biomimetic artificial surfaces quantitatively reproduce the water repellency of a lotus leaf
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DOI: 10.1002/adma.200800651 NICATIO
Biomimetic Artificial Surfaces Quantitatively Reproduce theWater Repellency of a Lotus Leaf**
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By Vassilia Zorba, Emmanuel Stratakis,* Marios Barberoglou, Emmanuel Spanakis,
Panagiotis Tzanetakis, Spiros H. Anastasiadis,* and Costas Fotakis
The water repellency and self-cleaning properties of many
plant surfaces have been qualitatively and sometimes quanti-
tatively attributed to not only the chemical constituency of
the cuticle covering their surface, which is composed by
soluble lipids embedded in a polyester matrix, but, even more
importantly, to the specially textured topography of the
surface.[1–3] It is understood that the microstructrured rough
surface enhances the effect of surface chemistry into super-
hydrophobicity, reduced particle adhesion and water repel-
lency. Actually, in the cases of the most famous water repellent
plant leaves like Nelumbo nucifera (the sacred Lotus) or
Colocasia esculenta, a dual scale roughness has been observed
on their surfaces created by papillose epidermal cells and an
additional layer of epicuticular waxes. The roughness of the
papillae leads to a reduced contact area between the surface
and a liquid drop (or a particle) with droplets residing only
on the tips of the epicuticular wax crystals on the top of the
[*] Dr. E. Stratakis, Prof. S. H. Anastasiadis, Dr. V. Zorba,M. Barberoglou, Dr. E. Spanakis, Prof. P. Tzanetakis,Prof. C. FotakisInstitute of Electronic Structure and LaserFoundation for Research & Technology – Hellas711 10 Heraklion (Greece)E-mail: [email protected]; [email protected]
Dr. V. Zorba, M. Barberoglou, Prof. P. Tzanetakis, Prof. C. FotakisDepartment of PhysicsUniversity of Crete714 09 Heraklion (Greece)
Dr. E. Stratakis, Dr. E. SpanakisDepartment of Materials Science and TechnologyUniversity of CreteHeraklion 710 03, Greece
Dr. E. Stratakis, Dr. E. SpanakisTechnological Educational Institute of Crete711 04 Heraklion (Greece)
Prof. S. H. AnastasiadisDepartment of Chemical EngineeringAristotle University of ThessalonikiThessaloniki 541 24 (Greece)
[**] This work was partially supported by the Integrated European LaserLaboratories LASERLAB-EUROPE (Contract No. RII3-CT-2003-506350),the Greek Ministry of Education (Herakleitos Programme), the GreekGeneral Secretariat of Research and Technology (PENED Programme),and NATO’s Scientific Affairs Division (Science for Peace Programme).The authors acknowledge Prof. Sophia Rhizopoulou of the University ofAthens, Department of Biology for kindly providing the Nelumbo nucifera(the sacred Lotus) leaves and Ms. Aleka Manousaki and Ms. AlexandraSiakouli for their continuous support with scanning electron microscopy.Supporting Information is available online fromWiley InterScience or fromthe authors.
Adv. Mater. 2008, 20, 4049–4054 � 2008 WILEY-VCH Verlag G
papillose epidermal cells. As a result, contaminating particles
can be picked up by the liquid and carried away as the droplet
rolls off the leaf; this was coined the ‘‘Lotus Effect’’ by
Barthlott and Neinhuis,[1] who then organized a consortium
trying to develop self-cleaning products.[4] Similar behavior has
been observed on other biological surfaces like the wings of
Cicada orni[5] or Rhinotermitidae[6] insects. A water repellent
surface exhibits certain remarkable wetting characteristics
originating from very high contact angles (in excess of 1508) and
very small values of contact angle hysteresis (less than 58):[7]
droplets roll down these surfaces at a speed faster than that of a
solid sphere rolling under gravity,[8] they can fully bounce after
impacting the surface[9,10] whereas the time of contact of an
impacting droplet with the surface is independent of its velocity.[10]
Water repellency was discovered very early when Boys
noticed that water deposited on a lycopodium layer rolled itself
up into perfect little balls.[11] However, it has attracted the
interest of the scientific community over the last ten years
following the observations of the microtextures of plant
surfaces and the first development of super-water-repellent
surfaces possessing a fractal microstructure;[12] the latter were
created spontaneously when alkylketene dimmer was solidified
from the melt. Water repellency as well as the development of
super-hydrophobic surfaces are currently the focus of con-
siderable research because of a range of potential applications,
such as the development of self-cleaning surfaces, micro-
fluidics, lab-on-chip devices, low friction coatings, water proof
and anti-rain textiles, etc.[13–15] The actual strategy consists in
mimicking superhydrophobic biosurfaces by designing rough
substrates out of hydrophobic materials.[16,17] This has been
implemented in a variety of bottom-up or bottom-down
approaches[18] like deposition of functionalized particles[19–22]
or micelles[23] on surfaces, solvent treatment of polymer
surfaces,[24] growth of aligned carbon nanotube[25–27] or ZnO
nanorod films,[28] deep silicon dry etching[29] or anisotropic
plasma etching (‘‘black silicon method’’)[30] as well as X-ray
lithography.[31]
Highly regular rough surfaces have been prepared on the
submicrometer scale where the importance of the length scale
and of the shapes of the protrusions has been recognized.[32]
Moreover, it has been suggested that more complex hier-
archically rough or fractal surfaces could render any surface
non-wettable.[33] Attempts to develop surfaces with dual-scale
roughness were successful in creating surfaces with high
contact angles (higher than �1608) and small contact angle
hysteresis of only 2.5–58.[15,26,27,31,34–37] Note that it has been
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Figure 1. a) Picture of a water droplet on the artificial structured silicon surface (dark area).b) Static contact angle measurement of a water droplet of 0.78mm radius on the artificial surface;the contact angle is 1548� 18. c) SEM image of the artificial surface comprising protrusions withconical or pyramidal asperities with average sizes of�10mmwith surface density 1.0� 106 cm�2
(scale bar 5mm). d) High magnification SEM image of a single protrusion depicting nanos-tructures of sizes up to few hundred nanometers on the slopes of the protrusions (scale bar1mm). The surface was structured in the presence of 500 Torr SF6 at a laser fluence of 2.47 J cm
�2
with an average of 500 pulses.
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emphatically pointed out that very low
values of the contact angle hysteresis are
especially important with respect to water
repellency whereas simply measuring high
values of the contact angles is not sufficient to
distinguish different materials and to decide
which one is the ‘‘best’’ for a given applica-
tion.[14]
In this work, we prepare artificial surfaces,
which can quantitatively mimic both the
structure and the water repellent character-
istics of the natural Lotus leaf. These surfaces
possess hierarchical micro- and nano-
structures and are prepared with a simple
one-step production process utilizing ultra-
fast (femtosecond) laser irradiation of a
silicon surface under a reactive gas atmo-
sphere.[38,39] This leads to a surface morphol-
ogy that mimics that of the sacred Lotus leaf.
Silanization of the dual-scale roughened
surface leads to contact angle values of
1548� 18 and to very small contact angle
hysteresis of 58� 28 both very similar to the
values of the Lotus leaf. The water repellency
of the surfaces and its relation to that of the
Lotus leaf was quantified by investigating the
restitution coefficient of water droplets of
various sizes bouncing off the surfaces as a
function of their impact velocity. It is found
that these structured surfaces constitute one
of the most water repellent artificial surfaces
ever reported, which are as efficient as the
sacred Lotus leaf. To our knowledge, this is
the first time such a direct comparison of
performance is made and it clearly demon-
strates the possibility of biomimicking the
real surface and, thus, identifying the subtle surface features,
which are responsible for the high water repellency observed.
Images of a water droplet lying on the artificial surface are
shown in Figure 1a and b, which can be directly compared to
those of a droplet lying on the surface of a natural leaf of
Nelumbo nucifera (Lotus leaf) shown in Figure 2a and b. The
static contact angle of water on the artificial surface is
measured as 1548� 18 with a contact angle hysteresis of 58� 28whereas those on the Lotus leaf as 1538� 18 and 48� 28,respectively. Figure 2c and d show scanning electron micro-
scopy (SEM) images of the surface of the Lotus leaf, which
comprises randomly distributed almost-hemispherically-topped
papillae with sizes 5–10mm (height to basal radius aspect ratio�1)
decorated with branch like protrusions with sizes of about 150 nm
(Fig. 2d); these observations are in agreement with earlier
reports.[3,16,26] Figure 1c and d show the SEM micrographs of
the most water repellent artificial surface, which exhibits the
highest contact angle and the lowest hysteresis among the different
surfaces obtained by varying the irradiation parameters. Its
morphology looks very similar to that of the Lotus leaf (Fig. 2c
www.advmat.de � 2008 WILEY-VCH Verlag GmbH &
and d) consisting of micro-scale conical features decorated with
nano-scale protrusions. The protrusions in this case have conical or
pyramidal asperities with average sizes of �10mm and aspect ratio
of�4. Nanostructures of sizes up to a few hundred nanometers are
clearly seen on the slopes of the protrusions (Fig. 1d). Thus, the
femtosecond laser irradiation under reactive SF6 atmosphere
was indeed able to produce a surface that mimics the structural
features of the Lotus leaves as well as their water contact angle
properties.
Figure 3 presents the time evolution of the shape of water
droplets impacting on the artificial structured silanized surface,
such as that of Figure 1, in comparison to a Lotus leaf surface
and to a silane coated flat Si surface. In particular, Figure 3a
and b show a selected time sequence of snapshots of 10mL
water droplets free-falling on the structured surface and on a
Lotus leaf, respectively. These drops impact the surface with a
velocity that corresponds to a dimensionless Weber number of
We¼ 3.5. The behavior of the falling droplet is quite similar on
the two surfaces. In particular, it is observed that the drop
shape changes significantly during impact as its kinetic energy
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Figure 2. a) Picture of water droplets on a Nelumbo nucifera (Lotus) leaf. b) Static contact anglemeasurement of a water droplet of 0.78mm radius on the Lotus leaf surface; the contact angle is1538� 18. c) SEM image of the leaf surface comprising almost-hemispherically-topped papillaewith sizes 5–10mm with surface density of 4.2� 105 cm�2 (scale bar 10mm). d) Highmagnification SEM image of a single papillose depicting branch like protrusions with sizes ofabout 150 nm (scale bar 1mm).
transforms into stored energy due to surface deformation. In
these cases, the deformation is strong, because the Weber
number, signifying the ratio of the arriving kinetic energy to the
intrinsic surface energy, is higher than unity. Despite this
deformation, both surfaces are so water-repellent that the drop
bounces back numerous times (see Supporting Information,
Movie 1S and Movie 2S). Figure 3c and d show similar series of
video frames of drops impacting the artificial structured silicon
and the Lotus leaf surface, respectively, for We¼ 0.7< 1, where
the drop is lightly deformed: the similarity in the two cases is
evident. On the other hand, no rebound is observed when the
drop impacts on the flat (unstructured) region of a silanized
silicon surface even for We¼ 3.5 (Fig. 3e). In this case the
surface is not water repellent and the drop has insufficient
momentum to leave the sample; as a result it remains stuck to
the surface.
The elasticity of the collisions observed on both the artificial
laser structured surface and that of the natural Lotus leaf is
remarkable, indicating a high degree of repellency. A direct
measure of this elasticity is the restitution coefficient, e¼V0/V,
defined as the ratio of the center of mass velocity just after
impact, V0, to that just before impact, V. This coefficient was
deduced from the recorded video images and is shown in
Figure 4 as a function of the impact velocity V for a series of
Adv. Mater. 2008, 20, 4049–4054 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA,
experiments, performed with drops of dif-
ferent volumes. The highest elasticity is
observed at intermediate velocities, from
�0.15 m s�1 to �0.25 m s�1, where the
restitution coefficient is found to exceed 0.90.
Its value matches that of the Lotus leaf and, to
our knowledge, is among the highest ever
reported.[9] Elasticity arises from the efficient
interchange between kinetic and surface
energy during drop deformation.[8,40] Accord-
ing to Richard and Quere,[9] even in the ideal
case of zero energy loss during collision, there
is a limit in elasticity, e< 1, due to the transfer
of a part of kinetic energy into drop vibra-
tions.[8] Indeed, in all cases it is observed that
the droplet vibrated after leaving the surface.
Thus, part of its initial kinetic energy is
transferred into vibrational energy after the
impact, and, subsequently, damping of the
bouncing motion occurs due to viscous
dissipation.
Although full rebounds occur at moderate
impact velocities, the situation is different
at small and large impact velocities V. For
small velocities, e decreases abruptly with
decreasing V and reaches zero at some
velocity that depends on the droplet
volume. This is the threshold that quantifies
the water repellency of the surface;[40] the
smaller this velocity, the more water
repellent the surface is. The threshold
velocity for the artificial surface is compa-
rable to that of the Lotus leaf (Fig. 4). The bouncing to
non-bouncing transition arises from the presence of surface
defects that become the main source of kinetic energy
dissipation. The contact line pins on such defects resulting in
a difference between the advancing and receding contact
angles, ua and ur, i.e., in hysteresis. The bigger the droplet the
longer this line is, resulting in higher hysteresis
and, therefore, in an increase of the anticipated threshold
velocity. This is exactly what is observed in Figure 4 for
both surfaces. The pinning force per unit length is[13,32,41]
F ¼ gLVðcos ur � cos uaÞ ¼ gLVDðcos uÞ, with gLV the liquid
surface tension; the energy dissipated will, thus, scale as
gLVR2Dðcos uÞ. The drop will bounce provided that its kinetic
energy, which scales as rR3V2, can overcome this dissipation.
An estimate of the threshold velocity for water repellency
can be obtained by equating the two energies. The relative
contact angles were measured as ua ¼ 1578 and ur ¼ 1528 for
the artificial structured silanized surface. For a drop with
radius R � 0.84 mm, the estimated threshold velocity is
calculated as �0.06 m s�1, a value close to the one observed
experimentally.
Finally, in the high velocity regime e slowly decreases with V
because of the large drop deformation followed by increased
internal vibration after impact. Studies on other structured
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Figure 3. Selected snapshots of the impact and rebound of millimetric water droplets: a) on an artificial silane-coated structured silicon surface impactingwith a velocity that corresponds to a Weber number, We, of 3.5; b) on a Lotus leaf surface impacting with We¼ 3.5; c) on an artificial silane-coatedstructured silicon surface impacting withWe¼ 0.7; d) on a Lotus leaf surface impacting withWe¼ 0.7; e) on an unstructured silane coated silicon surfaceimpacting with We¼ 3.5. In all cases the drop radius is R¼ 1.35mm corresponding to a drop volume of 10ml. In (a), (b), and (e) the drop is releasedfrom a height of 10mm so that its impact velocity is �0.44m s�1. In (c) and (d) the drop is released from a height of 1.9mm so that its impact velocity is�0.19m s�1. The dimensionless Weber number We is defined as We ¼ rV2R=gLV , where r is the liquid density, R the liquid drop radius, V its impactvelocity and gLV the liquid surface tension. The artificial surfaces in (a) and (c) were structured in the presence of 500 Torr SF6 at a laser fluence of 2.47 Jcm�2 with an average of 500 pulses.
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surfaces have shown that there exists an upper velocity above
which significant impalement of the drop occurs resulting in
part of it getting captured to completely wet the surface.[41,42]
The value of the highest velocity at which the surface remains
dry has also been used as a measure of the surface resistance
against wetting. In this context, the behavior of water droplets
with impact velocities up to 5 m s�1, a typical value for the
terminal velocity of millimetric raindrops,[41] was examined. In
this high velocity regime, the drop brakes apart into numerous
smaller droplets (see Supporting Information, Movie 3S and
Movie 4S). This behavior was never observed in the flat region
of the silanized silicon sample, indicating that structuring
favors the creation of tiny droplets in an effort to resist
penetration by the falling drop. It is noted that the surface was
thoroughly examined after each experiment with a high
resolution CCD optical system for signs of water impalement.
It was found that both the artificial structured surface and the
Lotus leaf were impervious to water penetration over the
entire range of attainable impact velocities (�0.1–5 m s�1).
Moreover, it should be emphasized that the low-adhesion and
www.advmat.de � 2008 WILEY-VCH Verlag GmbH &
high repellency of the artificial surfaces are maintained even
after rinsing and complete immersion in water for long periods
of time. Long-term endurance against wetting is a feature that
is always desirable in relevant applications.
In summary, a silicon-based water-repellent surface has
been constructed, partially mimicking nature. The surface
possesses a hierarchical morphology with two length-scale
roughnesses combined with a proper hydrophobic chemistry.
The water repelling characteristics of the surfaces were
quantified by investigating the bouncing of free-falling
water droplets impacting onto them as a function of impact
velocity. This repellency of the artificial surface can be very
favorably compared to that of the Lotus leaf in terms of
the threshold velocity sufficient to avoid sticking of the
droplets, the collision energy loss and the remnant wetting
of the surfaces at high velocities. The great similarities
observed in the water repellency between the artificial and
Lotus leaf surfaces are indicative of the critical role of
the two-scale hierarchical morphology, which combines
microscale and nanoscale features. These results provide
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0.50 0.75 1.00 1.25 1.50 0.50 0.75 1.00 1.25 1.50 1.75 2.00
0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
0.0
0.2
0.4
0.6
0.8
1.0
V (m s-1)
ε
We1/2
We1/2
Lotus leaf
Rdrop
=0.84 mm
0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
V (m s-1)
Lotus leaf
Rdrop
=1.35 mm
0.0
0.2
0.4
0.6
0.8
1.0a)
b)
c)
d)
ε
artificial surface
Rdrop
=0.84 mm
artificial surface
Rdrop
=1.35 mm
Figure 4. Restitution coefficient e¼V0/V, where V 0 is the center of mass velocity right afterimpact and V that right before impact, as a function of the impact velocity V for an artificialsilane-coated structured silicon surface (a,c) and a Lotus leaf surface (b,d) for two different sizesof falling water droplets with radii R of 0.84mm (a,b) and 1.35mm (c,d). The similarly highvalues of the restitution coefficients at intermediate velocities (exceeding 0.90) as well as thesimilarities in the threshold velocities necessary to avoid sticking of the drops between the twosurfaces are evident; the dashed lines signify the threshold velocities of 0.11m s�1 and 0.17ms�1 for the artificial surfaces for the two sizes of water droplets, respectively. The structuredsurfaces were structured in the presence of 500 Torr SF6 at a laser fluence of 2.47 J cm
�2 with anaverage of 500 pulses.
insight into the design of stable water repellent surfaces in
other materials, thus creating opportunities for various exciting
applications.
Experimental
The natural sacred Lotus (Nelumbo Nucifera) leaves were providedby the National Botanic Garden in Athens, Greece. Prior to themeasurements each leaf was cleaned by placing it under running water,which removed any contaminants due to its self-cleaning ability. Then asmall part was cut from the leaf and was attached to each measuringsetup using double-sided adhesive tape.
Microstructuring of the flat silicon (Si) substrate surfaces byultrafast lasers [38,43] under reactive gas (SF6) atmosphere was chosenas the method of surface micro- and nanostructuring because itproduces surface morphologies exhibiting two length scales [43]through a simple one-step process, without the need of a clean roomfacility and high-vacuum equipment requirements, and it can beapplied to a wide variety of materials [44–46]. Silicon substrates havebeen used in view of potential applications that can be readilycompatible with the existing semiconductor technology. Single crystaln-type Si(100) wafers with a resistivity of 2–8 Ohm cm were placedinside a vacuum chamber that was evacuated down to a base pressureof 10�2 mbar at the beginning of each run. SF6 gas was, then, introducedand maintained at a certain pressure (in the range of 50 to 1250 Torr) bymeans of a precision micro-valve. The irradiating source was aregenerative amplified Ti:Sapphire laser (l¼ 800 nm) delivering 180femtosecond pulses at a repetition rate of 1 kHz. The laser pulse fluencewas varied from 0.37 to 2.47 J cm�2 [39,47]; the laser fluence influences
Adv. Mater. 2008, 20, 4049–4054 � 2008 WILEY-VCH Verlag GmbH & Co. KGaA
both the surface density of the spikes on the texturedsurface [39] and the resulting values of the watercontact sliding angle [39,47]. Samples were mounted ona high-precision X-Y translation stage normal to theincident laser beam. A mechanical shutter wassynchronized to the stage motion to provide a uniformexposure of a 4� 4 mm2 area to a constant averagenumber of laser pulses (from 300 to 3000).
Following the irradiation process, the sampleswere first cleaned in ultrasonic baths of trichloro-ethylene, acetone and methanol followed by a 10%HF aqueous treatment in order to remove the oxidegrown on the surface. Then, the patterned surfaceswere silanized using dichlorodimethylsilane,(CH3)2SiCl2 (DMDCS), which can assemble intohigh quality conformal coatings on flat Si surfaces[48] resulting in a material with a low surface energythat can maintain its hydrophobic properties forlong periods of time and for a wide temperaturerange [49]. The samples were placed in a flaskcontaining 0.5 ml of DMDCS reagent wherehydrophobic DMDCS monolayers were depositedon their surface via adsorption reactions. Thesilanization process employed is similar to thatreported in the literature [50].
Static contact angle measurements were per-formed by an automated tensionmeter, using thesessile drop method [51]. A 2ml distilled, deionizedMillipore water droplet was gently positioned onthe surface using a microsyringe and images werecaptured to measure the angle formed at theliquid-solid interface. The mean value was calcu-lated from at least five individual measurements.Successive measurements were reproducible within�18. The error bars presented in the text areassociated with multiple measurements on differentspots on the same surface as well as on different
surfaces artificially structured under the same conditions. Contactangle hysteresis was determined by measuring the advancing andreceding angles from the drop-snapshot just before slippage occurred.On a drop about to slide, the wetting angle on the lower edge is theadvancing angle, whereas that on the upper edge is the receding angle.The dynamic behavior of water droplets free falling on flat or patternedsurfaces as well as on those of the Lotus leaf was examined using ahigh-speed camera at a frame rate of up to 1000 Hz. The velocitiesbefore and after each shock were calculated either from the distancetraveled between successive snapshots (at high impact speeds) or fromthe corresponding maximum heights attained (at low impact speeds).
The contact angle measured on the DMDCS-coated flat siliconsubstrate was 1048, close to that reported for total monolayer coverage[50]. Spectroscopic ellipsometry measurements on the flat region of thesamples show that the average thickness of the silane coating is about2.5 nm in agreement to other studies on similar coatings [48].Intermittent contact-atomic force microscopy was utilized to char-acterize the quality of the silane coatings on the flat part of the Sisamples, outside of the irradiating region. It showed a relativelyhomogeneous deposition of molecules over the entire area. Thecorresponding rms roughness, calculated by the image processingalgorithm, is �1 nm indicating dense, void-free coverage. All thesamples were morphologically characterized by scanning electronmicroscopy. Fresh leaf samples were affixed to aluminum stubs usingdouble-sided conductive tape and air dried. Then they were sputter-coated with a less than 20 nm thick Au/Pd coating.
Received: March 6, 2008Revised: May 22, 2008
Published online: September 22, 2008
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