Fabrication and characterization of a cotton candy like surface with superhydrophobicity

Applied Surface Science 257 (2011) 6044–6048

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Fabrication and characterization of a cotton candy like surface with superhydrophobicity You Hu a,∗ , Chengya Huang a , Dong Su a , Qiangwei Jiang a , Yunfeng Zhu b a b

School of Materials Science and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, PR China No. 5 XingYeDong Road, Zone A, Shishan Science & Technology Industry Park, Nanhai, Foshan, Guangdong 528225, PR China

a r t i c l e

i n f o

Article history: Received 21 December 2010 Received in revised form 25 January 2011 Accepted 25 January 2011 Available online 2 March 2011 Keywords: Superhydrophobic D5 Cotton candy Capillary force

a b s t r a c t Superhydrophobic thin films were prepared on glass by air-brushing the in situ polymerization compositions of D5 /SiO2 . The wettability and morphology were investigated by contact angle measurement and scanning electron microscopy. The most superhydrophobic samples prepared had a static water contact angle of 157◦ for a 5 ␮l droplet and a sliding angle of ∼1◦ for 10 ␮l droplet. Thermal stability analysis showed that the surface maintained superhydrophobic at temperature up to 450 ◦ C. Air trapping and capillary force on superhydrophobic behavior were evaluated. © 2011 Elsevier B.V. All rights reserved.

1. Introduction When a water droplet comes in contact with a clean glass surface, we will see that water spread on the glass surface; but if the water droplet is dropped on a Paraffin plate, it will be spherical, the former is usually called as “wetting”, the latter is “non-wetting” [1]. The contact angle, which is a very critical evaluate parameter for wettability, when it is higher than 150◦ and the sliding angle or tilt angle is less than 10◦ , it is defined as superhydrophobicity in the literatures, which exhibits great water repellency [2,3]. Recently, so-called superhydrophobic surfaces have attracted considerable interest for its practical applications, such as dust-free coating, prevention of snow sticking, and others [4]. Inspired in many cases by the natural world, especially the “lotus effect” [5], attributed to its significant micro- and nanohierarchical texturing coated with a waxy surface of low surface energy [6], many researches have done to realize superphydrophobicity in the latest decades. For example, there are sol–gel method combing with self-assembled monolayers (SAMs) [7–13]; forming of binary structural polymeric surface under ambient atmosphere [14]; template method [15,16]; synthesis of new complex nanostructures [17,18]; plasma technology [6,19]; electrochemistry [20]. However, the expensive low surface energy compounds which are used to modify the surface, such as fluorine and fluorine-containing

∗ Corresponding author. E-mail address: [email protected] (Y. Hu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.01.121

compounds, have become a distinct shortcoming, and the complex fabrication process in some reports also limit their practical usage. In the recent experiment, we prepared the ring-opening polymerization product of D5 (decamethylcyclopentasiloxane) (Fig. 1), the main chains of the polymerization product are composed of Si–O polar bonds, but the non-polar alkyl groups arrange outward directionally, which prevent water penetrating into the internal, rending it possess hydrophobicity [21]. This siloxane polymer has good water repellency, and the contact angle is about 101◦ when coated on the clean glass with a smooth surface. Literatures have also reported polymers containing the –Si(CH3 )2 groups possess excellent hydrophobicity [22]. Here, we wish to report a lotus-like superphydrophobic surface at the glass surface based on the above. The fabrication process is relative sample and the raw materials are inexpensive. The resulted surfaces have a micro–nanoscale binary structure and have a good temperature resistance. 2. Experimental 2.1. Materials (decamethylcyclopentasiloxane) and KH570 (␥D5 methacryloxypropyltrimethoxysilane) were purchased from GuangZhou JuChenZhaoYeYoujiguiyuanliao Co., Ltd. Sulfuric acid (98%), absolute ethanol (99.8%) and deionic water were purchased from GuangZhou Qianhui Bose Instrument Co., Ltd. SiO2 (HL-300, with a diameter of 15 nm) was kindly supplied by Guangzhou JBS High-High-tech & Industry Co., Ltd. All the reagents were used as received without further purification.

Y. Hu et al. / Applied Surface Science 257 (2011) 6044–6048

Fig. 1. Ring-opening polymerization of D5 .

2.2. Sample preparation D5 and ethanol were mixed with various weigh SiO2 particles, vibrated by ultrasonic washer for 10 min then added three drops of Sulfuric acid (about 0.06 g) and vibrated for another 10 min. The mixtures were placed in weighing bottles, sealed and stood for 24 h at room temperature. The weight of D5 and ethanol were constant 3.00 g and 0.50 g, respectively. The weight of SiO2 changed from 0, 0.01 g, 0.02 g, 0.03 g, 0.04 g, 0.05 g and 0.07 g, which were corresponding to different weight ratio of SiO2 particles to total solution, 0, 0.28%, 0.57%, 0.84%, 1.11%, 1.39% and 1.93%, respectively. Ethanol was used to dilute the concentration of sulfuric acid so that to slow down the ring-opening polymerization rate of D5 . Next, took 1.5 ml of the mixtures from the weighing bottle into a small beaker, added 0.04 g KH570 and vibrated by ultrasonic washer for 20 min just before usage. The resulted mixes were slight milk-like solution. Microscope slides (2.5 cm × 7.6 cm) were used as substrates: they were cleaned by washing in a standard RCAI solution (NH4 OH:H2 O2 :H2 O = 1:1:5 by volume) at 70 ◦ C for 10 min and then

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rinsed several times in deionic water. The films were prepared by air-brushing, during which the glasses substrate were kept at a distance of about 14 cm from the air-brush applicator, the gauge pressure was 0.15 MPa and 0.5 ml of the above solution were deposited for only one time. The different SiO2 weight ratio, 0, 0.28%, 0.56%, 0.84%, 1.11%, 1.39% and 1.93% were corresponding to samples A0, A1, A2, A3, A4, A5 and A6, respectively. The coated glass substrates were dried at room temperature for a few minutes then subjected to an oven at 140 ◦ C for 2 h. 2.3. Characterization Water contact angles (WCAs) were measured with a deionized water droplet of 5 ␮l (the diameter of a spherical droplet is about 2.1 mm) on a JY-82 (Dingsheng, Chende, China) instrument using a micropipette at room temperature. All the contact angles were determined by averaging values measured at five different points on each sample surface. The sliding angle was measured by tilting the sample stage from 0◦ to higher angles and then putting a droplet of 10 ␮l on the sample using a micropipette. When the droplet rolled off the surface, the angle of the sample stage was the sliding angle. Morphology of the surface was characterized with an EVO 18 (Carl Zeiss, Germany) scanning electron microscopy. FI-IR spectra were obtained on a Vertex-70 spectrometer (BRUKEROPTICS, Germany) using KBr pellets. The adhesion between the films and the glass substrate was measured according to GB/T-9286-

Fig. 2. Morphology of the coated surfaces with different SiO2 content. (A) 0.56 wt%; (B) 0.84 wt% and (C) 1.93 wt%; (D) is the magnification of the microscale protrusion.

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1998 with 3 M Scotch 810# adhesive tape. A Muffle Furnace was used for thermal stability analysis.

-1

170 0 160

b

150

1

140 2 130 120

3

110

a

4

100 90

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

5

Nanoparticles content (wt%) Fig. 4. Effect of nanoparticles content on contact angles and adhesion grade.

GB/T-9286-1998, the adhesion can meet the requirement in practical application when it is not lower than second grade. A value of 0.84 wt% SiO2 particles can be picked out as the most satisfying particles content, which simultaneously contained over-all properties of adhesion and hydrophobicity. And sliding angles (SAs) were measured by a simple stage tilting experiment with the droplets of 10 ␮l size. For samples with WCAs higher than 155◦ , the sliding angles ranged from 0 to 1◦ . This meets the second portion of the definition of superhydrophobic materials on the point that they have a water sliding angle of less than 10◦ . For samples A0, the sliding angle was about 40◦ ; for A1 and A2, water cannot slide off the surfaces even when the substrates were completely vertical. Comparing A0 with A1, we can see that WCAs between them almost have no change, but SAs were very different, the microscale protruding parts on the surfaces may implanted into the water droplet resulting in the water droplet adhere to the surface and difficult to move, this appearance is reasonably good agreement with the Wenzel model, which higher adhesion between water and surface can be generated. The superhydrophobic behavior observed is attributed to suspension of the water droplets and air trapping under the droplet caused by the binary roughness structure in conjunction with the hydrophobic siloxane polymer coating. The success to form air trapping at higher density of aggregates is believed to cause the increase in observed contact angles. In simplistic terms, one can imagine that lower density of aggregates result in a lower density of hillocks result in larger inter-hillock distances, which make it harder for droplet surface tension to support its weight [19]. Bhushan [23] also found that there exists a transition from Cassie State to Wenzel State when the spacing factor S (S = D/W, D and W are defined in Fig. 5 below) decreases. The water contact angle on an air trapping composite surface can be expressed by classical Cassie equation [24] cos A = f1 cos 0 − f2

Fig. 3. Water droplet profile on the superhydrophobic surface.

Adhesion grade

Fig. 2(A–C) shows the SEM images of the surfaces, it can be seen that there were several to dozens of microns width “hillocks” with general uniform distribution, while around the surface of the “maintains” there thick grew loosen texturing “branches” with a diameter about 100 nm, which can been observed in Fig. 2(D). This micro-nano binary structure looks like cotton candy. The formation of this composite structure may be explained as the following: (1) nanoscale particles themselves aggregated and/or the ringopening polymerization product of D5 containing activity terminal –Si(OH) groups cross-linked with particles, resulting in generating of microscale particles; (2) the cotton candy like nano-scale loose texturing around the sides and tops might come from the vacancies when unreacted D5 and ethanol are evaporated, and microphase seperation may take place. Apparently, the density of the aggregates increased with the weight content of SiO2 , but this growth trends stopped or almost unchanged when the weight content reach certain value, this may because the dispersion of SiO2 become worse with the increasing weight content of SiO2 and/or the solubility of SiO2 in D5 get saturated. And the thickness of the coating was determined by subtracting the original thickness of the glass substrate from the coated one, the result was about 7 ␮m for sample A3 and A4. Surface wettability was examined by contact angle measurements. Fig. 3 shows the photograph of a water droplet on the superhydrophobic surface. The shape of the water droplet was almost spherical. The results and the effect of the weight ratio of SiO2 nanoparticles to total solution on WCAs of the prepared films were shown in Fig. 4(curve a). It was indicated that the WCAs of the films were increased with increasing SiO2 weight ratio, when the ratio was not more than 0.84%, afterward, there were almost no changes. This growth trends were just like the density of the aggregates as seen in the SEM images. The average WCAs of the films were 101.0 ± 1.1◦ , 125.5 ± 0.8◦ , 155.9 ± 0.9◦ , 157.1 ± 0.8◦ , 157.4 ± 0.9◦ and 156.8 ± 1.0◦ , corresponding to the different weight ratio of the particles to total solution, 0.28%, 0.56%, 0.84%, 1.11% 1.39% and 1.93%, respectively. Samples with more than 0.84 wt% SiO2 particles may show a higher hydrophobicity but a lower adhesive attraction (Fig. 4 curve b). The adhesion is an important parameter of coating. The surface based on the ring-opening polymerization product of D5 has good adhesion on glass due to the glass has plenty of hydroxyls which can react with the hydroxyls of the polymerization product. However, the adhesion was found to be decreased with the increase of SiO2 particles ratio successively, possibly because of the decreased reactive point, hydroxyls of the polymerization product, reacted with the hydroxyls of substrate surface. According to

Contact angle(°)

3. Results and discussion

(1)

here  A and  0 are the contact angles on the cotton candy like film with a rough surface and on the native siloxane polymer film with a smooth surface, respectively; f1 and f2 are the corresponding fractions of the solid surface and air in contact with the liquid and f1 + f2 = 1. The WCA is increased by minimizing the solid/liquid fraction, f1 , and maximizing the air/liquid fraction, (1 − f1 ), thereby increasing the air trapping site under the droplet. In order to further illustrate air trapping on superhydrophobic behavior, an idealized 2-D model with hillock surfaces is shown in Fig. 5 along with water on it. Every hillock is modeled as a pulse and a water droplet (the red ring) is assumed to sit on top of the hillocks.

Y. Hu et al. / Applied Surface Science 257 (2011) 6044–6048

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Table 2 Thermal stability of the superhydrophobic surface with 1.39 wt% SiO2 after heating for 1.5 h at different temperatures. Temperature (◦ C)

The distance between two hillocks is W, the height of hillocks is H and the hillock width is D. This kind of 2D structure has been widely used to illustrate air trapping on a hydrophobic surface caused by roughness [19,25,26]. If a flat hydrophobic surface exhibits a water contact angle of , a droplet will form this same angle with the sidewall as shown in Fig. 5. As the contact angle decreases, a larger portion of the droplet will occupy the well formed by a hillock and its closest neighbor. We assume vertical features as shown in Fig. 5, neglect gravity, and apply Laplace’s equation for surface curvature. P =

2LV cos  = −4LV • R1 W

(2)

where R1 is the radii of curvature,  LV is the surface tension of water, P is the pressure difference across the surface and  is the contact angle on the native flat siloxane polymer film. When the contact angle goes approximately to 180◦ , we can assume the water droplet is still spherical, so the total contact area between water droplet and the composite surface, 2

2

S1 ≈ [R• cos(A − 90)] = • (R sin )

(3)

The total upward capillary force caused by the curved fluid surface,



F=



P × S = P × S1 × f = −

4LV • (R• sin A ) W

2•

cos  • f

This means there exists adhesion force between solid and the liquid, otherwise, water droplet may bound off, and the work of adhesion is proportional to the contact area between the solid and liquid [27]. The upward force caused by the curved fluid surface reducing the resultant of forces resulting in water droplet floating-like on the surface and moving very easily. Overall, in the Cassie state when there exists air trapping, increasing the fraction of air/liquid not only increases the contact angle, but also decreases the contact area between the solid and liquid, which results in decreasing of adhesion force of solid/liquid while increasing of capillary force, making motion of water droplet more easier. However, further studies should be done on the exact quantitative relationships between capillary force and contact angle and sliding angle. Thermal stability analysis was conducted by testing the contact angle and sliding angle after the sample was heated in the Muffle Furnace for 1.5 h at different temperatures. As shown in Table 2, a thermal treatment at 450 ◦ C demonstrated that the surface is stable to this temperature, which may attribute to the siloxane polymer containing silicon–oxygen bond, the bond energy of which is as high as 450.4 kJ/mol [21]. Further temperature increases resulted in a rapidly decrease of the contact angle and sliding angle. When the treatment temperature was 500 ◦ C, the surface was no longer superhydrophobic (contact angle = ∼0) as a result of the decomposition/oxidation of the siloxane polymer. IR spectrum of the surface was shown in Fig. 6, the intensity of bonds relating to Si–O–Si at around 1102 and 470 cm−1 were noticeable at different temperatures. The absorption of methyl groups at 2964 and 1264 cm−1 and the absorptions attributed to Si–C bonds of –Si(CH3 )2 groups at 849 cm−1 could be seen when temperature were 140 ◦ C and 400 ◦ C. However, when the treatment temperature got 500 ◦ C, the absorption of methyl groups and Si–C bonds completely disappeared, which revealed the decomposition of the siloxane polymer, this could be the reason that why it lost superhydrophobicity.

From Eq. (4), it implies that the F value is proportional to f, the fraction of air/liquid, and inversely proportional to W, the distance between two hillocks. For Cassis state f could be substituted by f2 [−4LV • (R × sin A )2 × cos  × f2 ] W

F values for sample A3 and A5 were shown in The calculated Table 1, from which we can see that capillary force was higher than the weight of water droplet with a volume of 5 ␮l that is 0.05 mN. Table 1  F value, contact angle and sliding angle. Calculated Wa (␮m)

f2

A3 A5

75 56

0.892 0.905



F (mN)

0.31 0.42

b

(5)



Sample

a

Contact angle (◦ )

Sliding angle (◦ )

155.9 ± 0.9 157.4 ± 0.9

∼1 ∼1

a W value selected is the longest lineal distance between two aggregates that there exists nomicroscale protrudes on the linking line.

Transmittance(%)

F=

∼1 ∼1 ∼2 Not measurable

(4)





Sliding angle (◦ )

156.9 ± 0.8 156.0 ± 0.8 155.4 ± 1.1 ∼0

300 400 450 500

Fig. 5. Idealized 2-D wetting model.

Contact angle (◦ )

c 2964 cm

-1

1264 cm -1

-1

849 cm 470 cm -1

1102 cm

4000

3000

2000

1000

-1

0

-1

Wavenumber(cm ) Fig. 6. FTIR of the surface at different temperatures (a) 500 ◦ C, (b) 400 ◦ C and (c) 140 ◦ C.

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Y. Hu et al. / Applied Surface Science 257 (2011) 6044–6048

4. Conclusions In summary, we have successfully fabricated a facile superhydrophobic film with a microscale and nanoscale hierarchical structure on glass substrates and non-fluorine-containing materials were used. Compared with other complex processes, this procedure is simple and potentially usable to fabricate large-area superhydrophobic surfaces with different curvatures for practical self-cleaning applications. More importantly, such surface is superhydrophobic with WCA higher than 155◦ and SA lower than 10◦ , and possesses good temperature resistance. And the capillary force on superhydrophobic behavior was qualitative studied. Acknowledgment The authors thank Dr. Jianqing Ling from South China University of Technology, who conducted the SEM measurements in this letter. References [1] Z.L. Wang, Y.P. Zhou, S.L. Li, J.J. Liu, Physical Chemistry, Higher Education Press, Beijing, 2001. [2] Sh.T. Wang, Y.L. Song, L. Jiang, Nanotechnology 18 (2007) 015103. [3] J. Yang, P. Pi, X. Wen, D. Zheng, M. Xu, J. Cheng, Z. Yang, Applied Surface Science 255 (2009) 3507. [4] W.K. Cho, S.J. Park, S.Y. Jon, I.S. Choi, Nanotechnology 18 (2007) 395602. [5] W. Barthlott, C. Neinhuis, Planta 202 (1997) 1. [6] A.D. Tserepi, M.-E. Vlachopoulou, E. Gogolides, Nanotechnology 17 (2006) 3977.

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