Production and characterization of stable superhydrophobic surfaces based on copper hydroxide nanoneedles mimicking the legs of water striders

Article abstract of DOI:10.1021/jp056969x

The present work reports a simple and economic route for production and characterization of stable superhydrophobic surfaces from thin copper layers coated on arbitrary solid substrates. The thin copper layer was anodized in a 2 M aqueous solution of potassium hydroxide to form a thin film of copper hydroxide nanoneedles; then the film was reacted with n-dodecanethiol to form a thermally stable Cu(SC₁₂H₂₅)₂ superhydrophobic coating. The contact angle of the modified nanoneedle surface was higher than 150°, and its tilt angle was smaller than 2°. Furthermore, the surface fabricated on copper foil kept its superhydrophobic property after heating at 160 °C in air for over 42 h. This technique has also been applied for fabrication of copper wire with superhydrophobic submicrofiber coating to mimic water strider legs. The maximal supporting force of the superhydrophobic copper column has also been investigated in comparison to real water striders.

Full text of DOI:10.1021/jp056969x

J. Phys. Chem. B 2006, 110, 11247-11252
11247
Production and Characterization of Stable Superhydrophobic Surfaces Based on Copper
Hydroxide Nanoneedles Mimicking the Legs of Water Striders
Xufeng Wu and Gaoquan Shi*
Department of Chemistry and Key Lab of Bio-organic Phosphorous, Tsinghua UniVersity,
Beijing 100084, P. R. China
ReceiVed: NoVember 30, 2005; In Final Form: March 5, 2006
The present work reports a simple and economic route for production and characterization of stable
superhydrophobic surfaces from thin copper layers coated on arbitrary solid substrates. The thin copper layer
was anodized in a 2 M aqueous solution of potassium hydroxide to form a thin film of copper hydroxide
25 2
nanoneedles; then the film was reacted with n-dodecanethiol to form a thermally stable Cu(SC12H )
superhydrophobic coating. The contact angle of the modified nanoneedle surface was higher than 150°, and
its tilt angle was smaller than 2°. Furthermore, the surface fabricated on copper foil kept its superhydrophobic
property after heating at 160 °C in air for over 42 h. This technique has also been applied for fabrication of
copper wire with superhydrophobic submicrofiber coating to mimic water strider legs. The maximal supporting
force of the superhydrophobic copper column has also been investigated in comparison to real water striders.
Introduction
water striders were focused on for their special ability to walk
2
3-25
quickly on the water surface.
It was reported that the legs
Superhydrophobic surfaces with water contact angles (CA)
larger than 150° and tilt angles (TA) less than 5° have attracted
extensive attention for their potential application in self-cleaning
surfaces, microfluidic devices, and many other industrial
of water striders were columns in shape and covered by large
numbers of submicrometer-sized hairs with fine nanogrooves.
The hierarchical structures of strider legs make them superhy-
drophobic and generate high surface tensions, which make the
water striders move quickly on the water surface.25 Superhy-
drophobic coatings composed of dendritic gold clusters have
recently been fabricated on a gold column by a layer-by-layer
1-3
processes. Various techniques for achieving superhydropho-
bic surfaces including roughening the surfaces of hydrophobic
materials (e.g., roughening hydrophobic polymer surfaces by
thermal or solvent treatment, template syntheses, electrospinning,
2
6
deposition method. However, to date, no method has been
developed for fabricating superhydrophobic submicrofiber coat-
ings on column substrates.
4
-7
plasma polymerization or etching, and solidification of melted
alkylketene dimmer, etc.) and modifying rough surfaces with
materials of low surface free energies (e.g., coating waterproof-
8
Recently, we developed a method for fabrication of copper
9
ing breathable fabrics or thiol layers on copper or gold fractal
2
7
hydroxide nanoneedles on copper foil. Here, we desire to
demonstrate a simple way to fabricate thermally stable, low-
cost, and large area superhydrophobic surfaces based on the
copper hydroxide nanoneedles for various substrates. In this
process a thin layer of copper was electroplated on a given
substrate, then the copper layer was transformed into a nano-
needle film of copper hydroxide by anodization in an aqueous
solution of potassium hydroxide, and finally the nanoneedle film
was reacted with n-dodecanethiol to form a superhydrophobic
surface. Replacing the copper foil with copper wire fabricates
a submicrofiber superhydrophobic structure similar to that of a
water strider’s leg.
surfaces,1
0,11
coating perfluorosilane
12-16
or poly(tetrafluoro-
1
7
ethylene) (PTFE) on various roughened solid substrates, etc.)
also have been developed in recent years. However, most of
these approaches involve expensive and complex time-consum-
ing procedures or are only applicable to special substrates, which
prevent practical application on a large scale. Furthermore, there
are few publications concerning the thermal stability of super-
hydrophobic surfaces. Recently, we successfully fabricated
superhydrophobic surfaces on copper foils under mild condi-
1
8
tions. However, in these cases, a thin gold layer has to be
deposited on the substrate for assembling a hydrophobic
monolayer of alkylthiol. On the other hand, it is well known
that electrochemical procedures can be accurately controlled and
easily magnified on the large scale in industry. Fabrication of
superhydrophobic surfaces through electrochemical approaches
Experimental Section
Synthesis of Cu(OH)2 Nanoneedle Film. The fabrication
has been reported also.1
electrochemical techniques were limited by the properties of
the substrates.
Biomimicry is also one of the most important research areas
in modern science.
9,20
2
procedure for Cu(OH) nanoneedle film was reported previ-
However, in these cases, the
2
7
ously. Typically, growth of this film was carried out at room
temperature in a one-compartment cell by use of a model 263
potentiostat-galvanostat (EG&G Princeton Applied Research)
under computer control. A copper foil (99.9%) (washed suc-
2
1,22
Studying the biological microstructures
-
1
cessively with a 2.0 mol L HCl aqueous solution, acetone,
and ethanol for ∼5 min each under ultrasonication; finally, it
was dried with a dry N2 stream before experiment) or a substrate
coated with a thin copper film was used as the working electrode
and mimicking the structures to make the materials useful for
human beings have attracted extensive attention.1
5,21
Recently,
*
To whom correspondence should be addressed. Phone: +86-10-6277-
2
3
743. Fax: +86-10-6277-1149. E-mail: gshi@tsinghua.edu.cn
(surface area ) 3 cm ). The counter electrode was a stainless
1
0.1021/jp056969x CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/19/2006

11248 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Wu and Shi
TABLE 1: Composition of the Electrolyte Used for
Electroplating Copper Film on Various Substrates
Thermogravimetric analysis was carried out using a TGA 2050
system (TA Co.). X-ray photoelectron spectra (XPS) were taken
on a PHI Quantera SXM (PE Co.) X-ray photoelectron
spectrometer.
components
amount
copper pyrophosphate hydrate (Cu
potassium pyrophosphate trihydrate K
triammonium citrate ((NH
2
P
2
O
7
)
2
6 g
4
P
O
7
‚3H
2
O
32 g
2.5 g
100 mL
8.5
Results and Discussion
4 3 6 5 7
) C H O )
2
H O
The synthesis and characterization of Cu(OH)2 nanoneedles
fabricated on copper foil have already been described previ-
pH
2
7
ously. The as-prepared Cu(OH)2 nanoneedle film has a high-
2
steel sheet (AISI 304, surface area ) 3.2 cm ). The electrolyte
energy surface with a contact angle close to 0°. To fabricate a
-
1
was a 2.0 mol L aqueous solution of KOH. The solutions
were deaerated by dry nitrogen bubbling and maintained at a
light overpressure during the experiments. Cu(OH)2 nanoneedle
film was electrochemically grown at a constant current density
superhydrophobic surface we treated the film with a 10 mmol
-1
L
ethanol solution of n-dodecanethiol for 1.5 h in order to
lower its surface energy. Figure 1 illustrates the SEM images
of the nanoneedle film before (A) and after (B) treatment with
n-dodecanethiol. It is clear from this figure that the morphology
of the thiol-modified nanoneedle surface (thiol-MNS) is differ-
-
2
-2
of 1.5 mA cm for 3 C cm at room temperature.
Synthesis of a Cu(OH)2 Submicrofiber Column. Growth
of submicrofibers on copper wires was carried out by anodiza-
tion of a copper wire at 20 °C in a one-compartment cell by
use of a model 273 potentiostat-galvanostat (EG&G Princeton
Applied Research) under computer control. The copper wire
was 140 µm in diameter, which is almost the same size as that
of a mature water strider’s leg. Copper wire was used as the
ent from that of pristine Cu(OH) nanoneedle film. The appear-
2
ance of the nanoneedle surfaces was changed, and the nano-
needles themselves adhered together into bundles to form a
highly rough surface, indicating reaction of n-dodecanethiol and
Cu(OH) nanoneedles. Chemical reaction caused the destruction
2
of the Cu(OH) nanocrystal and the morphology to change. The
2
2
working electrode (surface area ) 0.066 cm ). The counter
copper concentration of 25 mL of thiol solution after treating a
2
electrode was a stainless steel sheet (AISI 304, surface area )
3 cm copper foil with Cu(OH) nanoneedles was measured to
2
2
-1
-1
3
.2 cm ). The electrolyte was a 2.0 mol L aqueous solution
be only 0.25 mg L by an inductively coupled plasma-atomic
of KOH.
Growth of Copper Films on Different Solid Substrates.
emission spectrometer, indicating that the as-prepared thiol-
MNS is insoluble in the solvent. After thiol modification the
water contact angle of the nanoneedle surface was measured to
be as high as 155°, and its tilt angle was tested to be smaller
than 2° (see insert of Figure 1B). The contact angle of the droplet
changed from 155° to 160° after exposure to an ambient environ-
ment for 30 min, which indicates the surface is stable with time.
In comparison, a flat copper foil was also treated with n-do-
decanethiol through the same chemical procedures. The contact
angle of the modified flat copper surface was measured to be
only 110°. This result indicated that the nanostructures of the
-
3
Clean stainless steel and (AISI 304) silicon wafer (10 Ω cm)
were used without any previous treatment; the glass and PTFE
electrode was covered with a thin film of silver by silver mirror
reaction. Copper film was electrochemically deposited at a
-
2
-2
constant current density of 10 mA cm for 20 C cm at 50
C using copper foil as the counter electrode. The composition
°
of the electrolytes is listed in Table 1. The as-prepared copper
film electrode was washed with deionized water and dried with
N2 stream.
Surface Treatment. The as-prepared Cu(OH)2 film was
immersed in an ethanol solution of n-dodecanethiol (10 mmol
n-dodecanethiol-modified Cu(OH) nanoneedle surface (thiol-
2
MNS) strongly enhanced its hydrophobicity.
-1
L ) for 1.5 h and then washed with ethanol and dried with N2
stream before characterization. The as-prepared Cu(OH)2 sub-
microfiber column was immersed in an ethanol solution of
Furthermore, the superhydrophobic property of thiol-MNS
was tested to be thermally stable. The water contact angle of
thiol-MNS described above did not show any change even
when the surface had been kept in air at 200 °C for 2 h
(Supporting Information, Figure S1). When the modified
nanoneedle surface was heated at 160 °C in air for a very long
time (42 h), the surface still exhibited the superhydrophobic
property with a water contact angle as high as 154° and tilt
angle smaller than 2°. The surface will not decrease even when
bending the copper substrate to over 120°. After heat treatment
at 160 °C the surface retains its superhydrophobic property even
-
1
n-dodecanethiol (1 mmol L ) for 3.5 h and then washed with
ethanol and dried with N2 stream before characterization. Pristine
Cu(SC12H25)2 was prepared by direct reaction of CuCl2 aqueous
solution with n-dodecanethiol at a molar ratio of 1:2. Pristine
Cu(OH)2 was prepared by direct reaction of CuCl2 aqueous
solution with KOH at a molar ratio of 1:2. Flat Cu(OH)2 film
was fabricated by planish Cu(OH)2 particles under 5 MPa
pressure using a GX Fourier transform infrared (FT-IR)
spectrometer.
when the surface was rubbed by a piece of paper. Cu(OH) is
2
Characterizations. Water contact angles were measured by
a contact angle system, OCA 20, made by Dataphysics
Instruments GmbH (Germany). The size of the water droplet
was 5 µL. The static contact angle measurement was carried
out after the droplet deposited on the surface for 20 s. The
contact angle value was obtained by measuring five different
positions of the same sample. The tilt angles were measured
five times for each sample by tilting the sample with a 5 µL
drop on it until the drop began to slide. Scanning electron
micrographs (SEM) were taken out by use of a FEI Sirion 200
scanning electron microscope. Ion concentrations were measured
using an inductively coupled plasma-atomic emission spec-
trometer (ICP-AES), model Vista-MP (VARiAN Co.). X-ray
diffraction (XRD) was carried out using a D8 Advance (Bruker)
X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å).
thermally unstable and can be easily decomposed into CuO at
temperatures higher than 150 °C as demonstrated by the TGA
2
8
curve shown in Figure 2 (solid line). Therefore, the thermal
stability of modified film implies that n-dodecanethiol was not
simply adsorbed on the Cu(OH)2 surface. In fact, Cu(OH)2 can
react with n-dodecanethiol following the equation Cu(OH)2 +
2C12H25-SH f Cu(SC12H25)2 + 2H2O. Thus, the nanoneedles
were covered with hydrophobic alkyl chains after chemical treat-
ment. Cu(SC12H25)2 exhibits a decomposition temperature of
approximately 250 °C (Figure 2, dashed line); the Cu(SC12H25)2
coating enhanced the thermal stability of thiol-MNS. As shown
in Figure 2, dotted line, the thiol-modified nanoneedles de-
compose in two steps. The first step in the temperature range
of 170-200 °C is attributed to dehydration of Cu(OH)2 of
the nanoneedle host, and the second step in the temperature

Characterization of Stable Superhydrophobic Surfaces
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11249
Figure 1. SEM images of the film before (A) and after (B) treatment with an ethanol solution of n-dodecanethiol (10 mmol L-1) for 1.5 h (the
-
2
-1
-2
nanoneedles were deposited at a constant current density of 1.5 mA cm in 2.0 mol L KOH solution for 3C cm ). Insert of B: digital camera
image of a water droplet on the film after treatment at room temperature (drop weight ) 20 mg).
film.31 However, after treating with n-dodecanethiol, this O 1s
peak is greatly weakened while a strong C 1s peak of alkane at
2
84.8 eV appears. These results provided additional evidence
for reaction of Cu(OH)2 and n-dodecanethiol. There is a weak
carbon peak in the C 1s spectrum of as-prepared Cu(OH)2 film,
which is attributed to a trace amount of impurity.
As mentioned above, as-prepared Cu(OH)2 nanoneedles film
shows a superhydrophilic property. We found that the flat
surface of a pellet formed by pressing Cu(OH)2 particles under
5
MPa pressure (see Experimental Section) has a water contact
angle of 62°. A Wenzel equation is usually used to describe
the contact angle on the rough surface in this situation,32 cos
θ* ) r cos θ (θ* ) contact angle on the rough surface; θ )
contact angle on the flat surface, r ) ratio of the actual over
the apparent surface area of the substrate). Thus, r for the
superhydrophilic Cu(OH)2 nanoneedle surface was calculated
to be about 2.2. Modifying the film with n-dodecanethiol
produces a hydrophobic surface, and surface roughness enhances
its contact angle to over 150°. In this situation air trapped below
the water droplet can significantly enhance the surface hydro-
phobicity and the contact angle is usually described by3 cos
θ* ) -1 + æs(1 + cos θ) (θ* ) contact angle on the rough
surface, θ ) contact angle on the flat surface, æs ) fraction of
solid in contact with water). n-Dodecanethiol treated flat
Cu(OH)2 film mentioned above has a water contact angle of
about 110°; the thiol-MNS has a water contact angle of 155°.
Accordingly, æs of the thiol-MNS was calculated to be 14.2%,
indicating that about 86% of the total surface area was trapped
with air.
Figure 2. TGA traces for as-prepared Cu(OH)
line), the thiol-modified nanoneedles (dotted line), and pristine
Cu(SC12 (dashed line) prepared by direct reaction of CuCl and
25SH at a molar ratio of 1:2 (the samples were heated in air at a
2
nanoneedles (solid
H
25
)
2
2
12
C H
3,34
-
1
rate of 20 °C min ).
scale of 220-250 °C is attributed to decomposition of the
Cu(SC12H25)2 skins of the thiol-modified nanoneedles. Accord-
ing to the TGA data we can calculate that the film was com-
posed of 76% Cu(OH)2 and 24% Cu(SC12H25)2 (by weight).
It was found that the dehydration reaction changed the nano-
needle morphology little and the Cu(SC12H25)2 coating made
the superhydrophobic surface stable at temperatures as high as
2
00 °C.
It is well known that electroplating of copper on various solid
substrates such as a microprinting circuit or plastic can be easily
realized. Thus, the electrochemical approach for fabrication of
thermally stable superhydrophobic surfaces described above can
be extended to other substrates. Here, we used two conductive
(stainless steel sheet and silicon wafer) and two insulating (glass
and polyethylene tetrafluoride, PTFE) substrates as examples.
First, a thin layer of copper was electrochemically deposited
High-resolution X-ray photoelectron spectral results also
confirmed that the surfaces of Cu(OH)2 nanoneedles were
converted to Cu(SC12H25)2 after reaction with n-dodecanethiol.
The XPS spectra of the surfaces of as-prepared Cu(OH)2
nanoneedle film (a), thiol-modified nanoneedle film (b, thiol-
MNS), and a pure copper foil after thiol treatment (c) are shown
in Figure 3. As can be seen from this figure, no obvious S 2p3/2
peak was found in the spectrum of as-prepared Cu(OH)2
nanoneedle film. However, the spectrum of thiol-MNS showed
two S 2p3/2 peaks. The peak at 162 eV is related to the sulfur
of Cu(SC12H25)2, and that at 163.5 eV is associated with the
sulfur of free n-dodecanethiol.2 Accordingly, on thiol-MNS,
most of the n-dodecanethiol molecules changed into the
corresponding copper salt, while on the copper foil surface, most
of the thiol molecules are in the free state. Thus, it is reasonable
to conclude that n-dodecanethiol reacts with Cu(OH)2 much
easier than with metallic copper. Furthermore, there is a strong
O 1s peak at 531.2 eV in the spectrum of as-prepared Cu(OH)2
-
2
-2
on the surface of each substrate at 10 mA cm for 20 C cm .
The insulating substrates were precoated with a thin layer of
3
5
silver by use of the silver mirror reaction. Then, a surface of
Cu(OH)2 nanoneedles was achieved on the substrate through
the same electrochemical procedures described above. Figure
4A, B, C, and D shows typical SEM images of as-prepared
copper hydroxide nanoneedle film on a stainless steel, silicon
9,30
-
3
wafer (10 Ω cm), glass, and PTFE sheet, respectively. As
can be seen, the nanoneedles constructed a rough porous surface
on each substrate. The inserts of Figure 4A and B are profile
SEM images of the samples. They clearly demonstrate that the

11250 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Wu and Shi
Figure 4. SEM images of as-prepared Cu(OH)
2
nanoneedle films
-
2
-1
grown at a constant current density of 1.5 mA cm in 2.0 mol L
KOH solution and coated on different substrates: (A) stainless steel,
B) silicon wafer, (C) glass, (D) PTFE. Insets of A: optical picture of
(
a water droplet on the thiol-MNS on stainless steel (drop weight 5
mg) and a profile image of the film (the lower one). Inset of B: profile
images of the film coated on a silicon wafer.
Figure 3. High-resolution XPS spectra of as-prepared Cu(OH)
2
nanoneedle film (a) and the same film (b) or a copper foil (c) after
treatment with n-dodecanethiol.
film coated on stainless steel is compact, even when the surface
is lacerated by a knife, while the quality of the film coated on
silicon, glass, or PTFE was relatively worse. The thiol-MNS
fabricated from these surfaces also exhibited superhydropho-
bicity with a contact angle larger than 151° and tilt angle less
than 2° and also can resist a high temperature of 200 °C (Figure
2
Figure 5. SEM and TEM images of as-prepared Cu(OH) submicrofi-
-2
bers deposited at a constant current density of 2 mA cm in 2.0 mol
-
1
L
KOH solution for 1500 s. T ) 20 °C. (A) overall image,; (B)
magnified image, (C) TEM image. Inset: SAED pattern.
4
). After the hea treatment, the superhydrophobic layer still
graphs (SEM) of a copper column after anodization in a 2.0
-
1
adhered firmly on SS substrate while delamination occurred at
the interface of the superhydrophobic coating and silicon, glass,
or TFEE substrate due to their different mechanical and thermal
properties.
Anodization of a copper column was also successfully
achieved. Figure 5 shows the typical scanning electron micro-
mol L aqueous solution of KOH at a constant current density
-
2
-2
of 2 mA cm for 3 C cm . As can be seen, the copper wire
was coated compactly with submicrofibers. Figure 5B is a
magnified view of the as-prepared submicrofibers, which
demonstrated that they are ∼5 µm in length and 150-300 nm
in diameter. The morphology of the copper column surface is

Characterization of Stable Superhydrophobic Surfaces
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11251
Figure 7. Model constructed for mimicking a water strider.
the superhydrophobic copper column (which is similar to the
legs of water striders). To simplify the measurement, a model
was constructed to mimic the water striders. As shown in Figure
Figure 6. SEM images of as-prepared copper hydroxide submicrofibers
after treatment with an ethanol solution of n-dodecanethiol (1 mmol
7
, the body of the spider model was made of an oval plane of
-1
L
) for 3.5 h (the fibers were deposited at a constant current density
polyethylene tetrafluoride (PTFE). Six superhydrophobic copper
columns (blue parts, 1.5 cm each) were arranged symmetrically
at the two sides of the plane. The unmodified copper wires (1
cm each, yellow parts) were fixed 120° to the plane. The angle
between the superhydrophobic wire and the pristine copper wire
was also about 120°. To prevent the water surface from being
pierced by the sharp tip of the superhydrophobic copper wire,
part of the wire end (about 3 mm) was bent vertical to the water
surface. During the test process the model spider was put onto
the water surface carefully and additional weight was loaded
until its feet were completely immersed in the water. The
maximal supporting force of each superhydrophobic foot of the
spider model was measured to be about 83 dynes; this value is
-2
-1
-2
of 2 mA cm in 2.0 mol L KOH solution for 3 C cm ): (A) overall
view and (B) magnified view.
similar to that of water striders’ leg. Our energy-dispersive X-ray
spectroscopy (EDX) measurement affirms a strict stoichiometric
ratio of Cu:O ) 1:2, which reveals that the as-prepared
submicrofibers were composed of Cu(OH)2. XRD measurement
of the as-prepared product on the copper foil (instead of copper
wire) is in agreement with the orthorhombic phase Cu(OH)2
(JCPDS 13-420). Figure 5C is a transmission electron micro-
graph (TEM) of an as-prepared Cu(OH)2 submicrofiber. The
selected area electron diffraction (SAED) pattern of a 300 nm
wide fiber (inset, Figure 5C) indicates that the fiber is a single
crystal. According to the features of the diffraction pattern,
Cu(OH)2 fibers are arranged parallel to their (010) planes and
the preferential growth direction of the single-crystal fibers is
their [100] direction.
25
close to that of the real water striders. We also used thiol-
treated smooth copper wires (CA ) 110°) as the feet of a spider
model for comparison. The experimental results indicated that
the maximal supporting force of each foot was measured to be
only 29 dynes, which suggests that the superhydrophobic
submicrofibers morphology is the principal factor for water
repellency.
It is known that the weight of a static water strider model is
supported mainly by two kinds of forces. One is the buoyancy
force, Fb, deduced by integrating the hydrostatic pressure over
the body area in contact with the water; the other is the curvature
force, Fc, deduced by integrating the curvature pressure over
It is known that water striders can move quickly on the water
surface due to its superhydrophobic legs with submicrofibers.
To fabricate a superhydrophobic surface, the copper column
with Cu(OH)2 submicrofiber coating was immersed in an ethanol
-1
solution of n-dodecanethiol (1 mmol L ) for 3.5 h, then washed
with ethanol, and dried with N2 stream. Figure 6 illustrates the
SEM images of the submicrofibers fabricated on the copper
column after thiol treatment. The overall view of the thiol-
modified surface (Figure 6A) is similar to that of the column
surface with submicrofibers (Figure 5A). Figure 6B is a
magnified view of the column surface shown in Figure 6A. It
is clear from this image that the surface morphology of the thiol-
modified submicrofiber surfaces is obviously different from that
of pristine Cu(OH)2 fibers. The original smooth surfaces of
Cu(OH)2 fibers were roughened by the reaction with n-do-
decanethiol. When a water droplet was dropped onto the cop-
per wire surface it rolled off quickly. As it is hard to measure
the static contact angle of the wire surface, instead of the cop-
this area or equivalently the vertical component of the surface
23,36
tension, σ sin θ, along the contact perimeter.
Here, Fc is
the more important factor. When Mg ) Fb + Fc, the maximal
supporting force was obtained (where M is the mass sum of
the spider model and the additional load and g is the gravitation
constant). The superhydrophobic submicrofibers coatings on the
copper wire is effective for preserving air in the pores and
forming a cushion at the wire-water interface that prevents the
wire from being wetted and causes a large Fc. In contrast to
other superhydrophobic coatings, the submicrofiber coating has
deep pores in high number density, which makes the surface of
the wire-water interface hard to be wetted by water under static
pressure. This may help us to understand why water striders
choose the hierarchical structure of micrometer-sized tiny hairs
with fine nanogrooves.
2
per column, we used a flat copper foil with 1.5 × 1 cm for
measuring the water contact angle (after anodization and thiol
treatment, the surface morphology of the copper foil is similar
to that of the copper column described above). The contact angle
of the thiol-treated copper foil with submicrofibers was mea-
sured to be 155°, and its tilt angle was tested to be smaller than
Conclusion
2
°, which revealed that the surface was superhydrophobic.
To deeply understand the behavior of water striders on the
water surface, we investigated the maximal supporting force of
We developed a simple way to fabricate a thermally stable
superhydrophobic surface with high contact angles and a low

11252 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Wu and Shi
tilt angle by combination of the electrochemical technique and
surface reaction. The method described here can be easily
extended to any other substrates, such as stainless steel, silicon
wafer, glass, and PTFE, for fabricating thermally stable super-
hydrophobic coatings. Moreover, as electroplating of copper film
is a low-cost and mature technology in industry, the method
described here can be easily magnified to a large scale. Fur-
thermore, we successfully applied the fabrication method of
superhydrophobic copper wires with submicrofibers structures,
which is similar to the leg of water striders. The superhydro-
phobic wire causes a much larger curvature force than that of
a thiol-treated smooth copper wire. This work may help us to
understand why water strider can move quickly on the water
surface and why it chooses the hierarchical structure of
micrometer-sized tiny hairs with fine nanogrooves.
(7) Chen, W.; Fadeev, A. Y.; Heieh, M. C.; Oner, D.; Youngblood, J.;
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Acknowledgment. This work was supported by the National
Natural Science Foundation of China (50533030, 90401011,
(
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0374034, 50225311) and 973 project (2003CB615700). We
3
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thank Prof. X. Zhang and Mr. F. Shi (Tsinghua University
China) for useful discussions and help with the contact angle
measurements.
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Supporting Information Available: Detailed information
of the water contact angle at different treated temperature.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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