A simple strategy to realize biomimetic surfaces with controlled anisotropic wetting

Abstract

The study of anisotropic wetting has become one of the most important research areas in biomimicry. However, realization of controlled anisotropic surfaces remains challenging. Here we investigated anisotropic wetting on grooves with different linewidth, period, and height fabricated by laser interference lithography and found that the anisotropy strongly depended on the height. The anisotropy significantly increased from 9° to 48° when the height was changed from 100 nm to 1.3 μ m . This was interpreted by a thermodynamic model as a consequence of the increase of free energy barriers versus the height increase. According to the relationship, controlled anisotropic surfaces were rapidly realized by adjusting the grooves’ height that was simply accomplished by changing the resin thickness. Finally, the perpendicular contact angle was further enhanced to 131°±2° by surface modification, which was very close to 135°±3° of a common grass leaf.

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A simple strategy to realize biomimetic surfaces with controlled
anisotropic wetting
Dong Wu,1Qi-Dai Chen,1,a?Jia Yao,1Yong-Chao Guan,1Jian-Nan Wang,1Li-Gang Niu,1
Hong-Hua Fang,1and Hong-Bo Sun1,2,a?
1State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering,
Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
2College of Physics, Jilin University, 119 Jiefang Road, Changchun 130023, People’s Republic of China
?Received 22 October 2009; accepted 29 December 2009; published online 5 February 2010?
The study of anisotropic wetting has become one of the most important research areas in
biomimicry. However, realization of controlled anisotropic surfaces remains challenging. Here we
investigated anisotropic wetting on grooves with different linewidth, period, and height fabricated
by laser interference lithography and found that the anisotropy strongly depended on the height. The
anisotropy significantly increased from 9° to 48° when the height was changed from 100 nm to
1.3 ?m. This was interpreted by a thermodynamic model as a consequence of the increase of free
energy barriers versus the height increase. According to the relationship, controlled anisotropic
surfaces were rapidly realized by adjusting the grooves’ height that was simply accomplished by
changing the resin thickness. Finally, the perpendicular contact angle was further enhanced to
131°?2° by surface modification, which was very close to 135°?3° of a common grass leaf.
© 2010 American Institute of Physics. ?doi:10.1063/1.3297881?
A wealth of surfaces in nature not only exhibit good
hydrophobic ability but also behave directional wetting, such
as the rice leaf,1the wings of the butterfly,2the trionum
flower,3the duck feather,4and the shark skin.5This phenom-
enon, crucial for the natural species surviving, is known as
anisotropic wetting. It is considered that the directional
arrangement of microstructures was the key reason for
anisotropy. Surfaces with controlled anisotropic wetting re-
strict liquids flow to desired directions, and are expected
to greatly improve the functions of, for example, microflu-
idicdevices6,7
and evaporation-driven
nanopattern.8,9The potential applications have attracted last-
ing research efforts.1,9–13For example, Jiang et al.1grew
aligned carbon nanotubes to mimick the surface of a rice
leaf; Morita et al.10reported macroscopic anisotropy on line
patterned surfaces of fluoroalkylsilane monolayers by
vacuum ultraviolet lithography; Zhao et al.11prepared sub-
micrometer periodic grooved surface on azobenzene polymer
films by laser interference. Despite of these exciting works,
precisely controlling anisotropic wetting which is highly de-
sired in practical applications6–9remains challenging because
of the lack of a systematic study and deep insight into phys-
ics of anisotropic wetting. Until now it is unclear how to
control anisotropy and it is also unknown whether it is pos-
sible to develop a simple approach to realize controllable
anisotropy. In addition, the anisotropy on submicrometer
scale periodic grooves has been found weak, only about
10°,11as is out of general expectation.
In this letter, we systemically investigated anisotropic
wetting behaviors on groove structures with different peri-
ods, linewidths, and heights fabricated by interference
lithography.14–16Importantly, we found that the anisotropic
wetting strongly depended on the height, and weakly on the
period and linewidth in the common microfabrication scale,
formationof
about the magnitude of several micrometers. This agrees
well with the theoretical results calculated by a thermody-
namic model.17,18With optimal parameters, artificial biomi-
metic surfaces with controlled anisotropy were realized.
Shown in Figs. 1?a? and 1?c? are bird-view scanning
electron microscope ?SEM? images of 2 ?m period and 400
nm height grooves structures with different linewidths. The
grooves with several micrometers period were the most com-
mon and useful structures for studying anisotropic wetting,
here we also focused on this regime. Four kinds of grooves
with widths from 600 to 1500 nm were obtained by control-
ling the laser exposure dosage. The exposure time was
a?Authors to whom correspondence should be addressed. Electronic ad-
dresses: hbsun@jlu.edu.cn and chenqd@jlu.edu.cn.
FIG. 1. ?Color online? Anisotropic wetting on grooves with different line-
widths and periods. ?a? and ?c? SEM images of 2 ?m period and 400 nm
height grooves structures with different linewidths. ?b? and ?d? the corre-
sponding CA. ?e? 2.5 ?m period groove. ?f? The CA measurement curve.
APPLIED PHYSICS LETTERS 96, 053704 ?2010?
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96, 053704-1
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changed while the power keeps 50 mW. The durations were
14, 16, 18, and 21 s for groove widths 600 and 900 nm ?Fig.
1?a??, 1.2 and 1.5 ?m ?Fig. 1?c??, respectively. The contact
angle ?CA? measurement ?Figs. 1?b? and 1?d?? was made by
a Contact Angle System OCA 20 ?DataPhysics Instruments
GmbH, Germany?. ?1and ?2were defined as the CA mea-
sured in the directions perpendicular and parallel to the lon-
gitudinal axis of the grating, and ??=?1-?2was defined as
the degree of wetting anisotropy. We could find that the per-
pendicular CAs were evidently larger than the parallel ones
for all grooves because a water droplet tended to flow along
the groove. When it spreads along the perpendicular direc-
tion, it has to overcome the energy barrier exerted by the
grooves. However, it was noted that all the ?? was around
20° for different grooves and the change for both of ?1and
?2was not pronounced. Besides the groove linewidth, the
influence of period was also studied. The groove period
d=?/?2 sin ?? is determined by the incident angle ? and the
laser wavelength ?. Four kinds of periods 0.9, 1.5, 2, and
2.5 ?m ?Fig. 1?e?? was obtained under ?=11.4°, 6.8°, 5.1°,
and 4°, respectively. The ??’s ?Fig. 1?f?? for four kinds of
periods are between 21° and 23°. There are no obvious dif-
ferences in anisotropic wetting behaviors. The reason may be
that both of the parameters belong to the horizontal direction,
and the horizontal variations ??2 ?m? of the groove do not
significantly cause the increase of FE barriers, which is in
accordance with the results reported by Mortia et al.10
Height, which is controlled by the thickness of the resin
NOA 61, is the only parameter along the vertical direction of
groove. Shown in Fig. 2?a? is 45° titled-view SEM image of
the cross section of 1.3 ?m height ?the AFM image in the
inset of Fig. 2?a?? groove. To our surprise, the ?1reached as
much as 104°?2°, while the ?2kept 56°?1° ?Fig. 2?b??.
Due to the strong anisotropy, the droplet along the groove
direction became very long, as shown in Fig. 2?c?, which was
a sample with ten grooved regions ??600 mm2?. Due to the
scattering and diffraction of periodic grooves, the surface
shows brilliant iridescence which changes from purple to
green ?The inset of Fig. 2?c?? under different viewed angles.
Furthermore, an iridescent diffraction pattern ?Fig. 2?d?? was
obtained when the grooves were illuminated under white
light, just like the one of the Hibiscus trionum flowers.3
In order to better understand the physical mechanism of
anisotropic wetting on the groove, a thermodynamic model18
was developed to calculate the change in the surface FE as a
function of the instantaneous CA during the three-phase con-
tact line moving along the two orthogonal directions. The
wetting in our case is in the noncomposite state ?the inset in
Fig. 3?a??, where the water completely penetrates into the
grooves, because the ?1strongly depends on the height. The
magnitude of the FE barriers increases as the groove height
increases, which could lead to the increase in the degree of
wetting anisotropy. For the model, provided that the droplet
area was constant, an equation was deduced from geometri-
cal analysis to calculate the ? from A to B
?A? LA
2/sin2?A− LA
? ctg?B,
2? ctg?A= ?B? LB
2/sin2?B− LB
2
?1?
where the initial ?Ais chosen as 150°, LAis 1 mm, LB=LA
+a and ?0is the CA for the flat surface. Then, the relative FE
barrier for AB and BC section18was calculated and the
whole FE barrier curve was obtained.
The FE barrier with respect to CA for groove with
1.3 ?m height was calculated, as shown in Fig. 3?a?. The F
was normalized with respect to the surface tension rlabe-
tween liquid and gas state and the FE unit will be mm. From
the FE barrier curve, we see that there are a minimum FE at
101°, which means the equilibrium state for the water droplet
on the grooved surface because the least energy state is
steadiest for a system. The experimental value 104°?2°
agreed well with the theoretical result. With the same calcu-
lation method, the FE barrier for the previous 400 nm height
groove was obtained. The equilibrium CA was about 72°,
which was very close to the measured value 76°?2°. The
small difference ?4° was partially caused by the measure-
ment error. In addition, the groove shape we assumed in
FIG. 2. ?Color online? Strongly anisotropic wetting on a 1.3 ?m-height
groove and its optical properties. ?a? 45° tilted-view SEM image of the
groove. The inset is the AFM measurement. ?b? the corresponding CA. ?c? A
camera photo of the groove sample with ten grooved regions. ?d? An irides-
cent diffraction pattern of the groove.
FIG. 3. ?Color online? The theoretical calculation of surface FE and the
equilibrium CA by a thermodynamic analysis. ?a? The surface FE barrier
calculation with respect to CA for groove with 1.3 ?m height. The inset is
the two-dimensional model schematic cross section of groove surface. ?b?
The strongly dependence between the groove height and anisotropy.
053704-2Wu et al. Appl. Phys. Lett. 96, 053704 ?2010?
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numerical analysis was square,17,18which had small error
compared with the actual cross section of groove. This also
affected the difference of anisotropy. To further investigate
the relationship between the height and the anisotropy,
groove structures with different heights, e.g., 200, 600, and
1000 nm were prepared, and the measured CA’s ?Fig. 3?b??
were in accordance with the theoretical results. It was evi-
dently seen that the anisotropy strongly depended on the
groove height. This made it possible for us to precisely con-
trol the degree of the anisotropy by changing the groove
height.
To verify this possibility, we freely selected certain
angle, for example, 90°. Along the curve in Fig. 3?b?, we
deduced that the groove height was about 800 nm. Then, by
controlling the thickness of the resin, 800 nm height grooves
?Fig. 4?c?? was realized and the measured CA was 90°?2°,
just as what we expected. To further verify the possibility of
realizing controlled anisotropy, a smaller anisotropy, e.g., 65°
would be obtained when the height was around 100 nm ac-
cording to theoretical analysis. Experimentally, the measured
CA on 100 nm-height groove surface ?Fig. 4?a?? was about
64°?2° ?Fig. 4?b??. The anisotropy was only 9°, which was
very close to the results reported by Zhao et al.11They ob-
tained the degree of the anisotropy 9.8° when the height was
127 nm. According to our studies, we deduce that the weak
anisotropy on submicrometer grooves11is mainly caused by
the small height ??150 nm?. Likewise, the reason for the
weak anisotropy wetting on the line-patterned surface of
fluoroalkylsilane monolayer ??100 nm? reported by Morita
et al.10is also the small height.
For most natural anisotropic surfaces, the CAs along
both of the parallel and perpendicular directions are very
large ??120°? because larger CA is beneficial for water to
roll down. For example, the ?1and ?2are about 135°?3°
and 120°?2° ?the insets in Figs. 5?a? and 5?b?? for a com-
mon grass leaf. From the SEM images ?Figs. 5?a? and 5?b??,
we can find that the surface microstructures are in directional
arrangement ?the white arrow in Fig. 5?a??. Although the ??
is only 15°, it is enough for living species to control the
water movement direction. For our grooved surface, in order
to increase both of the ?1and ?2, we adopted low-surface-
energy fluoroalkysilane19,20to modify the 1.3 ?m height
groove. After surface modification, the ?1was increased
from 104°?2° ?Fig. 2?b?? to 131°?2° ?Fig. 5?c?? and ?2
was enhanced from 56°?1° ?the inset of Fig. 2?b?? to
108°?1° ?Fig. 5?d??. This result was very close to the the-
oretical ?1133° by the FE barrier calculation.
In conclusion, anisotropic wetting behaviors on groove
structures with different parameters were systemically stud-
ied. Specially, we found the strongly dependence between
the anisotropy and the height. Then, we proposed to pre-
cisely control anisotropy by adjusting the grooves’ height.
This method was very simple and easily realized by laser
interference lithography. Finally, the CAs were enhanced by
surface modification and the wetting property of artificial
grooved surfaces was more close to those natural anisotropic
surfaces. These results not only provide new insights into the
anisotropic wetting phenomenon but also are beneficial to
freely design anisotropic surfaces for bioinspired system ap-
plication.
This work was supported by NSFC ?Grant Nos.
60977025, 60525412, and 90923037?.
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FIG. 4. ?Color online? Controlled anisotropic wetting by changing the
groove height. ?a? The AFM image of a 100 nm height groove. ?b? The weak
anisotropy ?9° because of its smaller height. ?c? 45° tilted-view SEM image
of the 800 nm height groove. ?d? The measured CA was just as what we
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FIG. 5. ?Color online? ?a? and ?b? are SEM image of a common grass leaf
surface and its magnified SEM image. The insets are the measured ?1and
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on 1.3 ?m height groove surface.
053704-3Wu et al.Appl. Phys. Lett. 96, 053704 ?2010?
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