of aquatic plants with large, showy flowers resembling water lilies, commonly known as lotus. The generic name is derived from the Sinhalese word Nelum. There are only two known living species in the genus. The sacred lotus is native to Asia, and is the better known of the two...
). By the mid 1990s, Wilhelm Barthlott developed industrial products and trademarked the principle as the Lotus-Effect.
External links
- Animation of the lotus effect
- Scientific American
Scientific American is a popular science magazine. It is notable for its long history of presenting science monthly to an educated but not necessarily scientific public, through its careful attention to the clarity of its text as well as the quality of its specially commissioned color graphics...
article on Self Cleaning Materials