H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
789–793
INVESTIGATION OF THE STATIC ICING PROPERTY FOR
SUPER-HYDROPHOBIC COATINGS ON ALUMINIUM
PREISKAVA LASTNOSTI STATI^NE ZALEDENITVE PRI
SUPERHIDROFOBNIH PREVLEKAH NA ALUMINIJU
Hai Yun Jin1, Shi Chao Nie1, Yu Feng Li1, Tian Feng Xu1, Peng Zhang1, Wen Li2
1Xi’an Jiaotong University Xi’an, State Key Laboratory of Electrical Insulation and Power Equipment, 710049, China
2Hubei Polytechnic University, Hubei Key Laboratory of Mine Environmental Pollution Control & Remediation and School of Chemical and
Materials Engineering, Huangshi, 435003, China
lxfzzl@126.com
Prejem rokopisa – received: 2016-09-02; sprejem za objavo – accepted for publication: 2016-10-24
doi:10.17222/mit.2016.266
An anti-icing aluminium with a super-hydrophobic surface was fabricated. A static icing system was developed to investigate
the process of static icing for samples with different contact angles (CAs), and the contact state was also researched. The results
showed that the water droplets’ solidification time on a super-hydrophobic surface was much longer than for other surfaces at
the same temperature. A water droplet on the super-hydrophobic surface had a much smaller contact area than on another
surfaces. In addition, the air gap between the droplet and the super-hydrophobic surface retarded and prevented any exchange of
heat. The temperature affected the transition from the Cassie state to the Wenzel state for super-hydrophobic aluminium.
Keywords: aluminium coating, static icing, solidification time, super hydrophobicity
Narejeno je bilo prepre~evanje zaledenitve aluminija s superhidrofobno plastjo. Sistem stati~ne zaledenitve je bil razvit za
preiskavo procesa stati~ne zaledenitve za vzorce z razli~nimi kontaktnimi koti (angl. CA), preiskano pa je bilo tudi stanje
kontakta. Rezultati so pokazali, da je ~as strjevanja vodnih kapljic na superhidrofobni povr{ini precej dalj{i kot na drugih
povr{inah pri enaki temperaturi. Vodna kapljica je imela na superhidrofobni povr{ini precej manj{o kontaktno povr{ino kot na
drugih povr{inah. Poleg tega je zra~na re`a med kapljicami in superhidrofobno povr{ino zaustavljala in prepre~evala vsako
izmenjavo toplote. Temperatura je vplivala na prehod iz stanja Cassie v stanje Wenzel za superhidrofobni aluminij.
Klju~ne besede: aluminijeva prevleka, stati~na zaledenitev, ~as strjevanja, superhidrofobnost
1 INTRODUCTION
Icing is a natural phenomenon that occurs on the
surfaces of objects in the bad conditions of low tem-
peratures and freezing rain. An ice disaster is a serious
natural disaster for a power system,1,2 especially for
abnormal weather with the El Nino and La Nina pheno-
mena, the ice disaster became more frequently. In the
past, for a long time, de-icing methods were active, and
external energy was required to deal with the icing on the
surfaces.3,4 In recent years, anti-icing coatings that could
improve the icing performance effectively received more
attention, such as electric heating coatings, photo-ther-
mal coatings and hydrophobic coatings. Two features are
required for promising anti-icing materials. One is that
overcooled water droplets can roll off from the surface
easily before crystallization. And the other, is that the ice
adhesion will be weaker when the ice is accumulated on
the surface.5 Super-hydrophobic coatings are promising
materials due to the retardation and prevention of icing
and have become a focus of study.6–16
The researches on super-hydrophobic materials for
anti-icing are abundant; however, there are few reports
about the process in detail of water condensing and then
freezing to form ice on a cold super-hydrophobic
surface.14,15 M. He et al.14 fabricated super-hydrophobic
surfaces with isotactic polypropylene; the results showed
that the micro- and nanometre structures led to a delay of
the solidification of liquid water at the three phase line
region. M. He et al.14 fabricated super-hydrophobic
surfaces with ZnO nanorod arrays; the process of icing at
different temperatures below the freezing point was
studied.15 However, the effect on the mechanism is un-
clear and needs more systemic and deep research.
Furthermore, there are few reports about super-hydro-
phobic aluminium used in a power transmission line.
Aluminium has good thermal conductivity, so different
phenomenon may occur in the process of icing. S. L.
Zheng et al.17,18 fabricated super-hydrophobic aluminium
coatings using the Electrochemical Corrosion Method.
The adhesion strength of ice on coatings was researched,
but the process of solidification of the droplets was not
mentioned.
In this paper, super-hydrophobic coatings were fabri-
cated using the Chemical Etching Method and a static
icing system was developed to study the ice crystalliza-
tion process of super-cooled water droplets on different
coatings and different temperatures below the freezing
point. The results are helpful to provide theoretical basis
effectively for the design and fabrication of super-hydro-
phobic transmission aluminium conductors in the future.
Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793 789
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 620.1:67.017:669.058 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)789(2017)
2 EXPERIMENTAL PART
In order to fabricate samples with different hydro-
phobicities, a pre-treatment was necessary for the
electrically pure aluminium. First, 1000 mesh sandpaper
was used to polish the surface of the aluminium for the
purpose of removing dense oxide layer, then using water
and absolute ethyl alcohol to clean filth and grease on
the surface by ultrasonic cleaning for 10 min. After
pre-treatment, the aluminium can be used as a hydrophi-
lic aluminium surface samples (A1). Using stearic acid
to modify the aluminium surface, stearic acid is a kind of
long-chain fatty acid and has a lower free energy. As
shown in Figure 1, a dehydration reaction would happen
between hydroxyl groups on the surface of the
aluminium and the carboxyl group in stearic acid. Then a
tight and thin coating with low free energy would be
generated on the surface of the aluminium and hydro-
phobic aluminium surface samples (A2) could be
obtained. As for super-hydrophobic aluminium surface
samples, put the aluminium after pre-treatment to the
20 % of mass fractions of hydrochloric acid to etch for 1
min, after drying put it in the 1 % of mass fractions of
stearic acid to modify the surface energy for 15 min,
finally put the sample into the 90 °C oven to fabricate
super-hydrophobic aluminium surface samples (A3).19
Figure 2 shows that static icing system was made up
of a Peltier chip, the cooling fan, temperature sensors
and a temperature controller. The Peltier chip
(TEC-1206) was supplied with power by a 12V DC
powervsupply, the temperature difference between cold
end and hot end was 75 °C and the temperature on the
cold end could be –20 °C. The microprocessor in the
semiconductor refrigeration temperature controller
(TTC-B) was used to control the DC power in order to
realize the temperature control. The test of static icing
characteristics was completed at room temperature. In
order to compare the process of icing, three kinds of
typical aluminium wire samples (A1, A2 and A3) were
observed. Detailed test steps was as follows:
1) Setting the surrounding environment in a dark-
room, the entire Peltier cooling system was placed in a
simple studio, the illumination was backlit bright LED
projection mode, and the Peltier chip temperature was
kept constant by adjusting the parameters of the PID
control system (–5 °C± 0.5 °C, –3 °C ± 0.5 °C and –1 °C
± 0.5 °C), and recorded both the ambient temperature
and the humidity value;
2) Turning on the power of the Peltier cooling sys-
tem. When the temperature of the surface was constant,
we placed the samples on the Peltier surface, 15 μL
water droplets were injected onto the samples’ surface
rapidly by using a micro-syringe, and we recorded the
solidification process of droplets using a digital camera.
3 RESULTS AND DISCUSSION
Figure 3 shows the CA of droplets on 3 typical
aluminium samples at room temperature. The CA on A1,
A2 and A3 were 82°, 109° and 152°, respectively. Heat
transfer happened between the droplets and the cold
surface. Droplets placed on the surface of sample were
liquid at first, and because of the results of differences of
hydrophobicity, different contact states on three different
surfaces were observed. As shown in Figure 3, the
droplet on the surface of A1 was like a pan upside down,
it had largest contact area; the droplet on the surface of
A2 was like a semicircle, it had larger contact area and
droplet on the surface of A3 was like a ball, it had the
smallest contact area.
Figure 4 shows SEM images and 3D laser scanning
of A3, it can be seen that A3 had many micron structure
pits and protrusions, and the size of these pits and pro-
trusions was about 3 μm. From Figure 4b and 4d, a
rough structure similar with building blocks could be
observed, so the pits and protrusions connected with
each other. Besides, the micron pits and protrusions had
submicron even nano-sized rough structure on their
surfaces. This surface structure was easy to form super
hydrophobicity with a water droplet.20
Figure 5 showed droplets’ solidification process on
samples (surface temperature is –3 °C). Droplets on the
surface of samples were liquid at first, as the time pro-
H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
790 Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Contact angle of droplets on 3 typical aluminium samples at
room temperature: a) A1, b) A2 and c) A3
Figure 1: Modification mechanism of stearic acid on the aluminium
surface
Figure 2: Schematic diagram of static icing test system
longed, gas/liquid/solid three phase contact line between
droplet and surface of sample expanded, and the contact
area between the droplet and the surface increased,
which meant the CA decreasing as the time prolonged. A
phase transition occurred in the droplets because of the
heat transfer between the cold surfaces and the droplets,
which meant the transition from liquid to solid for the
droplets. Also shown was the process of solidification
after the phase transition, a clear liquid/solid line could
be observed and this line grew up from bottom to the top.
Under the liquid/solid line, the droplets were already
frozen ice. Above the line, the droplets were still liquid.
The liquid/solid line grew up at different speeds on three
typical aluminium surfaces; the line grew up quickly on
the A1 and A2 but relatively slowly on the A3. The line
went up to the top of the droplet, and finally, a peak on
the top of droplet was generated, which meant the end of
solidification process.
Figure 5 showed the phase transition time on diffe-
rent surfaces. The phase transition starting times for A1,
A2 and A3 were 20 s, 34 s and 348 s, respectively. The
phase-transition time on A3 was much longer than the
other two surfaces. The solidification time for total drop-
let for A1, A2 and A3 were 38 s, 49 s and 404 s,
respectively, the solidification time increased with
increasing CA. The solidification time of the droplet for
A3 was 10.6 times that of for A1 and 8.2 times that of
for A2. Therefore, the solidification process of water
H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793 791
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Microstructure of A3: a) The SEM image magnified 1000 times, b) the SEM image magnified 5000 times, c) the 3D laser scanning
magnified 10 times and d) the 3D laser scanning magnified 20 times
Figure 5: Droplets’ solidification process on the surfaces of 3 typical
aluminum wire samples, the red lines are the boundary of liquid and
solid: a) A1, b) A2 and c) A3
2 EXPERIMENTAL PART
In order to fabricate samples with different hydro-
phobicities, a pre-treatment was necessary for the
electrically pure aluminium. First, 1000 mesh sandpaper
was used to polish the surface of the aluminium for the
purpose of removing dense oxide layer, then using water
and absolute ethyl alcohol to clean filth and grease on
the surface by ultrasonic cleaning for 10 min. After
pre-treatment, the aluminium can be used as a hydrophi-
lic aluminium surface samples (A1). Using stearic acid
to modify the aluminium surface, stearic acid is a kind of
long-chain fatty acid and has a lower free energy. As
shown in Figure 1, a dehydration reaction would happen
between hydroxyl groups on the surface of the
aluminium and the carboxyl group in stearic acid. Then a
tight and thin coating with low free energy would be
generated on the surface of the aluminium and hydro-
phobic aluminium surface samples (A2) could be
obtained. As for super-hydrophobic aluminium surface
samples, put the aluminium after pre-treatment to the
20 % of mass fractions of hydrochloric acid to etch for 1
min, after drying put it in the 1 % of mass fractions of
stearic acid to modify the surface energy for 15 min,
finally put the sample into the 90 °C oven to fabricate
super-hydrophobic aluminium surface samples (A3).19
Figure 2 shows that static icing system was made up
of a Peltier chip, the cooling fan, temperature sensors
and a temperature controller. The Peltier chip
(TEC-1206) was supplied with power by a 12V DC
powervsupply, the temperature difference between cold
end and hot end was 75 °C and the temperature on the
cold end could be –20 °C. The microprocessor in the
semiconductor refrigeration temperature controller
(TTC-B) was used to control the DC power in order to
realize the temperature control. The test of static icing
characteristics was completed at room temperature. In
order to compare the process of icing, three kinds of
typical aluminium wire samples (A1, A2 and A3) were
observed. Detailed test steps was as follows:
1) Setting the surrounding environment in a dark-
room, the entire Peltier cooling system was placed in a
simple studio, the illumination was backlit bright LED
projection mode, and the Peltier chip temperature was
kept constant by adjusting the parameters of the PID
control system (–5 °C± 0.5 °C, –3 °C ± 0.5 °C and –1 °C
± 0.5 °C), and recorded both the ambient temperature
and the humidity value;
2) Turning on the power of the Peltier cooling sys-
tem. When the temperature of the surface was constant,
we placed the samples on the Peltier surface, 15 μL
water droplets were injected onto the samples’ surface
rapidly by using a micro-syringe, and we recorded the
solidification process of droplets using a digital camera.
3 RESULTS AND DISCUSSION
Figure 3 shows the CA of droplets on 3 typical
aluminium samples at room temperature. The CA on A1,
A2 and A3 were 82°, 109° and 152°, respectively. Heat
transfer happened between the droplets and the cold
surface. Droplets placed on the surface of sample were
liquid at first, and because of the results of differences of
hydrophobicity, different contact states on three different
surfaces were observed. As shown in Figure 3, the
droplet on the surface of A1 was like a pan upside down,
it had largest contact area; the droplet on the surface of
A2 was like a semicircle, it had larger contact area and
droplet on the surface of A3 was like a ball, it had the
smallest contact area.
Figure 4 shows SEM images and 3D laser scanning
of A3, it can be seen that A3 had many micron structure
pits and protrusions, and the size of these pits and pro-
trusions was about 3 μm. From Figure 4b and 4d, a
rough structure similar with building blocks could be
observed, so the pits and protrusions connected with
each other. Besides, the micron pits and protrusions had
submicron even nano-sized rough structure on their
surfaces. This surface structure was easy to form super
hydrophobicity with a water droplet.20
Figure 5 showed droplets’ solidification process on
samples (surface temperature is –3 °C). Droplets on the
surface of samples were liquid at first, as the time pro-
H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
790 Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Contact angle of droplets on 3 typical aluminium samples at
room temperature: a) A1, b) A2 and c) A3
Figure 1: Modification mechanism of stearic acid on the aluminium
surface
Figure 2: Schematic diagram of static icing test system
longed, gas/liquid/solid three phase contact line between
droplet and surface of sample expanded, and the contact
area between the droplet and the surface increased,
which meant the CA decreasing as the time prolonged. A
phase transition occurred in the droplets because of the
heat transfer between the cold surfaces and the droplets,
which meant the transition from liquid to solid for the
droplets. Also shown was the process of solidification
after the phase transition, a clear liquid/solid line could
be observed and this line grew up from bottom to the top.
Under the liquid/solid line, the droplets were already
frozen ice. Above the line, the droplets were still liquid.
The liquid/solid line grew up at different speeds on three
typical aluminium surfaces; the line grew up quickly on
the A1 and A2 but relatively slowly on the A3. The line
went up to the top of the droplet, and finally, a peak on
the top of droplet was generated, which meant the end of
solidification process.
Figure 5 showed the phase transition time on diffe-
rent surfaces. The phase transition starting times for A1,
A2 and A3 were 20 s, 34 s and 348 s, respectively. The
phase-transition time on A3 was much longer than the
other two surfaces. The solidification time for total drop-
let for A1, A2 and A3 were 38 s, 49 s and 404 s,
respectively, the solidification time increased with
increasing CA. The solidification time of the droplet for
A3 was 10.6 times that of for A1 and 8.2 times that of
for A2. Therefore, the solidification process of water
H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793 791
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 4: Microstructure of A3: a) The SEM image magnified 1000 times, b) the SEM image magnified 5000 times, c) the 3D laser scanning
magnified 10 times and d) the 3D laser scanning magnified 20 times
Figure 5: Droplets’ solidification process on the surfaces of 3 typical
aluminum wire samples, the red lines are the boundary of liquid and
solid: a) A1, b) A2 and c) A3
droplet could be greatly delayed by the super-hydro-
phobic surface (A3).
The results show that the CA directly affects the
freezing process of the droplets. For A1 and A2, the con-
tact states between droplets and surfaces were the
Wenzel state. The contact angle on A2 was larger than
on A1, so the contact area between the droplet and A2
was smaller. For A3, the contact state of ball-like droplet
made the smallest contact area. It was known that the
heat-transfer rate was related to the contact area21, solidi-
fication time on A3 was the longest. At the same time,
the vertical height of the droplet on A3 was greater,
which meant more distance and time should be required
for the solidification process from the bottom to the top
of the droplet, therefore, the solidification time on A3
was the longest. On the other hand, for A3, the contact
state was the Cassie state. There were a lot of air films in
the contact interface and the air films played a role of
energy barrier, which greatly delayed the heat exchange
process between the droplets and surfaces, so the soli-
dification time of droplets for A3 surface was also much
longer than the other two surfaces.
After solidification, the top of the droplet would form
a peak, and there were two main reasons for this: one
was that the volume of droplets increased from liquid to
solid; the other was that the droplet contained some of
air, the dissolved air would form bubbles and released
out of droplet during the process of phase transition and
solidification. However, the peak of the droplet on the
super-hydrophobic surface was more acute, which
indicated that there was other air in the droplet besides
the dissolved air, the explanation for this phenomenon
could be the involvement of an air film between the
droplet and the surface of the super-hydrophobic alumi-
nium sample. Results might indirectly prove that the
contact state of the droplet and the super-hydrophobic
aluminium surface was in the Cassie contact state.
Figure 6 shows the relationship between the solidifi-
cation time and surface temperature (the data of solidifi-
cation time was average data of five experiments). When
the surface temperature was –1 °C, the solidification
times for A1, A2 and A3 were 41.7 s, 50.7 s and 803.7 s,
respectively. When the surface temperature of the
samples was –3 °C, the solidification time for A1, A2
and A3 were 36 s, 47.7 s and 390 s respectively. When
the surface temperature was –5 °C, the data were 31.7 s,
35 s and 186 s, respectively. From Figure 6 it can be
seen that with the reduction of surface temperature, the
solidification time of the droplets on three typical sam-
ples should decrease. This was because of the increase of
the energy exchange speed and the decrease of
nucleation barrier of surfaces with decreasing surface
temperature. At different surface temperatures the soli-
dification time of the droplets on A3 was much longer
than A1 and A2, which showed the excellent static
anti-icing performance of A3. When the surface tem-
peratures were –1 °C, –3 °C and –5 °C, the droplets’
solidification times on A3 were 803.7 s, 390.0 s and
186.0 s, respectively. With a decreasing surface tempe-
rature, the solidification time of the droplets on A3 fell
more rapidly than the other two typical surfaces. Due to
the instability of the air film between the droplets and
surface,22 the contact state change from the Cassie state
to the Wenzel state could become easier for A3, with tem-
perature decreasing (Figure 5c) for 0 s and 5 min 17 s.
4 CONCLUSIONS
In summary, the super-hydrophobic coatings on alu-
minium (CA=152°) were fabricated by Chemical
Etching and Modification Method. For the super-hydro-
phobic surface, the solidification and phase-change time
of the droplets were much longer than the hydrophilic
aluminium surface and the hydrophobic aluminium
surface. This was because for the super-hydrophobic
aluminium surfaces the freezing process could be
retarded and prevented by both smaller contact area and
air film between droplet and surface (Cassie contact
state), which influenced the thermal exchange between
the droplet and the surface.
Due to that the air film latched by the droplet on the
super-hydrophobic surface was not stable, with the
temperature decreasing, the droplet transited from the
Cassie contact state to the Wenzel contact state more
easily.
Acknowledgements
This research was financially supported by the
National Natural Science Foundation of China (No.
51272208), the Natural Science Foundation of Hubei
Province (2010CDA026), the Key Program of Hubei
Provincial Department of Education (Z20104401), the
Outstanding Scientific and Technological Innovation
H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
792 Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Relationship between solidification time and surface tempe-
rature: the picture shows the initial liquid state and the final solid state
of the water droplets on A3 when the surface temperature is –3 °C
Team Project of Universities in Hubei Province
(T201423), and the Talent Program of Hubei Polytechnic
University (11yjz04R).
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num surface for corrosion-resistant, self-cleaning,and anti-icing
applications, Materials and Design, 93 (2016), 261–270,
doi:10.1016/j.matdes.2015.12.155
19 Z. Huang, Y. Li, P. Jin, M. Hu, B. He, N. Gao, H. Zhang, H. Jin,
Fabrication of Aluminum Superhydrophobic Surface with Facile
Chemical Etching Method, Materials Science Forum, 804 (2015),
103–106, doi:10.4028/www.scientific.net/MSF.804.103
20 L. Jiang, Nanostructured Materials with Super-hydrophobic Sur-
face-from Nature to Biomimesis, Chemical Industry and Engineering
Progress, 22 (2003) 12, 1258–1264, doi:10.3321/j.issn:1000-6613.
2003.12.002
21 O. R. Enriquez, A.G. Marin, K. G. Winkels, J. H. Snoeijer, Freezing
singularities in water drops, Physics of Fluids, 24 (2012) 9, 091102,
doi:10.1063/1.4747185
22 H. Y. Jin, Y. F. Li, P. Zhang, S. C. Nie, N. K. Gao, The investigation
of the wetting behavior on the red rose petal, Applied physics Letter,
108 (2016) 15, 151605, doi:10.1063/1.4947057
H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793 793
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
droplet could be greatly delayed by the super-hydro-
phobic surface (A3).
The results show that the CA directly affects the
freezing process of the droplets. For A1 and A2, the con-
tact states between droplets and surfaces were the
Wenzel state. The contact angle on A2 was larger than
on A1, so the contact area between the droplet and A2
was smaller. For A3, the contact state of ball-like droplet
made the smallest contact area. It was known that the
heat-transfer rate was related to the contact area21, solidi-
fication time on A3 was the longest. At the same time,
the vertical height of the droplet on A3 was greater,
which meant more distance and time should be required
for the solidification process from the bottom to the top
of the droplet, therefore, the solidification time on A3
was the longest. On the other hand, for A3, the contact
state was the Cassie state. There were a lot of air films in
the contact interface and the air films played a role of
energy barrier, which greatly delayed the heat exchange
process between the droplets and surfaces, so the soli-
dification time of droplets for A3 surface was also much
longer than the other two surfaces.
After solidification, the top of the droplet would form
a peak, and there were two main reasons for this: one
was that the volume of droplets increased from liquid to
solid; the other was that the droplet contained some of
air, the dissolved air would form bubbles and released
out of droplet during the process of phase transition and
solidification. However, the peak of the droplet on the
super-hydrophobic surface was more acute, which
indicated that there was other air in the droplet besides
the dissolved air, the explanation for this phenomenon
could be the involvement of an air film between the
droplet and the surface of the super-hydrophobic alumi-
nium sample. Results might indirectly prove that the
contact state of the droplet and the super-hydrophobic
aluminium surface was in the Cassie contact state.
Figure 6 shows the relationship between the solidifi-
cation time and surface temperature (the data of solidifi-
cation time was average data of five experiments). When
the surface temperature was –1 °C, the solidification
times for A1, A2 and A3 were 41.7 s, 50.7 s and 803.7 s,
respectively. When the surface temperature of the
samples was –3 °C, the solidification time for A1, A2
and A3 were 36 s, 47.7 s and 390 s respectively. When
the surface temperature was –5 °C, the data were 31.7 s,
35 s and 186 s, respectively. From Figure 6 it can be
seen that with the reduction of surface temperature, the
solidification time of the droplets on three typical sam-
ples should decrease. This was because of the increase of
the energy exchange speed and the decrease of
nucleation barrier of surfaces with decreasing surface
temperature. At different surface temperatures the soli-
dification time of the droplets on A3 was much longer
than A1 and A2, which showed the excellent static
anti-icing performance of A3. When the surface tem-
peratures were –1 °C, –3 °C and –5 °C, the droplets’
solidification times on A3 were 803.7 s, 390.0 s and
186.0 s, respectively. With a decreasing surface tempe-
rature, the solidification time of the droplets on A3 fell
more rapidly than the other two typical surfaces. Due to
the instability of the air film between the droplets and
surface,22 the contact state change from the Cassie state
to the Wenzel state could become easier for A3, with tem-
perature decreasing (Figure 5c) for 0 s and 5 min 17 s.
4 CONCLUSIONS
In summary, the super-hydrophobic coatings on alu-
minium (CA=152°) were fabricated by Chemical
Etching and Modification Method. For the super-hydro-
phobic surface, the solidification and phase-change time
of the droplets were much longer than the hydrophilic
aluminium surface and the hydrophobic aluminium
surface. This was because for the super-hydrophobic
aluminium surfaces the freezing process could be
retarded and prevented by both smaller contact area and
air film between droplet and surface (Cassie contact
state), which influenced the thermal exchange between
the droplet and the surface.
Due to that the air film latched by the droplet on the
super-hydrophobic surface was not stable, with the
temperature decreasing, the droplet transited from the
Cassie contact state to the Wenzel contact state more
easily.
Acknowledgements
This research was financially supported by the
National Natural Science Foundation of China (No.
51272208), the Natural Science Foundation of Hubei
Province (2010CDA026), the Key Program of Hubei
Provincial Department of Education (Z20104401), the
Outstanding Scientific and Technological Innovation
H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
792 Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Relationship between solidification time and surface tempe-
rature: the picture shows the initial liquid state and the final solid state
of the water droplets on A3 when the surface temperature is –3 °C
Team Project of Universities in Hubei Province
(T201423), and the Talent Program of Hubei Polytechnic
University (11yjz04R).
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H. Y. JIN et al.: INVESTIGATION OF THE STATIC ICING PROPERTY FOR SUPER-HYDROPHOBIC COATINGS ...
Materiali in tehnologije / Materials and technology 51 (2017) 5, 789–793 793
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS