Design and Fabrication of Hydrophobic Copper Mesh with Striking

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Design and Fabrication of Hydrophobic Copper Mesh with Striking Loading Capacity and Pressure Resistance Zai X. Jiang,†,‡ Lin Geng,† and Yu D. Huang*,‡ Materials Science and Engineering Postdoctoral Research Station, Harbin Institute of Technology, Harbin 150001, People’s Republic of China, and Department of Polymer Science and Technology, School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, People’s Republic of China ReceiVed: February 01, 2010; ReVised Manuscript ReceiVed: April 06, 2010

Mimicking the Asparagus setaceus, hydrophobic copper meshes were fabricated with chemical surface modifications. According to the reformed Cassie-Baxter equation, the superhydrophobicity of the copper meshes is explained and predicted. Water contact angles as high as 134.6° were achieved. Good agreement between the predictions and experiments was obtained. Dynamic contact angles are also performed, and the largest contact angle hysteresis is obtained with 1.0 wt % (heptadecafluoro-1,1,2,2,-tetradecyl)trimethoxysilane (HFTES) treated whereas the smallest hysteresis occurs for 8.0 wt % HFTES treated sample. This is explained in terms of sorption of liquid by the solid and penetration of liquid into the polymer film. Furthermore, the loading capacities and the water pressure resistances of the hydrophobic copper meshes were all performed. The highest loading weight, 18.59 g, and the deepest height of pressure resistance, 66.5 mm, were obtained, when the copper mesh was treated with 1.0 wt % HFTES. In addition, the capillary model was borrowed to explain the phenomena of copper mesh with water pressure resistance. From the capillary model, we predicted the height of pressure resistance, and the predicted and measured values were in good agreement. 1. Introduction In nature, many surfaces demonstrate superhydrophobic performance. Examples include the wings of butterflies and the leaves of plants such as a lotus leaf and Asparagus setaceus.1-4 Recently, technologies related to superhydrophobic treatments have attracted considerable attention due to their potential applications, such as contamination prevention, biocompatibility, enhanced lubricity, and durability of materials.5-8 A superhydrophobic surface is defined as having a water contact angle greater than 150°. The water-solid contact angle varies with the surface energy and roughness of the solid surface.8-12 The surface energy of a solid is determined by the surface chemistry, which in turn depends on the chemical composition and atomic arrangements at or near the surface.13 However, the chemical composition and atomic arrangements of solid itself only can result in a limited increase in contact angle, and is insufficient to make a surface superhydrophobic. Fluorochemicals are well-known with their low surface free energy, which are the most important class of water and oil repellent finishes and widely used for finishing. Furthermore, roughness of the solid surface is another important factor for contact angle increase. A further increase in contact angle and hydrophobicity, will require an increase in surface roughness.14-16 Increasing surface roughness of hydrophobic materials can enhance surface water repellency greatly. Thus, the superhydrophobic surface is obtained by two criteria: a low surface energy and an appropriated surface roughness.17-20 Significant research has been reported on the study and synthesis of superhydrophobic surfaces that combine selfassembled trifluoromethyl monolayers and introduce surface * To whom correspondence should be addressed, ydhuang.hit1@ yahoo.com.cn. † Materials Science and Engineering Postdoctoral Research Station. ‡ Department of Polymer Science and Technology, School of Chemical Engineering and Technology.

roughness. Cao and Hu21 developed a superhydrophobic surface via tuning surface roughness and achieved contact angles as high as 160°. Safaee and Sarkar22 developed a superhydrophobic copper surface by galvanic exchange reaction using thin nanostructured silver films. Michielsen and Lee23 designed a superhydrophobic surface using woven structures based on the Cassie-Baxter model. Tavana et al.24-26 have addressed in detail the three key problems in the interpretation of contact angles: determination of consistent and accurated solid surface tensions, “stick-slip” of the three-phase line in measurements of dynamic contact angles, and contact angle hysteresis. In particular, several authors, most recently the McCarthy group27-29 and Extrand group30-32 have commented on the importance of events that occur at the contact line during advancing and receding and the unimportance of interfacial free energy. Studies on artificial superhydrophobic surfaces have achieved great success and most of the artificial superhydrophobic surface fabrications mimicked the structure of a lotus leaf, with micro- and nanostructure.33-35 Unfortunately, the effects of macroscopic structures on superhydrobicity are ignored, such as in the case of Asparagus setaceus, as seen in Figure 1.36,37 There are few artificial superhydrobic surfaces fabricated from the mimicking of the Asparagus setaceus macroscopic structures. The macroscopic mechanisms of superhydrophobicity are also unclear. In the study discussed below, a hydrophobic surface, which is mimicking the Asparagus setaceus leaf, is prepared using a copper mesh to which a low surface tension materials such as (heptadecafluoro-1,1,2,2,-tetradecyl)trimethoxysilane (HFTES) is grafted, as shown in Figure 1c. A study of mechanisms of the copper mesh superhydrophobicities is somewhat based on the microscopic mechanisms of superhydrophobicity. The reason is mentioned above, i.e., there is much work on the microscopic mechanisms of the superhydrophobicity and the research techniques for microscopic mechanisms are extremely effective. Thus, the reformed Cassie-Baxter model is used to explain and

10.1021/jp1009516  2010 American Chemical Society Published on Web 05/04/2010

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Figure 1. Asparagus setaceus leaf and mimicking materials: (a) Asparagus setaceus leaf; (b) superhydrophobic copper branches mimicking the Asparagus setaceus leaf; (c) hydrophobic copper mesh mimicking the Asparagus setaceus leaf .

predict the statistic contact angle of the copper mesh. The dynamic contact angles are also investigated in order to provide further investigation in hydrophobicity of the copper mesh. Furthermore, based on the design and fabrication of the hydrophobic copper mesh, characterizations of the copper mesh, such as loading capacities and pressure resistances, are also performed. The fabricated copper meshes have lots of special characteristics, such as superhydrophobicity, striking loading capacity and pressure resistance, low flow resistance, low cost, easily preparation, and so on. They have great potential applications in fields such as aquatic robots, environmental surveillance, and microfluidity. Among them the aquatic robots that mimick the ephydrid, and microfluid mesh pipe have been fabricated by our group using the treated meshes directly. 2. Experimental Methods 2.1. Materials. The copper mesh used in this study were obtained from Hebei Anhua Hardware & Mesh Product Co., Ltd., People’s Republic of China, and were knitted by copper wires about 73 µm. The pore sizes of these mesh were about 200 µm × 400 µm. The (heptadecafluoro-1,1,2,2,-tetradecyl)trimethoxysilane (HFTES), provided by Dow Corning Co., Ltd., USA, was dissolved in a mixture of ethanol (95.0 wt %) and ultrapure water. HFTES used was 0.5, 1.0, 2.0, 4.0, and 8.0 wt %. Then the solutions were treated in an ultrasonic bath at room temperature for 30 min. Finally, these solutions were used to treat the copper mesh. 2.2. Fabrication of Hydrophobic Copper Mesh and Miniature Mesh Boat. The copper mesh was successively washed in an ultrasonic bath with acetone for 5 min, ultrapure water for 5 min, and 1.0 M NaOH for 5 min to remove surface impurities. After being cleaned by ultrapure water in ultrasonic bath for 10 min and dried for 30 min in ambient temperature, these mesh were immersed into HFTES solution. After that, the mesh were heat treated at 100 °C in air for 3 h. The untreated and treated sheets of copper mesh were cut to a size of 60 mm ×55 mm. The above obtained copper mesh sheets were folded into a boat of 40 mm ×35 mm × 10 mm. The schematic plan of the copper mesh boat dimensions is shown in Figure 2. The weights of these boats were 0.686 ( 0.013 g. 2.3. Characterizations of the Hydrophobic Copper Meshes. 2.3.1. SEM ObserWation. The topographical microstructures of the copper meshes were observed by scanning electron microscopy (Royal Dutch Philips Electronics Ltd., Netherlands). 2.3.2. Contact Angle Measurements. Contact angles were measured to study the hydrophobic properties of the treated copper mesh surfaces. The contact angles on the prepared copper mesh surfaces with sessile water drops were monitored using a Sony digital camera (DSC-W290, Sony Corp., Japan), and were

Figure 2. Schematic plan of copper mesh boat dimension.

calculated using a contact angle meter (The First Optics Factory, Chuangchun, People’s Republic of China). The volume of the applied drops of ultrapure water was 10 µL. For each sample a minimum of four different readings were recorded. Advancing contact angles were measured after sequential deposition with a small syringe and needle. A small drop was deposited on the surface, and then additional water was added to advance the contact line. The needle was inserted into the drop during injection to prevent drops from moving on the mesh surfaces. For receding contact angles, water was withdrawn until the contact line retracted. Measurements were made from more than four droplets and then averaged.38 2.3.3. Loading Capacity EWaluation. The maximum loading capacity of each miniature boat was measured according to the reference report.39 Sand was carefully added into the mesh boat until its upper edges were flooded by water and the boat started to sink. Then the sand was collected, dried, and weighed. 2.3.4. Pressure Resistance EWaluation. The pressure resistance of the copper mesh is also characterized. A sketch map of homemade testing systems is shown in Figure 3. The testing system was composed of a motor controller, stepper motor, camera, container, and sample. The camera and sample were synchronous, so the fall image of sample in container can be real-time recorded. The sample included the cramping apparatus, observation window, sealing plate, and indicator. The indicator used here was tetraiodofluorescein, and it was put into a funnel made from filter paper, as shown in Figure 3. The indicator will be become red immediately when it is in contact with water. 3. Results and Discussion 3.1. Preparation of Hydrophobic Copper Mesh. The term hydrophobic, which was originally applied only to water, is often used to describe the contact of a solid surface with any liquid. It is well-known that the characterization of hydrophobicity is through the measurement of contact angle. The value of the contact angle is given by Young’s equation

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Figure 3. Schematic plan of a homemade testing system.

Figure 4. Schematic plan of a water drop sitting on copper mesh. The red area represents the projected area of the copper wires in contact with water.

cos θe )

γSL - γSG γLG

(1)

where θe is an equilibrium contact angle on a smooth surface and γSL, γSG, and γLG are interface energies (or surface tensions) for the solid-liquid, solid-gas, and liquid-gas interfaces. Wenzel found that the contact angle of a liquid with a rough surface is different from that with a smooth surface.40 Wenzel showed that roughness makes a significant contribution to the wetting behavior of a solid surface. For a droplet of constant volume in contact with a rough surface without air pockets, the cosine of the contact angle is given as a ratio of the differentials of the liquid-air contact area and the area under the droplet, based on the geometrical considerations, which is given as41

cos θW )

dALA ASL dALA ) ) Rf cos θe dAF AF dASL

ASL AF

AC ) fSLAC + fLAAC ) RfASL + fLAAC

(3)

Cassie and Baxter extended eq 2 for the composite interface.42 In this model, a liquid sits on a composite surface made of a solid and air. Using fractional flat geometrical areas of the

(4)

The cosine of the contact angle can be calculated in a similar manner as in eq 2; however, the differential of the liquid-air interface area under the droplet, fLA dAC, should be subtracted from the differential of the total liquid-air area dALA, which yields41

cos θCB )

dALA - fLAdAC dASL dAF dALA ) - fLA ) dAC dAF dAC dASL RffSL cos θe - fLA (5)

Recently, eq 5 has been rewritten as follows32,43

(2)

where θe is the contact angle for smooth surface and ASL and ALA are the solid-liquid and liquid-gas contact areas, AF is the flat solid-liquid contact area (a projection of the solid-liquid area ASL on the horizontal plane). Rf is a roughness factor defined as41

Rf )

solid-liquid and liquid-gas interfaces under the droplet, fSL and fLA, the flat area of the composite interface is41

CB

cos θ

fSL ) Rff

(6)

fLA ) 1 - f

(7)

) Rff cos θe + f - 1

(8)

As mentioned above, we first need to make the surface hydrophobic and create the appropriate roughness to make a hydrophobic surface. According to Cassie-Baxter and Marmur and as is evident from eq 5,43 the Wenzel model is a special case of the Cassie-Baxter equation where fSL ) 1 and fLA ) 0. However, the minimization of the free energy requires that, for a hydrophobic surface with fSL ) 1, θe ) 180°. Since the only material known with θe ) 180° is vacuum, fSL cannot be equal to 1. In order to develop hydrophobic surfaces, we need to use a different approach, namely, the Cassie-Baxter model.32



Figure 5. SEM micrographs of copper mesh: (a) top view and (b) side view.

TABLE 1: Comparison of Predicted and Measured Contact Angles contact angles (deg) 0.5 wt % HFTES grafted 1.0 wt % HFTES grafted 2.0 wt % HFTES grafted 4.0 wt % HFTES grafted 8.0 wt % HFTES grafted samples

predicted

measured

predicted

measured

predicted

measured

predicted

measured

predicted

measured

copper film copper mesh

142.14

61.6 122.2 ( 1.4

150.13

65.2 134.6 ( 1.3

148.24

64.0 130.5 ( 1.5

142.35

62.2 128.6 ( 1.3

141.95

61.0 120.6 ( 1.2

The roughness of copper meshes used is fixed, so we begin with calculation of the copper mesh roughness. The analysis method is following the Michielsens’ reference.32 We first analyze the Rf, the roughness of the copper mesh. The model of copper mesh contacted with water, viewed normal to the cylinders’ axes, is shown in Figure 4. The roughness of the surface in contact with water is the length of the chord in contact with water, Rβ divided by the projected area of the chord in contact with water, R sin β, i.e., Rf ) β/sin β, where β is the angle between the top of the cylinder and the liquid contact line.32,42 Then, the f is calculated following Marmur’s derivation. Marmur showed that f is the length of the red area in Figure 4 divided by the project area. Thus, the f is

f)

2R sin βL 4(R + d)(R + L)

(9)

Substituting f and Rf into eq 8 results in32

cos θCB ) -

RLβ cos β RL sin β + -1 2(R + d)(R + L) 2(R + d)(R + L) (10)

According to the Marmurs’ computation, β ) π - θe. Substituting it for β, the following equation is obtained32,42

cos θCB )

RL(π - θe) cos θe RL sin θe + -1 2(R + d)(R + L) 2(R + d)(R + L) (11)

In order to calculate the θCB, the θe should be determined first, as seen from eq 11. So the second step of design of hydrophobic copper mesh is to determine the contact angles placed upon clean, smooth copper film surfaces. The contact angle of a clean and smooth copper film surface is measured to be 44.5°. In order to make the film surface hydrophobic, the HFTES with different concentrations were grafted onto the copper film surface as described in the Experimental Section. The contact angles measured on the treated copper films are shown in Table 1. As the θe is determined, we begin to

determine other parameters in eq 11. SEM micrographs of the copper meshes are shown in Figure 5. The mesh exhibits the shape of a rectangle, and the length and width of the mesh are about 400 and 200 µm. The radius of the copper wires is about 73 µm. Substituting these values into eq 11 along with the measured contact angles from the flat copper films, and we find the predicted contact values, as shown in Table 1. As seen, the superhydrophobic copper mesh will be obtained when the θe is larger than 65°. For the treated copper mesh, the mesh treated with 1.0 wt % HFTES can achieve hydrophobic character as predicted. The measured contact angles are shown in Table 1. As seen in Table 1, these values are in good agreement with the predicted values, though the measured values are slightly lower than our predicted values, and the superhydrophobicity of the copper mesh could not be realized. This is probably due to the existence of H, as shown in Figure 4. Though the water cannot wet the copper wire, a water drop will somewhat have contact with the copper wire because of gravity. Thus, in the measurement of contact angles through its two dimension photos, the bottom margin, the red area as seen in Figure 4, cannot be seen, which leads to the low measured contact angles. For example, the contact angles seen from three-dimension photos are higher than the contact angles measured from twodimension photos shown in Figure 6. It is well understood that there is no unique contact angle to characterize any given surface. The observed equilibrium contact angles always fall between the advancing and receding contact angles. Thus, both advancing and receding contact angles should be reported to characterize a surface; one static, metastable angle is less meaningful and not enough. The contact angle hysteresis for each solid-liquid system was determined as the difference between the advancing and the extrapolated receding angles and the results are presented in Figure 7. Furthermore, the representative images of dynamic contact angle of mesh treated with 4.0 wt % HFTES are shown in Figure 8. Overall, the contact angle hysteresis of water on treated mesh exhibits the same trend with the statistic contact angles. Historically, contact angle

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Figure 6. Tilt-view photographs for water droplets on the HFTES treated copper mesh surface: (a) 0.5 wt % HFTES; (b) 1.0 wt % HFTES solution; (c) 2.0 wt % HFTES solution; (d) 4.0 wt % HFTES solution; (e) 8.0 wt % HFTES solution. The inserts show the corresponding shape of a water droplet in two dimensions.

hysteresis was attributed to surface roughness16,29 and surface heterogeneity.10,18 However, it is significant to note that hysteresis is not limited only to rough and heterogeneous surfaces; a number of different causes are identified for hysteresis.26,35 To understand the effect factors of contact angle

hysteresis, Tavana et al. prepared high-quality films of four fluoropolymers.31 Then, they showed that the largest hysteresis is obtained with amorphous polymers whereas the smallest hysteresis occurs for polymers with ordered molecular chains. This is explained in terms of sorption of liquid by the solid and



Figure 7. Contact angle hysteresis for each specimen. The inset shows the difference between the advancing and receding contact angles for each specimen.

Figure 8. Typical image of dynamic contact angle of sample treated with 4.0 wt % HFTES: (a) advancing contact angle image; (b) receding contact angle image.

penetration of liquid into the polymer film. For our samples, the roughness indeed is one of the important effect factors, however, when the three-phase lines of samples are all hindered by the copper wires, the characteristics of fluoropolymers become the main effect factors. With the HFTES concentration increase from 1.0 to 8.0 wt %, the polymer chains became wellordered and form a layered structure, which is confirmed in the Tavana article.31 This organization of HFTES chains was also confirmed by molecular modeling.44 Thus, the sorption of liquid by the solid and penetration of liquid into the polymer film occur, and the contact angle hysteresis decreases with the increase of HFTES. 3.2. Loading Capacity Measurements. That a water droplet can rest on a mesh is most probably due to an air film trapped under the water drop. The hydrophobic properties of the copper mesh are attributed to the formation of an air film under the water drop. The water cannot wet the copper filament of copper mesh, i.e., the interface is copper filament and air instead of an interface of copper filament and water, and accordingly, the water can rest on the surface of copper mesh with the aid of surface tension. The wonder phenomenon is that the miniature boat fabricated with hydrophobic copper mesh not only can float on the water surface but also has remarkable loading capacity. Optical images of a miniature boat floating on the water surface (treated with 8.0 wt % HFTES solution) are shown in Figure 9. As shown in Figure 9, the miniature boat can freely float on the water surface, and the water does not penetrate into the boat during the process

of loading weight. Furthermore, it is seen that the boat can keep floating even if its upper edges are below the horizontal surface, as shown in part b. In addition, in the process of this work, it is found that the boat will have marked movement when only a small force is applied to it. The maximal loading capacities of the resulting boats are calculated through quantifying the loaded sand, as shown in Figure 10. Here, it should be noted that the untreated copper mesh boat cannot float over the water surface, so it has no loading capacity and the following discussion about loading capacity refers to the treated copper mesh boat. The largest loading capacity belongs to the miniature boat fabricated from copper mesh treated with 1.0 wt % HFTES solution, while miniature boat treated with 8.0 wt % HFTES solution has the lowest loading capacity. Interestingly, for a copper mesh boat treated with 1.0 wt % HFTES solution, for example, the maximal loading weight is about 27 times greater than the weight of the boat itself, which is almost impossible to achieve with a similar boat made with a copper sheet. As demonstrated above, the copper mesh boat, which can float over water surface, can be obtained by HFTES treatment, and has a striking loading capacity. The large loading capacity of these boats arises from their hydrophobic surfaces. Owing to the hydrophobic nature of the copper mesh surface, an air layer will surround the copper mesh surfaces, which prevents the boats from being wetted and/or penetrated by water. Furthermore, the loading capacities of these treated boats are all larger than 14.00 g, which exceeds the maximum buoyancy

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Figure 9. Optical images of a miniature boat floating on water surface (treated with 8.0 wt % HFTES solution): (a) top view; (b and c) side view; (d) bottom view.

contribution to the loading capacity is the buoyancy force (Fb) related to boat volumes; the second contribution is sufficient additional buoyancy force produced by the air surrounding the boat (Fa), which is the source of extra loading capacity of the copper mesh boat; the third part is the weight of the boat itself (Fi). Thus, the loading weight (FL) of the boat having a size of a × b × c (a length, b width, and c height) can be expressed as

FL ) Fb + Fa - Fi ) Fgabc + Fg[ab + 2(ac + bc)]H - mg (12) Here, g is the gravitational constant and H is the thickness of the air film. The extra supporting force (Fa) for a boat can be illustrated as

Fa ) Fg[ab + 2(ac + bc)]H Figure 10. Effect of HFTES concentration on the mesh loading capacities.

force calculated for the boat (i.e., 14.00 g). The maximal extra loading weight of resulting copper mesh boat is also shown in Figure 10 (the red part of the loading weight). The highest extra loading weight (4.59 g) is also achieved by the copper mesh boat treated with 1.0 wt % HFTES solution. The extra loading capacity is attributed to the existence of air film. On the basis of the above analysis, the loading weight of the copper mesh boat can be divided into three parts:39 the first

(13)

Thus, from eq 13, the H can be calculated. The H is 0.39-1.58 mm for the treated copper mesh boats; i.e., there is a air film with thickness of 0.39-1.58 mm around the treated copper mesh boats. If not for the air film, water would be permeating into the boat. 3.3. Pressure Resistance Measurements. Another interesting characteristic of hydrophobic surfaces is pressure resistance performances. Water pressure resistance of these mesh is performed, and the results obtained are summarized in Table 2. Photos of water pressure resistance of copper mesh treated with 1.0 wt % HFTES are shown in Figure 11. Here, we borrow the ideas from capillary phenomenon to explain the pressure



TABLE 2: Comparison of Measured and Predicted Depths HFTES concentrations h (mm)

0.5 wt %

1.0 wt %

2.0 wt %

4.0 wt %

8.0 wt %

measured values predicted values

56.0 34.7

66.5 45.7

63.4 42.3

59.2 40.6

55.1 33.1

Figure 11. Photo of water pressure resistances of mesh treated with 1.0 wt % HFTES.

resistance values obtained. The copper mesh can be considered as a combination of countless capillaries, and the liquid level decrease in capillaries can be calculated as

h)-

2γ cos θ Fgr

(14)

where γ is the surface tension of liquid, θ is the contact angle between capillary and liquid, F is the liquid density, g is the gravitational constant, and r is the radius of the capillary. Namely, we can consider that the capillary margin could bear water pressure as high as Fgh. So the pressure resistance of copper mesh can be calculated. For copper mesh with a rectangle shape, the diagonal line of the rectangle is chosen as the diameter of the mesh. Thus, eq 14 becomes

h)-

2γ cos θCB FgR

(15)

Substituting the values into eq 15 along with the measured contact angles shown in Table 1, we find the values of h for the treated meshes, which are shown in Table 2. As seen, the measured values are larger than the predicted values. This is probably due to determination of θCB and R. As discuss above, the measured contact values may be lower than the true values because of the limitations of the measured methods. And the rectangle mesh is smaller than the circumcircle; i.e., the radius of mesh is smaller than the R we chose. 4. Conclusion A hydrophobic surface is obtained by two criteria: a low surface energy and an appropriate surface roughness. In order to make copper mesh hydrophobic, a low surface energy material, HFTES, was grafted onto copper mesh surface. The hydrophobic copper meshes were designed and fabricated according to the reformed Cassie-Baxter equation by Marmur. The apparent contact angles of the copper mesh were also predicted. Good agreement between the predicted and the measured values is obtained, which indicate that the reformed Cassie-Baxter model is suitable for the explanation of the macroscopic mechanism. In addition, the dynamic contact angles were also characterized. The largest contact angle hysteresis is

obtained with 1.0 wt % HFTES treated sample whereas the smallest hysteresis occurs for 8.0 wt % HFTES treated sample. This is explained in terms of sorption of liquid by the solid and penetration of liquid into the polymer film. Furthermore, the characterizations of these hydrophobic meshes, loading capacities and pressure resistances, were performed. The copper mesh boats not only can float freely on a water surface but also exhibit a large loading capacity, and the highest loading weight, 18.59 g, is achieved when the treated concentration of HFTES solution is 1.0 wt %. The hydrophobic copper meshes also have the capacities of water pressure resistances. The deepest height of pressure resistance is 66.5 mm, which is also obtained by the mesh treated with 1.0 wt % HFTES. The capillary model was borrowed to explanation the phenomena of copper mesh with water pressure resistance. From the capillary model, we predict the height of pressure resistance. The predicted and the measured values are in good agreement. Acknowledgment. The authors thank the China Postdoctoral Science Foundation (No. 20090460067) (Project HIT.NSRIF.2009123) supported by the Natural Scientific Research Innovation Foundation at Harbin Institute of Technology and the Heilongjiang Postdoctorial Financial Assistance (No. LBHZ08185) for financial support. References and Notes (1) Barthlott, W.; Neinhuis, C. The Purity of Sacred Lotus or Escape from Contamination in Biological Surfaces. Planta 1997, 202, 1–8. (2) Li, X. M.; Reinhoudt, D. What do We Need for a Superhydrophobic Surface? A Review on the Recent Progress in the Preparation of Superhydrophobic Surfaces. Chem. Soc. ReV. 2007, 36, 1350–1368. (3) Tan, S. X.; Xie, Q. D. One Step Preparation of Superhydrophobic Polymeric Surface with Polystyrene under Ambient Atmosphere. J. Colloid Interface Sci. 2008, 322, 1–5. (4) Coffinier, Y.; Janel, S. Preparation of Superhydrophobic Silicon Oxide Nanowire Surfaces. Langmuir 2007, 23, 1608–1611. (5) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Bioinspired Surfaces with Special Wettability. Acc. Chem. Res. 2005, 38, 644–652. (6) Roura, P.; Fort, J. Comment on “Effects of the Surface Roughness on Sliding Angles of Water Droplets on Superhydrophobic Surfaces. Langmuir 2002, 18, 566–569. (7) Xiu, Y. H.; Zhu, L. B.; Hess, D. W.; Wong, C. P. Relationship between Work of Adhesion and Contact Angle Hysteresis on Superhydrophobic Surfaces. J. Phys. Chem. C 2008, 112, 11403–11407. (8) Lafuma, A.; Quere, D. Superhydrophobic States. Nat. Mater. 2003, 2, 457–460. (9) McHale, G.; Shirtcliffe, N. J.; Newton, M. I. Contact-angle Hysteresis on Super-hydrophobic Surfaces. Langmuir 2004, 20, 10146– 10149. (10) Wolansky, G.; Marmur, A. The Actual Contact Angle on a Heterogeneous Rough Surface in Three Dimensions. Langmuir 1998, 14, 5292–5297. (11) Dettre, R. H.; Johnson, J. R. E. Contact Angle Hysteresis. IV. Contact Angle Measurements on Heterogeneous Surfaces. J. Phys. Chem. 1965, 69, 1507–1515. (12) Neumann, A. W.; Good, R. J. Thermodynamics of Contact Angles. I. Heterogeneous Solid Surfaces. J. Colloid Interface Sci. 1972, 38, 341– 358. (13) Steinberger, A.; Cottin, B. C.; Kleimann, P.; Charlaix, E. High Friction on a Bubble Mattress. Nat. Mater. 2007, 6, 665–668. (14) Wu, W. C.; Chen, W.; Liang, S. Superhydrophobic Surface from Cu-Zn alloy by One Step O2 Concentration Dependent Etching. J. Colloid Interface Sci. 2008, 326, 478–482. (15) Tuteja, A.; Choi, W.; Ma, M. L.; Mabry, J. M.; Mazzella, S. A.; Rutledge, G. C.; McKinley, G. H.; Cohen, R. E. Designing Superoleophobic Surfaces. Science 2007, 318, 1618–1622.

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JP1009516

Design and Fabrication of Hydrophobic Copper Mesh with Striking

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