H. GAO et al.: PREPARATIONS AND CHARACTERIZATIONS OF SUPER-HYDROPHOBIC SURFACES ...
299–306
PREPARATIONS AND CHARACTERIZATIONS OF
SUPER-HYDROPHOBIC SURFACES ON Al ALLOYS AND THEIR
ANTI-ICING PROPERTIES
PRIPRAVA IN KARAKTERIZACIJA SUPERHIDROFOBNIH
POVR[IN NA Al ZLITINAH IN NJIHOVE PROTIZALEDENITVENE
LASTNOSTI
Hairong Gao
1
, Ruiying Wang
2
, Haijie Sun
1
, Aijuan Zhao
1
, Ling Wang
1
1
Zhengzhou Normal University, School of Chemistry and Chemical Engineering, Zhengzhou 450044, Henan, China
2
Henan Vocational College of Chemical Technology, School of Chemistry and Chemical Engineering, Zhengzhou 450042, Henan, China
sunhaijie406@zznu.edu.cn
Prejem rokopisa – received: 2017-07-25; sprejem za objavo – accepted for publication: 2017-11-15
doi:10.17222/mit.2017.125
A low-cost and simple methodology is successfully developed for the preparation of super-hydrophobic surfaces on Al alloys by
a facile hydrothermal reaction method at 60°C after a simple chemical modification. Using such a robust method, a relatively
large contact angle of 167° and a small roll angle of 3° can be achieved. The icing behavior of the surfaces was investigated in
–10 °C environment. It is found that such super-hydrophobic surfaces exhibit remarkable anti-icing characteristics. In particular,
the icing time on the surfaces can be delayed considerably. Therefore, our study suggests that the present methodology and the
prepared super-hydrophobic surfaces could be applied in various engineering fields.
Keywords: Al alloys, contact angle, icing, super-hydrophobic, anti-icing
Avtorji prispevka so uspe{no razvili enostavno metodologijo za pripravo super hidrofobnih povr{in na Al zlitinah z enostavno
metodo hidrotermalne reakcije pri 60 °C po enostavni kemi~ni modifikaciji. Z uporabo te robustne metode so dosegli relativno
velik kontaktni kot 167° in kotalni kot manj{i od 3°. Proti- zamrzovalne lastnosti povr{in so ugotavljali v okolju pri –10°C.
Ugotovili so, da imajo tak{ne super hidrofobne povr{ine odli~ne lastnosti, ki prepre~ujejo zamrzovanje oz. zaledenitev. [e
posebej je dose`ena pomembna zakasnitev ~asa zaledenitve na teh povr{inah. Avtorji tega prispevka zato ugotavljajo, da je
predstavljeno metodologijo in pripravljene povr{ine mo`no uporabiti na razli~nih in`enirskih podro~jih.
Klju~ne besede: Al zlitine, kontaktni kot, zdrs, super hidrofobnost, prepre~evanje zaledenitve
1 INTRODUCTION
The accumulation of ice on exposed surfaces may
cause serious problems to the integrity of outdoor
equipment such as high-voltage cables, wind-turbine
blades, buildings, telecommunication towers and aircraft.
Two main strategies can be used to deal with this issue.
One is active deicing methods, and another is passive
methods.
1,2
Current active deicing methods usually based
on melting or breaking of already formed ice layers. In
addition to their undesired weight and design com-
plexity, active anti-icing approaches require substantial
energy for operation. The passive methods based on
protecting the exposed surface by elaborating coatings
with ice-phobicity characteristics to significantly extend
the freezing time and reduce the ice adhesion strength.
Compared to active methods, passive ones are environ-
mentally friendly and low-cost. Passive ice-phobic coat-
ings have also been proposed during the past decades,
yet the expected results have not been achieved.
3,4
To protect exposed surfaces against excessive ice
accumulation, ice-phobic coatings must be highly reli-
able and durable. While some coatings have excellent
ice-phobic properties, the balance between ice-phobic
characteristics, excellent mechanical properties and
actual application value is required. Growing films di-
rectly from a substrate considerably improves the
adherence and the mechanical stability of the resulting
films, comparing with colloidal-deposition techniques.
5
However, as far as we know, there has been almost no
research of the in-situ growth of the ice-phobic film with
micro/nanoscale binary structure on aluminum substrates
up to now.
In recent years, the achievement of the "Lotus effect"
super-hydrophobic surface research made people see the
light of overcoming anti-icing difficulties. Inspired by
the outstanding super-hydrophobicity and self-cleaning
property of lotus leaf, the wettability of solid surfaces
has several applications in reducing the resistance shell
of submarines, ships, resisting water coalescence and fog
condensation, preventing the obstruction of the oil pipes,
micro-injection needle, and being catalysts, optical and
sensors, with the increasing biocompatibility, lubricity,
and durability.
6–8
Contact angle (CA) and sliding angle (SA) are two
indexes for the evaluation of wettability. In general, the
super-hydrophobic surface has a water contact angle
Materiali in tehnologije / Materials and technology 52 (2018) 3, 299–306 299
UDK 620.1:62-4-024.33:669.715.67.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(3)299(2018)

greater than 150° and a very low sliding angle, typically
below 10°.
9–12
Different kinds of engineered super-hydro-
phobic surfaces have been created on various metal
substrates, such as gold,
13
silver,
14
copper,
15–18
alloy,
19,20
and stainless steel.
21,22
The super-hydrophobicity and
ice-phobicity of aluminum surfaces have also attracted
more interest because of their practical or potential appli-
cations.
23–31
Several groups have studied the adhesion of
ice on super-hydrophobic surfaces and found good cor-
relations between the hydrophobicity of surfaces and
their ice-phobic behaviour.
32–35
Layered double hydro-
xides (LDHs), also well known as anionic clays or
hydrotalcite-like materials, are an important kind of
layered materials. The composition of LDH can be
represented by the general formula
[] MMO H A HO
1
23
22 −
++
+
−
⋅
x x
x
n
xn
m ()()
/
, where the metal
cations M
2+
and M
3+
occupy the octahedral holes in a
brucite-like layer and the anion A
n-
locates in the hyd-
rated galleries between the layers.
36,37
This paper presented a very simple, time-saving, and
inexpensive method for growing Zn–Al LDH nanowalls
and bayerite microrods on aluminum substrates in situ.
Aluminum substrates were employed to provide Al
3+
for
the growth of Zn-Al LDH and bayerite as well as to
collect relatively ordered micro/nanostructures rather
than random powders in the synthetic process. The
resulting film changed from super-hydrophilic and easy
icy to super-hydrophobic and ice-phobic after modified
stearic acid.
2. EXPERIMENTAL PART
2.1 Materials
All the reagents in the experiment were analytical
grade and used without further purification. Al sheets
with a size of 20 mm × 20 mm × 0.15 mm were cut from
a rolled sheet (99 %, Shanghai, Chemical Reagent
Co.,China), Zinc acetate [Zn(CH
3
COO)
2
, 99 %, Tianjin,
Chemical Reagent Co., China], ammonia (25–28 %), and
stearic acid. Deionized water was exclusively used in all
aqueous solutions and rinsing procedures.
2.2 Preparation of super-hydrophobic surfaces
In a typical process, Al sheets were dipped into
acetone and deionized water and cleaned by ultrasonic
washer. Any substances physically and weakly attached
to the substrate after the reaction was removed during
ultrasonication for 3 min. Then, they were immersed in a
1.5-M aqueous solution of NaOH for 1 min to get rid of
the surface impurity, and completely washed with
deionized water. In a typical experimental procedure,
0.00125 mol Zn(CH
3
COO)
2
·2H
2
O was added into 100
mL of deionized water under vigorous magnetic stirring
to form a homogeneous solution. An appropriate amount
of ammonia was added to this solution dropwise (the
molar ratio of ammonia/Zn
2+
was 0.25:1-4:1). The mix-
ture solution became turbid with the adding ammonia.
Meanwhile, a milk-like white solution was obtained,
which was then transferred into a Teflon-lined stainless-
steel autoclave. The cleaned Al sheets were vertically
placed in the reaction solution. The autoclave was sealed
tightly and maintained at 60 °C for 5 h. After the reac-
tion, the Al sheet was taken out, washed with deionized
water and dried in the air. To achieve surface super-hyd-
rophobicity, the as-prepared films were immersed in an
n-hexane solution of 5-mM stearic acids for5hatroom
temperature. Finally, the modified substrates were rinsed
with ethanol 3 times and then dried in the air at room
temperature.
2.3 Delay time experiment
The reference drops (8 μL, 21 °C, tap water) were
placed on these surfaces and the temperature was con-
trolled at –10°C, and observed in situ by a digital
camera. Initially, these drops were all transparent on the
surfaces. As the time passed, the drops on the surfaces
were frozen and became non-transparent. The delay
times (DTs)
35
were defined as the time that the drop
changed from a water drop to tiny ice cubes. The three
surfaces (untreated Al sheet, Zn-Al LDH super-hydro-
phobic surface, bayerite/Zn-Al LDH super-hydrophobic
surface) were observed in situ at the same time, the delay
times were recorded and values were an average of five
times. The room temperature and relative humidity were
22±2 °C and 60±5 %, respectively.
2.4 Ice-adhesion force measurement
The ice-adhesion evaluation tests of the above-men-
tioned three samples were conducted in a Spinner Coater
(Figure 1). The nozzle of a commercially available
manual sprayer equipped with tap water was kept at
about 8 cm from the substrates, and the tap water was
sprayed until the surface was fully covered with a liquid
layer, and then placed the substrates covered with liquid
layer in refrigerator at –10 °C until frozen. The spinning
speed was gradually raised to spin off the ice on the
substrate surfaces; the accurate rotating speed could be
obtained from the display screen of the Spinner Coater.
Ice mass and area were carefully evaluated and recorded.
The masses of the glass slide (m
1
), the substrate (m
2
) and
the substrate covered with ice (m
3
) were measured in
turn, the ice mass (m) was calculated as m = m
3
– m
2
–
m
1
. The artificially iced samples were then spun in the
Spinner Coater placed in a refrigerator at –10 °C to
determine the speed at which ice detachment from the
sample surface occurred. At the moment of the detach-
ment, the adhesion strength of ice was assumed to be
equal to the centripetal force, F = mv
2
/r, where m is the
ice mass, v is the rotation speed and r is the distance
between Al sheet and the center. The shear stress,
correspondingly, was calculated as  = F/A, where A is
the de-iced area and equal to the area of the substrate.
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300 Materiali in tehnologije / Materials and technology 52 (2018) 3, 299–306

The adhesion reduction factor, ARF,
29
was calculated as
the ratio of ice shear stress on the untreated Al sheet to
the treated Al sheet, which could reduce effectively the
influence of experimental errors.
2.5 Characterizations
The phases of the samples were characterized with
X-ray powder diffraction (XRD) using the Rigaku
D/max 2500 diffractometer with Cu-K
 radiation (k =
0.15406 nm). The surface morphology was observed
using a JOEL JSM-6610LV scanning electron micro-
scope. Static contact angles and sliding angles were
measured based on the sessile drop measuring method
with 5-μL droplets of distilled water using a Model 250
(p/n 250-F1) goniometer (ramé-hart instrument Co.,
USA) at ambient temperature. The average CA was
obtained by measuring the same sample at five different
positions and every position 15 times. The sliding-angle
measurements were carried out using a mechanical level
goniometer, which recorded when the deposited water
droplet started to roll off. The temperatures of the sample
surfaces and the environment were recorded by an Infra-
red Thermometer (CASON-CA390). Ice-phobic proper-
ties were studied by using a refrigerator (Mei Ling,
China). Ice-adhesion strengths were evaluated using a
Spinner Coater (KW-4A) referred to the centrifuge
adhesion test (CAT) method.
29
3 RESULTS AND DISCUSSION
3.1 Formation of micro/nanoscale hierarchical struc-
tures
Figure 2 shows the SEM images of the prepared
films using different molar ratios between Zn(CH
3
COO)
2
and NH
3
·H
2
O with different magnifications. It can be
seen from Figures 2a, 2b, 2c) that there are two different
morphologies: one is 3D flowerlike microspheres with
diameters of 5–20 μm and the sheet thickness of the
microspheres is about 50 nm; the other is a homoge-
neous nanowalls structure. The panoramic morphology
shows that the film is also composed of flower-like
protrusions (Figure 2d) with diameters of 5–20 μm
when the ammonia concentration doubled (Figure 2e),
but the flat area with the thickness 100 nm of the nano-
walls become more porous (Figure 2f). With the
increasing of the concentration of ammonia, the film
preferentially grow on the Al surface, forming a pattern
of high resolution, is constructed with stacked nano-
sheets of about 50 nm in thickness, resulting in a highly
porous surface (Figure 2g and 2h).
We found that the concentration of ammonia plays a
vital role in tailoring the morphologies of the resultant
films (Figure 3). It is clear that the resulting surface
exhibits two shapes of architectures. One is flower-like
club-shaped microrods, another is nanowall porosity.
Flower-like club-shaped and porous architectures are
different with the change of molar ratios between
Zn(CH
3
COO)
2
and NH
3
·H
2
O. The microrods have the
shape of stepped construction. We can see that the
flower-like microspheres have disappeared and become
H. GAO et al.: PREPARATIONS AND CHARACTERIZATIONS OF SUPER-HYDROPHOBIC SURFACES ...
Materiali in tehnologije / Materials and technology 52 (2018) 3, 299–306 301
Figure 1: a) Prepared sample in spinner coater set-up measuring ice
adhesion, b) partial enlarged drawing: 1) sample, 2) glass slide and 3)
Al sheet of equal mass with sample.
Figure 2: SEM images of: a) the prepared film at low magnification
under ammonia/Zn
2+
=0.25, b) the sphere at high magnification, c) the
flat area at high magnification, d), e) high-magnification image of flat
area and a hemispherical protrusion, respectively, f) the prepared film
at low magnification under ammonia/Zn
2+
=0.5:1, g) higher-magnifica-
tion images of the surface, h) the prepared film under ammo-
nia/Zn
2+
=0.75:1

more porous when the molar ratio of ammonia/Zn
2+
increases to 1:1 from Figure 3a. When the molar ratio of
ammonia/Zn
2+
is increased to 1.25:1, the flower-like
microspheres disappear and the microrods begin to
appear (Figure 3b). From the high-resolution image of
the bottom of the microrods (Figure 3c), it can be seen
that the microrods along with the relatively flat regions
grow on the curly nanowalls; the nanowalls are
uniformly crooked and assemble into a porous structure.
From the high-resolution image of the top of the micro-
rods (Figure 3d), we can see that the microrods are uni-
form wound by extremely thin nanosheet-like crystallites
even construct a tower-like superstructure. When the
molar ratio of ammonia/Zn
2+
is further increased to
1.5:1, densely packed microrods grow on the nanowalls
in situ and almost cover it (Figure 3e). From the high-
resolution image of the top of the flower-like microrods
(Figure 3f), it can be seen that the surface of each
microrod is not smooth but scraggly and even construct
the shape of a stepped superstructure. We can infer that
the SEM images showing the prepared surface is com-
posed of micro/nanoscale binary structure, which is
similar to the structure of a lotus leaf.
Figure 4 shows the X-ray diffraction (XRD) patterns
of the as-prepared Zn-Al LDH film and bayerite/Zn-Al
LDH film on the surface of Al sheets under the different
molar ratios of the ammonia/Zn
2+
. For comparison
purposes, the XRD pattern of the pure Al sheet was also
recorded, as shown in Figure 4a. We can see in Figure
4b that the hierarchical structures can be determined by
the mixture of Zn
6
Al
2
(OH)
16
CO
3
·4H
2
O (JCPDS 38-0486;
space group R3m, A = 0.3076 nm, c = 2.280 nm). The
strong peaks at 11.54°, 23.28°, 39° and 46.8°, which
correspond to the crystal orientation, are (003), (006),
(015), and (018), confirming the layer structure of LDH
materials.
38
The diffraction peaks in Figure 4c show the
strong peaks at 18.78°, 20.20°, 27.82°, 40.52°, and
53.12°, which correspond to the bayerite phase
according to PDF card no. 20-0011.
39
The X-ray and the
SEM results indicated that Zn-Al LDH films grew on the
surface of Al sheet first, and then transformed into
microrod bayerite films gradually with the increasing
molar ratio of the ammonia/Zn
2+
. From careful analyses
the peak at 11.85°, 23.68° and the SEM (Figure 3d), we
can get the information that there are some Zn-Al LDH
nanowalls mixing in bayerite microrods when the molar
ratio of the ammonia/Zn was 1.25:1.
The Al sheets play an important role in the formation
of the hierarchical Zn-Al LDH and bayerite micro/nano-
scale structures, which is employed in the synthetic
process to provide Al
3+
for the growth of Zn-Al LDH and
bayerite in situ and to collect relatively ordered micro/
nanoscale hierarchical structures rather than random
powders. Conventionally, Zn-Al LDH are prepared by
the coprecipitation of solutions of Zn
2+
and Al
3+
salts
with alkali, followed by aging at a certain temperature
for a period of time. In that case, the prepared films are
usually aggregates composed of many platelet-like 2D
LDH nanocrystallites, without preferential orientation.
Herein, the Al substrate was introduced to the experi-
ments to solve this problem as well as improve the
adhesion of the film and the substrate effectively. These
experiments results confirmed that the formation of
hydrotalcite-like compounds and bayerite on the Al
sheets is linked to the chemical reactions at the inter-
face.
40,41
NH
3
·H
2
O=NH
4
+
+OH
–
(1)
H. GAO et al.: PREPARATIONS AND CHARACTERIZATIONS OF SUPER-HYDROPHOBIC SURFACES ...
302 Materiali in tehnologije / Materials and technology 52 (2018) 3, 299–306
Figure 3: SEM images of the resultant films prepared at various molar
ratios of ammonia/Zn
2+
of: a) 1:1; b), c), d) 1.25:1, e), f) 1.5:1 while
the reaction time was 5 h and the reaction temperature was 60 °C
Figure 4: XRD patterns of: a) the pure Al sheet, b) the Zn-Al LDH
film on Al, c) bayerite/Zn-Al LDH films on Al

2Al + 2OH
–
+6H
2
O = 2Al(OH)
4
–
+3H
2
(2)
Zn
2+
+ 4OH
–
= [Zn(OH)
4
]
2–
(3)
Al(OH)
4
–
+ [Zn(OH)
4
]
2–
CO
3
2–
+H
2
O = Zn – AlLDH
(4)
NH
4
+
+CH
3
COO
–
=CH
3
COOH + NH
3
(5)
Al(OH)
4
–
+CH
3
COO = Al(OH)
3
+CH
3
COO
–
+H
2
O
(6)
As we can see, two major stages are involved in our
synthetic process. The growth of well-defined arrays of
2D LDH in situ is the first stage. At the initial stage,
NH
3
·H
2
O becomes dissociated (Equation (1)). The dis-
solving Al surface is a prerequisite to start the nucleation
and growth of LDH. Al(OH)
4
–
is first formed at the inter-
face by the reaction between OH
–
and Al (Equation (2)),
at the same time, [Zn(OH)
4
]
2–
is generated in solution
(Equation (3)). The next reaction between Al(OH)
4
–
and
[Zn(OH)
4
]
2–
(Equation (4)) will readily give rise to Zn-Al
LDH. Apparently, the high nucleation density correlated
to the high concentration of Al(OH)
4
–
released from the
Al substrate in the interfacial regions plays a very impor-
tant role in the formation of such oriented nanowalls.
The hetero-nucleation and growth of Al(OH)
3
microrods
is the second stage. The as-formed LDH begins to sim-
ply decrease and gradually grow well-ordered Al(OH)
3
nanorods (Equations (5) and (6)). The concentration of
OH
–
influences the formed amount of Al(OH)
4
–
. The
more Al(OH)
4
–
produced larger porosity on aluminum
substrates, so the porosity of Zn-Al LDH became larger
and larger with an increase of the concentration of
NH
3
·H
2
O, as shown in Figure 2. But the concentration of
Zn
2+
gradually fades away and results in the limited
growth of LDH, meanwhile, the reaction between
CH
3
COO
–
and Al(OH)
4
–
continued and gradually be-
came dominant, which led to a decrease in LDH and an
increase in bayerite.
3.2 Hydrophobic properties
Concerning the porous structures of the product, the
wettability of the film on the aluminum substrate and the
effect of ammonia concentration on the wettability of the
film was investigated. Figure 5 indicates that the contact
angles changed regularly with increasing the molar ratio
of ammonia/Zn
2+
. We can see that the CA first increases
and then decreases slowly with the ammonia concentra-
tion increasing. According to the curve, a clear contrast
can be found that the molar ratio of 1.25:1 is the best of
all. To sum up, the optimal parameters of the preparation
of super-hydrophobic surface were obtained. It is found
that the wettability of all the surfaces changed from
super-hydrophilic to super-hydrophobic after the treat-
ments with stearic acid. Herein, the sample is selected as
a typical case. Without chemical modification, the water
droplet is adsorbed immediately by the treated surface
shows super-hydrophilic (CA < 5°) (Figure 6a) and the
original aluminum sheet shows hydrophilic with a water
CA 85.2°, as shown in Figure 6a. However, after the
simple chemical modification with stearic acid, the water
CA increases to 167.3° (Figure 6b) with a low sliding
angle < 3° (Figure 6c, 6d, 6e, and 6f). In addition, the
CA of the untreated surface after modification with
stearic acid is 98.2°. Therefore, the changes in the CA
observed are due to Al surface roughening not to the
stearic acid crystals.
It is well known that the surface roughness plays a
key role in determining the super-hydrophobic behavior
of a surface. By increasing the surface roughness of the
material we can significantly improve the surface hydro-
phobicity and hydrophilicity. According to previous
research, a hierarchical structure can dramatically
decrease the area of contact between the liquid and the
solid, which can reduce the adhesion of a liquid droplet
H. GAO et al.: PREPARATIONS AND CHARACTERIZATIONS OF SUPER-HYDROPHOBIC SURFACES ...
Materiali in tehnologije / Materials and technology 52 (2018) 3, 299–306 303
Figure 6: a) the CA of original Al sheets, b) The CA of the treated
surface after modification with stearic acid, c), d), e), f) Snapshot
photographs of a 5-μL water droplet rolling off of the resultant
super-hydrophobic surface as the substrate is tilted by ~3.0°, g) the
CA of the treated surface before modification with stearic acid, h) the
CA of the untreated surface after modification with stearic acid
Figure 5: WCA of the resulting surfaces with different molar ratio of
ammonia/Zn
2+
in 5 h reaction time after modified

to the solid surface and sliding angle. Additionally, M.
Nosonovsky
42
demonstrated that a hierarchical roughness
could effectively improve the stability of a composite
interface. Our group also found out that the secondary
structure (i.e., submicrostructure or nanostructure) could
effectively resist the droplets into troughs of the rough
structures.
43,44
To explain the super-hydrophobic me-
chanism of these micro/nanoscale hierarchical structures,
here we present a brief theoretical explanation. The
super-hydrophobic behavior can be characterized in
terms of the Cassie–Baxter model,
45
which is generally
valid for the heterogeneous surfaces composed of air and
a solid exhibiting super-hydrophobicity. Cassie and
Baxter proposed an equation to describe the relationship
between the water CA on a flat surface ( ) and a rough
surface ( ’) composed of a solid and air (Equation (7)):
46
cos ' cos  =− ff
12
(7)
In this equation, f
1
and f
2
are the fractional area
estimated for the solid and air on the surface, respect-
ively, f
1
+ f
2
= 1. The value of ’and of Al sheet surface
in Figure 6a and 6b was 167.3° and 85.2°, successively.
From the Cassie–Baxter model, the value of f
2
is about
0.977. This means that almost the whole surface area is
covered by the air. Due to the large fraction of air
trapped within the interstices of the hierarchical nano-
walls and nanorods, which greatly increase the air/water
interface, preventing the penetration of water droplets
into the grooves effectively, and resulting in super-hydro-
phobicity with a low sliding angle. These results indicate
that the surface micro/nanoscale hierarchical morpho-
logy plays a significant role in the preparation of a
super-hydrophobic surface.
3.3 Ice-phobic properties
Numerous scientific experiments show super-hydro-
phobic surfaces can lead to high ice-phobic properties.
47
Usually, super-hydrophobic films exhibiting a contact
angle hysteresis lower than 5° lead to very high ice-
phobic properties. Reference drops (8 μL) are placed on
the surfaces in order to observe how long it takes for the
drops to freeze on the cold surfaces. The delay time
(DT), the time taken for the drop to freeze, on the surface
at –10°C is roughly recorded by observing the non-trans-
parency of the drop. From Figure 7 we can see that all
the drops are initially transparent (Frame 1), after freez-
ing time (FT) ~260 s, the drop on pure Al surface
becomes non-transparent (Frame 2). After a DT of
~249 s, two drops are non-transparent, on the pure Al
and Zn-Al LDH surfaces, respectively (Frame 3). After a
DT of about ~380 s, another drop is also non-transparent
on the bayerite/Zn-Al LDH surface (Frame 4). We can
see that the bayerite/ Zn-Al LDH surface has a much
longer DT of ~380 s than the Zn-Al LDH surface with a
DT of 249 s. This implies that the differences between
the DTs are attributable to the different structural topo-
logies of the surfaces.
48
As for bayerite/Zn-Al LDH
surface, the case of the micro/nanostructure, because of
combining both nanostructure and microstructure, a high
ratio of trapped-air appears on the micro/nanostructure
surface, thus the drop is greatly suspended. Concerning
the case of water-condensed drops, the tiny condensed
drop, a result of the environment temperature gradient,
would first sit on the nano-steps and then grow upwards,
and is subsequently suspended onto the microrods with
nano-steps to decrease the wet contact with the solid
surface. In addition, the thermal conductivity of solid is
better than air, so the freezing time of micro/nanoscale
hierarchical bayerite/Zn-Al LDH structure with less
contact area is longer than pure Al and LDH, which just
coincides with the experiment results.
The ice adhesion was evaluated by spinning the sam-
ples at constantly increasing speed until it delaminated.
The ice-adhesion reduction factors relative to the type of
sample and the corresponding CA and SA are displayed
in Figure 8. All the values are averaged on 5 experi-
ments. The adhesion-reduction factors were computed
using the following equation: AFR =  Al
/ film
, where  Al
and  film
are the ice shearing force of pure Al and treated
surfaces. The AFR of Zn-Al LDH and bayerite/Zn-Al
LDH surfaces were 3.05 and 8, respectively. From
Figure 9 we can see that the surfaces remain active after
several ice-shedding events. The morphology of Zn-Al
LDH and bayerite/Zn-Al LDH surfaces have no changes
nearly after the multiple thaw cycles. In addition, the CA
of bayerite/Zn-Al LDH surfaces can still reach 156.3°.
These results indicated that the prepared super-hydro-
phobic and ice-phobic surface possessed excellent
mechanically and chemically stable properties. While
some coatings have greater ice-phobic properties, the
balance between mechanical and ice-phobic properties is
needed. We found that the adhesion strength of the ice on
these samples depends not only on the CA but also on
the SA, which has a significant reduction with a decrease
of the SA. In comparison with the untreated samples, the
super-hydrophobic film on the aluminum surface can
effectively mitigate the ice accretion. During ice
accumulation, the water droplets did not penetrate the
H. GAO et al.: PREPARATIONS AND CHARACTERIZATIONS OF SUPER-HYDROPHOBIC SURFACES ...
304 Materiali in tehnologije / Materials and technology 52 (2018) 3, 299–306
Figure 7: a) In-situ observation of ice formations on pure Al, Zn-Al
LDH, and bayerite/Zn-Al LDH surfaces at –10 °C with delay times
(DT), b) contrast on delay times of ice formation on different surfaces
at a sub-zero temperature of –10 °C

micro/nanoscale hierarchical structures, which resulted
in air pockets being formed at the ice–solid interface,
consequently leading to poor ice-adhesion strengths.
Therefore, the higher surface roughness is responsible
for super-hydrophobic properties and ice-phobic proper-
ties, which can be verified well by the SEM observation
and the hydrophobic and ice-phobic results.
4 CONCLUSIONS
A super-hydrophobic and ice-phobic surface com-
posed of Zn-Al LDH nanowalls and bayerite/Zn-Al LDH
nanorods with micro/nanoscale hierarchical structures
was fabricated on aluminum by using a simple hyd-
rothermal reaction method after simple chemical
modification. After processing, the wettability of the
micro/nanoscale hierarchical structure surface can be
changed from super-hydrophilic to super-hydrophobic
with a CA as high as 167.3° and a low sliding angle < 3°.
The experiments of the static analysis of the freezing
time and ice adhesion on different surfaces proved that
the prepared films possess highly ice-phobicity. In addi-
tion, they remained active after ten ice-shedding events
using an aggressive Spinner Coater technique, which
shows the potential for protecting new, high-voltage
overhead aluminum cables against excessive ice or snow
accumulation. Further investigations have indicated that
the ammonia concentration plays a major role in the
surface structure, super-hydrophobicity and ice-phobcity.
The present study opens up a very simple and econo-
mical way for the fabrication of super-hydrophobic and
ice-phobic surfaces with the biomimic micro/nanostruc-
tures on engineering metallic materials.
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