Lotus effect

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Water on the surface of a lotus leaf. LotusEffect1.jpg
Water on the surface of a lotus leaf.
Water droplets on taro leaf with lotus effect (upper), and taro leaf surface magnified (0-1 is one millimetre span) showing a number of small protrusions (lower). Lotus effect on leaf of taro and leaf magnify.jpg
Water droplets on taro leaf with lotus effect (upper), and taro leaf surface magnified (0–1 is one millimetre span) showing a number of small protrusions (lower).
Computer graphic of a lotus leaf surface. Lotus3.jpg
Computer graphic of a lotus leaf surface.
A water drop on a lotus surface showing contact angles of approximately 147deg. DropConnectionAngel.jpg
A water drop on a lotus surface showing contact angles of approximately 147°.

The lotus effect refers to self-cleaning properties that are a result of ultrahydrophobicity as exhibited by the leaves of Nelumbo , the lotus flower. [1] Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to that surface. Ultrahydrophobicity and self-cleaning properties are also found in other plants, such as Tropaeolum (nasturtium), Opuntia (prickly pear), Alchemilla , cane, and also on the wings of certain insects. [2]

Contents

The phenomenon of ultrahydrophobicity was first studied by Dettre and Johnson in 1964 [3] 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 ultrahydrophobic micro-nanostructured surfaces was studied by Wilhelm Barthlott and Ehler in 1977, [4] who described such self-cleaning and ultrahydrophobic properties for the first time as the "lotus effect"; perfluoroalkyl and perfluoropolyether ultrahydrophobic materials were developed by Brown in 1986 for handling chemical and biological fluids. [5] Other biotechnical applications have emerged since the 1990s. [6] [7] [8] [9] [10] [11]

Functional principle

The high surface tension of water causes droplets to assume a nearly spherical shape, since a sphere has minimal surface area, and this shape therefore minimizes the solid-liquid surface energy. On contact of liquid with a surface, adhesion forces result in wetting of the surface. Either complete or incomplete wetting may occur depending on the structure of the surface and the fluid tension of the droplet. [12] The cause of self-cleaning properties is the hydrophobic water-repellent double structure of the surface. [13] This enables the contact area and the adhesion force between surface and droplet to be significantly reduced, resulting in a self-cleaning process. [14] [15] [16] 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 10 μm to 20 μm in height and 10 μm 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. This system regenerates. This biochemical property is responsible for the functioning of the water repellency of the surface.

The hydrophobicity of a surface can be measured by its contact angle. 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 ultrahydrophobic, meaning that only 2–3% of the surface of a droplet (of typical size) is in contact. Plants with a double structured surface like the lotus can reach a contact angle of 170°, whereby the droplet's 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. This cleaning effect has been demonstrated on common materials such as stainless steel when a superhydrophobic surface is produced. [17] As this self-cleaning effect is based on the high surface tension of water it does not work with organic solvents. Therefore, the hydrophobicity of a surface is no protection against graffiti.

This effect is of a great importance for plants as a protection against pathogens like fungi or algae 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 reduced photosynthesis.

Technical application

When it was discovered that the self-cleaning qualities of ultrahydrophobic surfaces come from physical-chemical properties at the microscopic to nanoscopic scale rather than from the specific chemical properties of the leaf surface, [18] [19] [20] the possibility arose of using this effect in manmade surfaces, by mimicking nature in a general way rather than a specific one.

Some nanotechnologists have developed treatments, coatings, paints, roof tiles, fabrics and other surfaces that can stay dry and clean themselves by replicating in a technical manner the self-cleaning properties of plants, such as the lotus plant. This can usually be achieved using special fluorochemical or silicone treatments on structured surfaces or with compositions containing micro-scale particulates.

In addition to chemical surface treatments, which can be removed over time, metals have been sculpted with femtosecond pulse lasers to produce the lotus effect. [21] The materials are uniformly black at any angle, which combined with the self-cleaning properties might produce very low maintenance solar thermal energy collectors, while the high durability of the metals could be used for self-cleaning latrines to reduce disease transmission. [22]

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).[ citation needed ] The Swiss companies HeiQ and Schoeller Textil have developed stain-resistant textiles under the brand names "HeiQ Eco Dry" and "nanosphere" respectively. In October 2005, tests of the Hohenstein Research Institute showed that clothes treated with NanoSphere technology allowed tomato sauce, coffee and red wine to be easily washed away even after a few washes. Another possible application is thus with self-cleaning awnings, tarpaulins and sails, which otherwise quickly become dirty and difficult to clean.

Superhydrophobic coatings applied to microwave antennas can significantly reduce rain fade and the buildup of ice and snow. "Easy to clean" products in ads are often mistaken in the name of the self-cleaning properties of hydrophobic or ultrahydrophobic surfaces. Patterned ultrahydrophobic surfaces also show promise for "lab-on-a-chip" microfluidic devices and can greatly improve surface-based bioanalysis. [23]

Superhydrophobic or hydrophobic properties have been used in dew harvesting, or the funneling of water to a basin for use in irrigation. The Groasis Waterboxx has a lid with a microscopic pyramidal structure based on the ultrahydrophobic properties that funnel condensation and rainwater into a basin for release to a growing plant's roots. [24]

Research history

Although the self-cleaning phenomenon of the lotus was possibly known in Asia long before (reference to the lotus effect is found in the Bhagavad Gita [25] ), its mechanism was explained only in the early 1970s after the introduction of the scanning electron microscope. [4] [16] Studies were performed with leaves of Tropaeolum and lotus (Nelumbo). [6] Similar to lotus effect, a recent study has revealed honeycomb-like micro-structures on the taro leaf, which makes the leaf superhydrophobic. The measured contact angle on this leaf in this study is around 148 degrees. [26]

See also

Related Research Articles

<span class="mw-page-title-main">Hydrophobe</span> Molecule or surface that has no attraction to water

In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.

<span class="mw-page-title-main">Biomimetics</span> Imitation of biological systems for the solving of human problems

Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from Ancient Greek: βίος (bios), life, and μίμησις (mīmēsis), imitation, from μιμεῖσθαι (mīmeisthai), to imitate, from μῖμος (mimos), actor. A closely related field is bionics.

<span class="mw-page-title-main">Soap bubble</span> Thin film of soapy water enclosing air

A soap bubble is an extremely thin film of soap or detergent and water enclosing air that forms a hollow sphere with an iridescent surface. Soap bubbles usually last for only a few seconds before bursting, either on their own or on contact with another object. They are often used for children's enjoyment, but they are also used in artistic performances. Assembling many bubbles results in foam.

Electrowetting is the modification of the wetting properties of a surface with an applied electric field.

<i>Nelumbo</i> Genus of aquatic flowering plants known as "lotus."

Nelumbo is a genus of aquatic plants with large, showy flowers. Members are commonly called lotus, though the name is also applied to various other plants and plant groups, including the unrelated genus Lotus. Members outwardly resemble those in the family Nymphaeaceae, but Nelumbo is actually very distant from that family.

<span class="mw-page-title-main">Wetting</span> Ability of a liquid to maintain contact with a solid surface

Wetting is the ability of a liquid to displace gas to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with the first one. The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces. There are two types of wetting: non-reactive wetting and reactive wetting.

<span class="mw-page-title-main">Contact angle</span> Angle between a liquid–vapor interface and a solid surface

The contact angle is the angle between a liquid surface and a solid surface where they meet. More specifically, it is the angle between the surface tangent on the liquid–vapor interface and the tangent on the solid–liquid interface at their intersection. It quantifies the wettability of a solid surface by a liquid via the Young equation.

<span class="mw-page-title-main">Cassie's law</span>

Cassie's law, or the Cassie equation, describes the effective contact angle θc for a liquid on a chemically heterogeneous surface, i.e. the surface of a composite material consisting of different chemistries, that is, non-uniform throughout. Contact angles are important as they quantify a surface's wettability, the nature of solid-fluid intermolecular interactions. Cassie's law is reserved for when a liquid completely covers both smooth and rough heterogeneous surfaces.

<span class="mw-page-title-main">Ultrahydrophobicity</span> Material property of extreme resistance to wetting

In chemistry and materials science, ultrahydrophobic surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the superhydrophobic leaves of the lotus plant. A droplet striking these kinds of surfaces can fully rebound like an elastic ball. Interactions of bouncing drops can be further reduced using special superhydrophobic surfaces that promote symmetry breaking, pancake bouncing or waterbowl bouncing.

<span class="mw-page-title-main">Coffee ring effect</span> Capillary flow effect

In physics, a "coffee ring" is a pattern left by a puddle of particle-laden liquid after it evaporates. The phenomenon is named for the characteristic ring-like deposit along the perimeter of a spill of coffee. It is also commonly seen after spilling red wine. The mechanism behind the formation of these and similar rings is known as the coffee ring effect or in some instances, the coffee stain effect, or simply ring stain.

Hydrophobic soil is a soil whose particles repel water. The layer of hydrophobicity is commonly found at or a few centimeters below the surface, parallel to the soil profile. This layer can vary in thickness and abundance and is typically covered by a layer of ash or burned soil.

<span class="mw-page-title-main">Plant cuticle</span> Waterproof covering of aerial plant organs

A plant cuticle is a protecting film covering the outermost skin layer (epidermis) of leaves, young shoots and other aerial plant organs that have no periderm. The film consists of lipid and hydrocarbon polymers infused with wax, and is synthesized exclusively by the epidermal cells.

Self-cleaning glass is a specific type of glass with a surface that keeps itself free of dirt and grime.

<span class="mw-page-title-main">Water-repellent glass</span>

Water-repellent glass (WRG) is a transparent coating film fabricated onto glass, enabling the glass to exhibit hydrophobicity and durability. WRGs are often manufactured out of materials including derivatives from per- and polyfluoroalkyl substances (PFAS), tetraethylorthosilicate (TEOS), polydimethylsilicone (PDMS), and fluorocarbons. In order to prepare WRGs, sol-gel processes involving dual-layer enrichments of large size glasses are commonly implemented.

<span class="mw-page-title-main">Wilhelm Barthlott</span> German botanist

Wilhelm Barthlott is a German botanist and biomimetic materials scientist. His official botanical author citation is Barthlott.

<span class="mw-page-title-main">Superhydrophobic coating</span> Water-repellant coating

A superhydrophobic coating is a thin surface layer that repels water. It is made from superhydrophobic materials, and typically cause an almost imperceptibly thin layer of air to form on top of a surface. Droplets hitting this kind of coating can fully rebound. Generally speaking, superhydrophobic coatings are made from composite materials where one component provides the roughness and the other provides low surface energy.

Icephobicity is the ability of a solid surface to repel ice or prevent ice formation due to a certain topographical structure of the surface. The word “icephobic” was used for the first time at least in 1950; however, the progress in micropatterned surfaces resulted in growing interest towards icephobicity since the 2000s.

The Salvinia effect describes the permanent stabilization of an air layer upon a hierarchically structured surface submerged in water. Based on biological models, biomimetic Salvinia-surfaces are used as drag reducing coatings (up to 30% reduction were previously measured on the first prototypes. When applied to a ship hull, the coating would allow the boat to float on an air-layer, reducing energy consumption and emissions. Such surfaces require an extremely water repellent super-hydrophobic surface and an elastic hairy structure in the millimeter range to entrap air while submerged. The Salvinia effect was discovered by the biologist and botanist Wilhelm Barthlott and his colleagues and has been investigated on several plants and animals since 2002. Publications and patents were published between 2006 and 2016. The best biological models are the floating ferns with highly sophisticated hierarchically structured hairy surfaces, and the back swimmers with a complex double structure of hairs and microvilli. Three of the ten known Salvinia species show a paradoxical chemical heterogeneity: hydrophilic hair tips, in addition to the super-hydrophobic plant surface, further stabilizing the air layer.

Surface tension is one of the areas of interest in biomimetics research. Surface tension forces will only begin to dominate gravitational forces below length scales on the order of the fluid's capillary length, which for water is about 2 millimeters. Because of this scaling, biomimetic devices that utilize surface tension will generally be very small, however there are many ways in which such devices could be used.

Self-cleaning surfaces are a class of materials with the inherent ability to remove any debris or bacteria from their surfaces in a variety of ways. The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few. The majority of self-cleaning surfaces can be placed into three categories:

  1. superhydrophobic
  2. superhydrophilic
  3. photocatalytic.

References

  1. ↑ Lafuma, A.; Quere, D. (2003). "Superhydrophobic states". Nature Materials . 2 (7): 457–460. Bibcode:2003NatMa...2..457L. doi:10.1038/nmat924. PMID   12819775. S2CID   19652818.
  2. ↑ Barthlott, W. (2023): “The Discovery of the Lotus Effect as a Key Innovation for Biomimetic Technologies” -  in: Handbook of Self-Cleaning Surfaces and Materials: From Fundamentals to Applications, Chapter 15, pp. 359-369 - Wiley-VCH, https://doi.org/10.1002/9783527690688.ch15
  3. ↑ Rulon E. JohnsonJr.; Robert H. Dettre (1964). "Contact Angle Hysteresis. III. Study of an Idealized Heterogeneous Surface". J. Phys. Chem. 68 (7): 1744–1750. doi:10.1021/j100789a012.
  4. 1 2 Barthlott, Wilhelm; Ehler, N. (1977). "Raster-Elektronenmikroskopie der Epidermis-Oberflächen von Spermatophyten". Tropische und Subtropische Pflanzenwelt. 19: 110.
  5. ↑ Brown Laboratory vessel having hydrophobic coating and process for manufacturing same Archived 2017-01-22 at the Wayback Machine U.S. patent 5,853,894 , Issued December 29, 1998
  6. 1 2 Barthlott, Wilhelm; C. Neinhuis (1997). "The purity of sacred lotus or escape from contamination in biological surfaces". Planta . 202: 1–8. doi:10.1007/s004250050096. S2CID   37872229.
  7. ↑ Barthlott, W., Mail, M., Bhushan, B., & K. Koch. (2017). Plant Surfaces: Structures and Functions for Biomimetic Innovations. Nano-Micro Letters, 9(23), doi:10.1007/s40820-016-0125-1.
  8. ↑ Cheng, Y. T.; Rodak, D. E. (2005). "Is the lotus leaf superhydrophobic?". Appl. Phys. Lett. 86 (14): 144101. Bibcode:2005ApPhL..86n4101C. doi:10.1063/1.1895487.
  9. ↑ Narhe, R. D.; Beysens, D. A. (2006). "Water condensation on a super-hydrophobic spike surface". Europhys. Lett. 75 (1): 98–104. Bibcode:2006EL.....75...98N. doi:10.1209/epl/i2006-10069-9.
  10. ↑ Lai, S.C.S. "Mimicking nature: Physical basis and artificial synthesis of the Lotus effect" (PDF). Archived from the original (PDF) on 2007-09-30.
  11. ↑ Koch, K.; Bhushan, B.; Barthlott, W. (2008). "Diversity of structure, Morphology and Wetting of Plant Surfaces. Soft matter". Soft Matter. 4 (10): 1943. Bibcode:2008SMat....4.1943K. doi:10.1039/b804854a.
  12. ↑ von Baeyer; H. C. (2000). "The Lotus Effect". The Sciences . 40: 12–15. doi:10.1002/j.2326-1951.2000.tb03461.x.
  13. ↑ Neinhuis, C.; Barthlott, W. (1997). "Characterization and distribution of water-repellent, self-cleaning plant surfaces". Annals of Botany . 79 (6): 667–677. doi: 10.1006/anbo.1997.0400 .
  14. ↑ Barthlott, Wilhelm; Neinhuis, C. (2001). "The lotus-effect: nature's model for self cleaning surfaces". International Textile Bulletin . 1: 8–12.
  15. ↑ Forbes, P. (2005). The Gecko's Foot, Bio-inspiration – Engineering New Materials and devices from Nature. London: Fourth Estate. p. 272. ISBN   978-0-00-717990-9.
  16. 1 2 Forbes, P. (2008). "Self-Cleaning Materials". Scientific American . 299 (2): 67–75. Bibcode:2008SciAm.299b..88F. doi:10.1038/scientificamerican0808-88. PMID   18666684.
  17. ↑ Serles, Peter; Nikumb, Suwas; Bordatchev, Evgueni (2018-06-15). "Superhydrophobic and superhydrophilic functionalized surfaces by picosecond laser texturing". Journal of Laser Applications. 30 (3): 032505. Bibcode:2018JLasA..30c2505S. doi:10.2351/1.5040641. ISSN   1042-346X.
  18. ↑ Solga, A.; Cerman, Z.; Striffler, B. F.; Spaeth, M.; Barthlott, W. (2007). "The dream of staying clean: Lotus and biomimetic surfaces". Bioinspiration & Biomimetics . 2 (4): S126–S134. Bibcode:2007BiBi....2..126S. CiteSeerX   10.1.1.477.693 . doi:10.1088/1748-3182/2/4/S02. PMID   18037722.
  19. ↑ Mueller, T. (April 2008). "Biomimetics, Design by Nature". National Geographic Magazine : 68.
  20. ↑ Guo, Z.; Zhou, F.; Hao, J.; Liu, W. (2005). "Stable Biomimetic Super-Hydrophobic Engineering Materials". J. Am. Chem. Soc. 127 (45): 15670–15671. doi:10.1021/ja0547836. PMID   16277486.
  21. ↑ Vorobyev, A. Y.; Guo, Chunlei (2015). "Multifunctional surfaces produced by femtosecond laser pulses". Journal of Applied Physics. 117 (3): 033103. Bibcode:2015JAP...117c3103V. doi:10.1063/1.4905616.
  22. ↑ Borghino, Dario (21 January 2015). "Lasers help create water-repelling, light-absorbing, self-cleaning metals". gizmag.com.
  23. ↑ Ressine, A.; Marko-Varga, G.; Laurell, T. (2007). Porous silicon protein microarray technology and ultra-/superhydrophobic states for improved bioanalytical readout. Biotechnology Annual Review. Vol. 13. pp. 149–200. doi:10.1016/S1387-2656(07)13007-6. ISBN   978-0-444-53032-5. PMID   17875477.
  24. ↑ "The different forms of condensation - Technology".
  25. ↑ Bhagavad Gita 5.10 Archived 2012-09-10 at the Wayback Machine
  26. ↑ Kumar, Manish; Bhardwaj (2020). "Wetting characteristics of Colocasia esculenta (Taro) leaf and a bioinspired surface thereof". Scientific Reports. 10 (1): 935. Bibcode:2020NatSR..10..935K. doi:10.1038/s41598-020-57410-2. PMC   6976613 . PMID   31969578.
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