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1141
RESONANCE December 2008
GENERAL ARTICLE
The water-repellent surface of lotus (Nelumbo nucifera) leaf
and flower is due to nanosized wax papillae on the upper side
of each epidermal cell. As a result, raindrops make a high
contact angle with the papillae and roll off carrying dust and
dirt particles, leaving the surface clean. This self-cleaning
property of highly hydrophobic surfaces, termed as the lotus
effect, has opened the possibilities of fabricating
superhydrophobic surfaces for a variety of products.
Lotus, botanically named Nelumbo nucifera, is regarded as a
sacred plant in Hindu mythology. Lotus is also India’s national
flower (Figure 1) and is regarded as a symbol of purity. Its leaf
and flower are water-repellant. A falling raindrop turns into a
water bead and rolls off, taking along with it the dust and the dirt
particle. Hence, despite growing in muddy waters, the lotus leaf
surface stays relatively clean (Figure 2).
Although the water repellency of lotus had long been recognized,
its scientific basis was understood only in 1997 [1] when two
botanists, Wilhelm Barthlott (Figure 3) and Christoph Neinhuis,
at the University of Bonn in Germany examined leaf surfaces of
lotus and several other plants using a scanning electron micro-
scope which resolves structures as small as 1–20 nm (one nm =
billionth or 10
–9
of a meter). They established that the self-
cleaning property is due to the presence of convex papillae on the
surface of leaves, coated with wax crystals of nanoscopic dimen-
sion: ~10 to ~100 nm (Figure 4). The papilla greatly reduces the
contact area of water droplets with it. The water droplet, as for
example due to rain or fog or dew, is dislodged, often coalescing
into a bigger drop at the center of leaf surface that falls off with
swaying of the leaf.
Wax is comprised of a mixture of long-chain hydrocarbons:
B Karthick is a research
scholar in the Centre for
Ecological Sciences, Indian
Institute of Science,
Bangalore. His research
interests are in taxonomy
and ecology of diatoms and
ecology of wetlands.
Ramesh Maheshwari is a
former professor in
biochemistry at the Indian
Institute of Science.
Keywords
Lotus, self-cleaning, hydropho-
bicity, nanotechnology.
Lotus-Inspired Nanotechnology Applications
B Karthick and Ramesh Maheshwari
Figure 1. Lotus,the national
flower of India featured on
a postage stamp issued on
September 1, 1977.
1142 RESONANCE December 2008
GENERAL ARTICLE
primary and secondary alcohols, aldehydes and triterpenes. Since
the phenomenon of water repellency and self-cleaning is best
studied in the lotus, it has come to be known as the ‘Lotus effect’.
Barthlott andhis associates examined over 13,000 plants. Barthlott
is quoted as advising, “Do trust your own eyes and not the
textbooks, and if your observation is repeatedly confirmed, pub-
lish it. But take a deep breath – expect rejections of your manu-
script” [2].
A great diversity of structures has been observed on surfaces of
the above-ground parts of the plants. Although the presently used
system of plant classification relies on the sexual (floral) charac-
teristics devised by the great Swedish botanist, Carl von Linnaeus
(1707–1778), it is thought that once a uniform terminology of
describing these structures is evolved, it is thought that micro-
morphologies of plant surfaces can be used as an aid in plant
taxonomy. Here we note that the world is celebrating the
tricentenary of the birth of Linnaeus who proposed the
binomial system of nomenclature and classification of
life forms.
Incidence of Water-Repellent Plant Surfaces
Interestingly, a majority of plants in the wetlands have
water-repellent leaves. If this were not so, wetting
would interfere with gas exchange through the stomata
located on the upper (exposed) side of the floating
Figure 2. Floating lotus leaves and a blooming flower in a pond
(Left); Water drops forming beads on the leaf surface (Right). Photo
courtesy: K V Gururaja and M Boominathan.
Figure 3. ProfessorWilhelm
Barthlott, Director of Bo-
tanical Garden of the Uni-
versity of Bonn, discovered
the lotus effect and con-
ceived the fabrication of
surfaces with nanoscopic
bumps to make them self-
cleaning and patented Lo-
tus Effect in 1980.
Photo courtesy: Kerstin Koch.
Figure 4. Scanning elec-
tron microscope image of
leaf surface of Nelumbo
nucifera.
Photo courtesy: Prof. Wilhelm
Barthlott.
1143
RESONANCE December 2008
GENERAL ARTICLE
leaves. Another advantage of a water-repellant surface could be
in reducing the risk of infection. Since spores of several patho-
genic fungi require free water and air for germination, the evolu-
tion of a water-repellent surface by superimposing wax mol-
ecules on cuticle (the outermost covering) probably evolved to
reduce the risk of infection by pathogenic bacteria and fungi.
Water-repellency is not restricted to plants alone. It is manifested
also by insects with large wings (such as butterflies and dragon-
flies) which cannot clean their flying structures by legs. In this
case lotus effect works not only for the removal of particles, but
also for maintenance of flight capability of insects, which other-
wise is lost due to an unequal load on the wings.
Water-Repellant Nanostructures on Leaf Replica
Lotus effect can be reproduced on artificially made
superhydrophobic films. In one of the procedures, a rough surface
was etched into polydimethylsiloxane (PDMS). Using the lotus
leaf, a negative PDMS template was made and the negative
template then used to make a positive PDMS reproduced as a
replica sheet of the original lotus leaf. The positive PDMS
template had the same surface structures and extreme water
repellency (superhydrophobic) as the lotus leaf. A plant epider-
mal cell extrudes wax molecules which self-assemble and crys-
tallize as nanosized pillars. Thus, the hydrophobic cuticle has a
double structure: the cuticle (a polyester called cutin), and crys-
talline wax on the cuticle. On a water-repellant leaf cuticle, the
dirt particles, like a ‘fakir lying on a carpet of nails’ make only a
minuscule contact with the top of the bumps present on the plant
surface. A rolling water drop on contaminated surfaces easily
picks up the dirt particle.
Physical Basis
Four classes of surface wettability are recognized (Figure 5):
namely, superhydrophobic, hydrophobic, hydrophilic and
superhydrophilic. Lotus leaf and petal surface exhibits
superhydrophobicity, i.e., the contact angle (Figure 6) is high –
Superhydrophobicity,
a prerequisite for
self-cleaning, is
characterized by very
high contact angles
of nanostructured
surfaces.
The discovery of
Lotus effect is a ‘by
product’ of research
on diversity of
structure,
morphology and
wetting of plant
surfaces.
1144 RESONANCE December 2008
GENERAL ARTICLE
Figure 5. The four classes
of surface wettability types
of leaf surface based on
their interaction with aque-
ous droplets.
Redrawn from Koch et al, 2008.
Figure 6. Physics of self-
cleaning property of lotus.
The large contact angle re-
sults from nanoscopic
bumps that trapairbetween
the water and the surface
minimizing the contactwith
the surface. A drop of water
rolls downwards on leaf
surface, picking up dirt
(Based on Forbes 2008).
exceeding 150 degrees. The drop, instead of sliding and spread-
ing the dirt particles which have fallen on the cuticle, rolls off
taking with it dirt (Figure 6). Lotus effect is inspiring material
scientists into sculpturing superhydrophobic surfaces that will
mimic this effect, minimizing labour and expense involved in
frequent painting of facade, particularly in the high-rise build-
ings.
CA=90
o
–10
o
Left: On smooth surface the dirt particles are redistributed
Right: On rough surface water droplet rolls off taking dirt
1145
RESONANCE December 2008
GENERAL ARTICLE
Fabrication of Hydrophobic Surfaces
Optimizing water repellency requires consideration of the geom-
etry of pillars (size, height) and spacing. Rough, nanoscopic
finish can be exploited for several applications. Among the
numerous applications of lotus effect are, non-wettable rain wear
and sails for boats, paints for kitchen roofs and walls that make
them soot-free, windows in high-rise buildings and glass for
greenhouses avoiding their expensive and cumbersome cleaning,
water repellant fibers for garments etc. Other applications can be
in sanitary products in bathrooms and toilets and motor vehicle
windshields for reducing sticking of dirt matter and easier clean-
ing.
Acknowledgement
We are thankful to Kerstin Koch, K V Gururaja and M
Boominathan for allowing us to use their photographs.
Suggested Reading
[1] W Barthlott, C Neinhuis, Purity of the sacred lotus, or escape from
contamination in biological surfaces, Planta, Vol.202, pp.1–8, 1997.
[2] P Forbes, Self-cleaning materials, Sci. Amer., August 2008.
[3] Y T Cheng, D E Rodak, C A Wong and C A Hayden, Effects of micro-
and nano-structures on the self-cleaning behaviour of lotus leaves,
Nanotechnology, Vol.17, pp.1359–1362, 2006.
[4] K Koch, B Bhushan and W Barthlott, Diversity of structure, morphol-
ogy and wetting of plant surfaces, Soft Matter, Vol.4, pp.1943–1963,
2008.
[5] http://www.bionik.tu-berlin.de/institut/shanlec2.ppt#3
Lotus effect is
caused by
combination of
waxiness of leaf
surface and the
nanoscopic bumps
that cover it.
Address for Correspondence
B Karthick
Centre for Ecological Sciences
Indian Institute of Science
Bangalore 560 012, India.
Email: *diatomist@gmail.com
Ramesh Maheshwari
Email: ramesh.maheshwari01
@gmail.com
