design and fabrication of teflon-coated tungsten nanorods for tunable hydrophobicity

Published: March 15, 2011

r 2011 American Chemical Society 4661 dx.doi.org/10.1021/la104891u | Langmuir 2011, 27, 4661–4668

ARTICLE

pubs.acs.org/Langmuir

Design and Fabrication of Teflon-Coated Tungsten Nanorodsfor Tunable HydrophobicityKhedir R. Khedir,† Ganesh K. Kannarpady,*,† Hidetaka Ishihara,† Justin Woo,† Charles Ryerson,‡ andAlexandru S. Biris*,†

†Nanotechnology Center, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock,Arkansas 72204, United States‡Terrestrial and Cryospheric Sciences Branch Cold Regions Research & Engineering Laboratory Engineer Research and Development,Center U.S. Army Corps of Engineers, Hanover, New Hampshire 03755-1290, United States

1. INTRODUCTION

Geometrical modification of surfaces and subsequent surfacetreatment with low surface energy materials is a conventionalstrategy for producing hydrophobic [AWCA (apparent watercontact angles) > 90�] or superhydrophobic (AWCA > 150�)surfaces from hydrophilic surfaces [WCA (water contact angle) <90�] such as metals.1�5 However, with the high degree of surfaceroughness, beyond the threshold roughness4 and controlledtopography,6 the conversion of high surface energy materials toexhibit water repellency properties7�10 with AWCA as high as178� has also been reported.7 Similarly, the anti-icing property wasobserved on superhydrophobic surfaces with contact angles in therange of 150�160� on polymer composites11 and Si nanostruc-tures.12Nevertheless, the robustness and longevity of themodifiedsurfaces still pose major challenges for the overall development ofsuperhydrophobic metallic surfaces to be used in harsh environ-mental applications.

The modification of metallic surfaces such as W to exhibitsuperhydrophobic properties that could prevent ice formation11

in aerospace applications would have enormous impact botheconomically and from a safety perspective. In order to achievesuch materials, the main challenge is to develop metallic surfaces,such as tungsten, with extremely high surface energy of more than3000 mN m�1. Reduction in the surface area’s exposure to the

environment is the key to minimizing the possibility of degrada-tion of the water repellency property as a result of corrosion andwearing of the coating. Therefore, fabrication of metallic surfaceswith both a low degree of roughness and controlled geometricalmorphologies may advance their overall water repellency abilities.

As directly related to wettability, roughness in both micro- andnanoscale and a combination of both can be imparted to themetallic surfaces using variousmethods,10,13�15 including glancing-angle deposition (GLAD),1 electrodeposition,9 chemical etching,8

femtosecond laser2 treatment, and sol�gel process.4 From theapplication point of view, fabrication cost and ease, scaling upabilities, and surface robustness are themeasures of reliability of thegenerated rough surfaces produced by such methods. In order toaddress all of these considerations, the growth of morphologicallycontrolled WNR surfaces by GLAD16 is proposed, since thismethod is capable of producing one-dimensional nanostructureswith tunable surface morphology and porosity.

To enhance the water repellency of metallic surfaces bygenerating nanorods using the GLAD technique, we have pre-viously reported an AWCA of 138� for both Al and W nanorod

Received: December 8, 2010Revised: February 21, 2011

ABSTRACT: The nature of water interaction with tungstennanorods (WNRs) fabricated by the glancing-angle depositiontechnique (GLAD)—using RF magnetron sputtering undervarious Ar pressures and substrate tilting angles and thensubsequent coating with Teflon—has been studied and re-ported. Such nanostructured surfaces have shown strong waterrepellency properties with apparent water contact angles(AWCA) of as high as 160�, which were found to dependstrongly upon the fabrication conditions. Variations in Arpressure and the substrate tilting angle resulted in the genera-tion of WNRs with different surface roughness and porosityproperties. A theoretical model has been proposed to predict the observed high AWCAs measured at the nanostructure interfaces.The unique pyramidal tip geometry ofWNRs generated at low Ar pressure with a high oblique angle reduced the solid fraction at thewater interface, explaining the high AWCA measured on such surfaces. It was also found that the top geometrical morphologiescontrolling the total solid fraction of the WNRs are dependent upon and controlled by both the Ar pressure and substrate tiltingangle. The water repellency of the tungsten nanorods with contact angles as high as 160� suggests that these coatings have enormouspotential for robust superhydrophobic and anti-icing applications in harsh environments.

4662 dx.doi.org/10.1021/la104891u |Langmuir 2011, 27, 4661–4668

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surfaces coated with a thin layer of silane.1 In addition, Fan et al.17

used the GLAD technique to fabricate Si nanorods with varyingdiameters. Hydrophobic surfaces with an AWCA of 142.7� wereobtained on Si nanorods with heights superior to 500 nm, aftertreating the prepared surfaces with hydrofluoric acid. To our bestknowledge, very limited research has been conducted usingthe GLAD technique to alter the metallic surface properties,such as roughness and porosity, by controlling the characteristicsof the metal nanorods with the ability to produce scalable androbust superhydrophobic surfaces.

In this work, WNRs with various surface morphologies andporosities were generated by RF magnetron sputtering, usingthe GLAD technique, under different Ar pressures and withvarious substrate tilting angles. The details about the influence ofAr pressure on the morphological control of WNRs werepresented in our previous work.18 After an additional coatingwith a thin Teflon layer, theWNRs of various morphologies haveshown water repellency properties with AWCAs ranging from122.7� to 160�. Such findings indicate the ability to tailor thesurface energy and the hydrophobic properties of such nano-structural films based on their morphologies and structuralcharacteristics that can be directly correlated to the growthparameters. Scanning electron microscopy and atomic forcemicroscopy were used to characterize the surface morphologyof the generated WNR surfaces and were further used in thedevelopment of a theoretical model proposed to predict theobserved high AWCA for the WNRs. To our knowledge, thisexcellent degree of control over the morphology and structuralproperties of such metal nanorods during the growth process andtheir direct impact on the corresponding surface energy of thefilms, resulting in tunable interactions with the water droplets,has never been shown before. The novel results highlighted bythis work could find significant applications in the area of water orice mitigation, biomedical implants, high-sensitivity sensing,and/or aeronautics.

2. EXPERIMENTAL PROCEDURE

2.1. Tungsten Nanorod Fabrication. First, glass substrates(2.5 cm �2.5 cm) were cleaned with both acetone and methanol in asonication bath for 10 min each. The cleaned substrates were blown withnitrogen gas and mounted on a rotatable substrate holder, 150 mm awayfrom the target. Then the chamber was pumped down using a cryopumpsupported by a mechanical pump until it reached the base pressure of5.0� 10�7 Torr. TheW target with purity of 99.9% purchased from ACIAlloys, Inc., was used. The depositions were carried out under a constantpower density of 7.64 W cm�2, using RF magnetron sputtering. Inthe GLAD technique, the atoms are sputtered onto the substrate with ahigh oblique angle between the substrate surface normal and the targetsurface normal, in addition to the substrate rotation with an angular speedof 30 rpm around its surface normal. The Ar gas was introduced into thechamber at working gas pressure through an injection ring just above thetarget with a constant flow rate of 10 sccm. In this work, two distinctfactors of deposition were manipulated to generate WNR surfaces withsignificant differences in surface morphology and porosity. First,WNRthin films were deposited on the glass substrate under various Arpressures of 0.38, 0.5, 0.7, 1.0, 5, 10, and 20 mTorr and a constantsubstrate tilting angle of 85� for 60min. Second,WNRswere generated attwo different oblique angles of 85� and 89�, but constant Ar pressure of1.0 mTorr, with three different thicknesses of 200, 400, and 600 nm foreach tilting angle. All of the depositions were carried out with nointentional heating of the substrates.

2.2. Surface Modification Using Teflon Nanolayer. Thesurface energy of the as-depositedWNR samples prepared under variousAr pressures and substrate tilting angles was modified by coating with athin layer of Teflon AF2400 using an effusion cell. The coating wascarried out with no substrate holder tilting (normal incidence) under thechamber pressure of less than 3� 10�7 Torr. In order for the samples tobe chemically modified under the same conditions, the whole set of theas-deposited WNRs was coated with Teflon AF2400, simultaneously.2.3. Contact Angle Measurements. The AWCA was measured

by the drop method using an EasyDrop (DSA1) device (Kruss Co.). Foreach sample, distilled water droplets of 2 μL were gently dispensed,using a computer-controlled automated syringe, in five random places.The water droplet images were digitally captured via a CCD camera withthe capability of recording 60 fps at a 780 � 580 resolution. For eachsample, five corresponding contact angles of the water droplet imageswere averaged and presented with minimum standard deviation possi-ble. This process was repeated for selected samples at intervals of severaldays, and consistent results were observed. All of themeasurements werecarried out at room temperature.2.4. SurfaceMorphology. A scanning electronmicroscope (JEOL

SEM7000FE) was used to characterize the surface topography of the as-deposited WNRs on the glass substrate under various Ar pressures andsubstrate tilting angles. The uniformity of the WNRs was investigated bytaking images from three different locations chosen randomly over thesurface of the WNR films. In addition, the surface roughness and surfacemorphology analyses were carried out using a Nanoscope 3100 atomicforce microscope. The tapping mode was utilized to scan the surface ofthe WNRs. The roughness factors (the ratio of increase in the surfacearea) of the three scanned locations were averaged, and a reasonablecorrelation among the results was achieved.

3. RESULTS AND DISCUSSIONS

3.1. TungstenNanorods Synthesis and TheirWater Hydro-phobicity Properties. 3.1.1. WNRs Generated under Various ArPressures. In the first part of this work, the as-deposited WNRsfabricated by the GLAD technique under various Ar pressuresexhibited various morphologies and surface roughness, in addi-tion to the variation in their porosity, as shown in Figure 1. Thetop-view SEM images for WNRs deposited at Ar pressures lowerthan 1 mTorr are not shown here; however, they are comparableto the images of WNRs obtained at 1 mTorr in terms of surfacemorphology and porosity. The increase in Ar pressure caused areduction in the lateral size of the WNRs and a visible increase intheir density-forming large agglomerates.18 TheWNRs generatedby the GLAD technique were observed to present naturallyoccurring pyramidal tips at their top ends. These pyramidal endsare more significant and visible for the isolated WNRs fabricatedat low Ar pressures but were found to gradually lose theirgeometrical structures as the Ar pressure increased, as shown inFigure 1a. This fact has been reported by other researchers, aswell.1,19 After chemicalmodification of the as-depositedWNRs bycoating them with a thin layer of Teflon, tunable water repellentmetallic surfaces with AWCAs ranging from 122.7� to 150.7�were obtained. The graphic representation of the steps followedto fabricate WNRs with hydrophobic properties is presented inFigure 2. The systematic increase in the water repellency of theprepared surfaces was due to the control of surface morphologyand porosity of the WNR films by varying the Ar pressure duringdeposition. As can be seen from the top view SEM images shownin Figure 1A, the as-deposited WNRs generated at relatively lowAr pressures possess larger lateral sizes but lower densities withisolated nanopillars.18 However, the opposite scenario occurred


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Figure 1. (a) Top-view SEM images ofWNRs deposited on glass substrate at various Ar pressures. The insets are the corresponding AFM images of thesurfacemorphology ofWNRs. The dimensions in AFM images areX = Y = 1μmand Z= 100 nm. (b) Top-view SEM images ofWNRs deposited on glasssubstrate at constant Ar pressure of 1 mTorr and different substrate tilting angle of 85� and 89�with height profile of 200, 400, and 600 nm, respectively.The dimensions in AFM images are X = Y = 1 μm and Z = 100 nm.

Figure 2. Schematic representation of steps to generate WNRs with water repellent properties.


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with the WNRs fabricated at high Ar pressures: their densityincreased to the extent of agglomeration—eventually leading tothe formation of thin films at Ar pressure of 20 mTorr. Moreover,the side view SEM images that are not shown here revealed thatthe height of the fabricated WNRs was less than 200 nm. To ourknowledge, high AWCAs in the vicinity of superhydrophobicityon nanopillar surfaces with height profiles of less than 200 nmhave not been reported previously.3.1.2. WNRs Generated at Different Substrate Tilting Angles

with Different Thicknesses. In the second part of this work,another set of samples was generated by changing the substratetilting angle (85� and 89�) under a constant Ar pressure of 1.0mTorr. Since the WNRs generated at 1 mTorr showed a higherdegree of alignment and isolation, we chose this pressure tocontrol the nanostructure further by varying the tilting angle. Inaddition, the thin films were deposited with different thicknessesof 200, 400, and 600 nm at both substrate tilting angles. Anincrease in the substrate tilting angle causes the lateral componentof the flux to shadow more and more and hence the verticalcomponent of the flux to grow more selectively.20 Also, with theincrease in the height of WNRs, more premature WNRs failed todevelop fully during deposition. Consequently, taller nanorodsgrow at the expense of shorter nanorods. The two factors ofextreme substrate tilting angle and the increasing heights of thenanorods promoted the fabrication of WNRs with significantenhancement in the porosity and sharpness of their pyramidaltips. The gradual variation in the porosity and sharpness in the

pyramidal tips ofWNRs can be seen in Figure 1b. As shown in thisfigure, the films deposited at an 89� tilt angle showedmuch betterporosity and sharpness in the pyramidal tips than did thosedeposited at an 85� tilt angle. Eventually, after coating the surfacesof the prepared samples with a thin layer of Teflon AF2400, asignificant enhancement in their water repellent behavior wasobserved with AWCAs ranging from 150� to 160�. The highestcontact angle of 160� was observed for the nanorods with a filmthickness of 600 nm deposited at an 89� tilt angle due to thereduction in the density of nanorods, as seen in the correspondingSEM picture in Figure 1b. The samples prepared at a substratetilting angle of 85� showed no significant decrease in the densityof nanorods (no solid fraction decrease) with the increase in theheight. As a result, their water repellency properties remainedalmost the same, with an AWCA of around 150�.In this work, the variation of deposition pressure and substrate

tilting angle enabled us to generate WNRs with various surfacemorphologies and consequent controllable water repellencystarting from the moderately hydrophobic to the superhydro-phobicity after chemical treatment of their surfaces. In order tounderstand the mechanism of water interaction with thesenanomodified surfaces and their nature in adopting a specificregime, the predicted AWCAs were analyzed by both theWenzeland Cassie methods as the classic proposed models for roughsurfaces. Such studies can be carried out in terms of twoparameters: the increase in surface area (roughness) and theratio of the top solid fraction of the nanopillars.3.2. Theoretical Wenzel and Cassie�Baxter Models Over-

view. The study of the interaction of liquid with the roughsurfaces was fundamentally proposed by both Wenzel21 andCassie�Baxter.22 In the Wenzel model, the rough surface area isin direct contact with the liquid, and the wetabillity of the surface,AWCA, can be predicted using the following equation:

cos θW ¼ r cos θY ð1Þwhere θW is the AWCA and θY the intrinsic WCA for thecorresponding flat surfaces, which can be deduced using theYoung equation. The roughness factor, r, is the ratio of the realover apparent area of the surface. The homogeneity of theinteraction at the interface of both the liquid drop and roughsurface has limited the validity of the Wenzel model to surfaceswith hydrophilic and mild hydrophobic properties. For a betterillustration of theWenzelmodel and the influence of roughness onthe AWCA, Figure 3a shows how hydrophilic surfaces with θ <90� become more hydrophilic with increase in roughness, whilehydrophobic surfaces with θ > 90� tend to have enhancedhydrophobicity.At a specific degree of roughness, which is also known as the

threshold roughness, air pockets may be trapped between thesolid voids under the liquid, promoting composite interfaces ofliquid�solid and liquid�air. In that case, the Cassie�Baxtermodel will be applied:

cos θCB ¼ rsfs cos θY þ fa cos θa ð2Þwhere fs and fa are the fraction of solid�liquid and air�liquidinterfaces, respectively, and rs is the roughness factor over the topof the solid fraction that is in contact with water droplet. Also fa =1 � fs and θa = 180� as the contact angle at the air�liquid inter-face. In the case of total wetting, fs = 1 and rs = r, and the Wenzelmodel will be recovered. From eq 2, it can be seen that the solidfraction fs, which is the counterpart of the roughness in the

Figure 3. (a) The Wenzel model and (b) the Cassie model for θ < 90�(hydrophilic), θ = 90�, and θ > 90� (hydrophobic).


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Wenzel model, controls the contact angle variation. With thesame strategy that we followed above in the case of the Wenzelmodel, the variation of both a hydrophilic surface and a hydro-phobic surface with the decrease in solid fraction is shown inFigure 3b. Unlike the Wenzel model, with both types of surfaces,the decrease in solid fraction fs caused an increase in themeasuredAWCA. Therefore, by controlling only the solid fraction, thehydrophilic surfaces can be modified to exhibit water repellencyphenomena without chemical modification of the surface.3.3. Proposed Theoretical Model for the Prediction of the

Experimentally Measured AWCAs. As mentioned above, theWenzel model is largely a reliable approach to predict the AWCAsfor surfaces with hydrophilic or moderate hydrophobic proper-ties. Therefore, it would be more reasonable to start first with thepredictions that can be made by this model with the associatedroughness obtained on the prepared WNR surfaces. The rough-ness factors of the WNRs generated under various Ar pressuresand the associated AWCAs predicted by the Wenzel modelagainst the depositionpressures are plotted and shown in Figure 4.The observed AWCAs for the WNRs generated by the one-step GLAD technique are much higher compared to the onespredicted by the Wenzel model (θY = 120�), for a flat sur-face coated with Teflon), except for the samples deposited at20mTorr. Also, from Figure 3a, according to theWenzel model, a

more than 70% increase of surface area (r > 1.7) is required toreach the limit of θ ∼ 150�. AWCAs of 150� on the chemicallymodified WNRs deposited at 1 mTorr in both the cases ofsubstrate tilting angles (85� and 89�) with the thickness of200 nm were obtained with only a 30%�50% increase in thesurface area (1.3�1.5 roughness factor). Therefore, the increasein the height of WNRs, which definitely increases their surfaceroughness, would not promote theWenzel state. To the contrary,that would enhance the stability of the metastable Cassie state byincreasing the energy barrier between the two states. As a result, itwould be insufficient to explain the nature of interactions betweenthe water droplets and the nanostructural WNR surfaces with theWenzel model only. It can be deduced, therefore, that there is astrong possibility of complex interactions (excluding the filmsdeposited at 20 mTorr) that could also be explained by theCassie model.In the Cassie model, the solid fraction fs is the major factor

predicting the AWCA values and “stickiness” of water dropletadhesion to the surface. With the progress in understanding theinteraction of the liquid with the surface, the Cassie regime is alsosubdivided into two categories: the lotus effect (high AWCA butlow adhesion)23 and the petal effect (high AWCA and also highadhesion to the surface).24 The limits of stability and metast-ability of this state are still not very well understood; however,although very important, neither the stickiness of water dropletsto the top surfaces of the prepared WNRs nor measurement ofthe contact angle hysteresis (different between receding andadvancing contact angle) were addressed in this study.For investigating the validity of the Cassie state occurrence,

the solid fraction factor must be determined. For this purpose,image analysis technique was utilized to determine the solidfraction factors of the WNR surfaces. This was carried out byconverting the grayscale top-view SEM images to binary imagesafter subtracting the background and adjusting the image con-trast. In the binary image, the white areas represent the flat top-view of WNRs, and the black areas represent spaces among theobjects. Eventually, the solid fraction factor was obtained by thesummation of all the areas covered by the objects (white areas)divided by the entire area of the image. Moreover, small areaswith poor brightness were excluded, because those areas repre-sent nanorods that, during the growth process, were terminatedprematurely and did not reach the surface. This procedure wasaccomplished by developing a Matlab program using functionsfrom the image processing toolbox available in Matlab 7.9.0(R2009b). Fan et al.17 followed a similar strategy to calculate theimage threshold of the top-view SEM images in order to estimatethe solid fraction factor of the silicon nanorods fabricated bythe GLAD technique. A typical example of the image conversionsteps obtained through the use of the Matlab program developedin this work is shown in Figure 5.To examine the possibility that the Cassie regime might

explain the results obtained for the WNR surfaces, the calculatedsolid fractions obtained by the described image processingapproach were substituted in eq 2. The AWCAs predicted bythe Cassie model associated with their solid fraction factors areshown in Figure 6. It can be noticed that the predicted AWCAsare still lower than those observed on the surface of WNRs—particularly for the WNRs fabricated at low Ar pressures, from0.38 to 5.0 mTorr, and an extreme oblique angle of 89� withvarious thicknesses, as well. This indicates that the water dropletsits partially on the very top portion of the WNRs’ tips with asolid fraction less than the one obtained through the image

Figure 4. The variation of AWCA on the surfaces of chemicallymodified WNRs with different roughness factors generated at variousAr pressures of 1.0, 5.0, 10, and 20 mTorr versus AWCAs predicted bythe Wenzel model.

Figure 5. Illustration of image analysis steps in the developed Matlabprogram to estimate the solid fraction of WNR surfaces: (a) top-viewSEM image for WNRs deposited at 1.0 mTorr and (b) a binary image,showing white areas (objects) and black areas (spaces), after backgroundsubtraction and contrast adjustment.


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analysis technique, which considers the top of nanorods as a flatsurface with (rs = 1). From the SEM images of the WNRsgenerated under various Ar pressures and substrate tilting anglesresulting in nanorods with consequent different surface mor-phologies (shown in Figure 1), the natural pyramidal tip ofWNRs can be clearly seen, particularly for the isolated WNRsdeposited at low Ar pressures of 1.0 mTorr. However, thesharpness of the pyramidal tip is more noticeable with theincrease in the height of WNRs deposited at the substrate tiltingangle of 89�. Therefore, in addition to the increase in the degreeof porosity of the WNRs deposited at an extreme oblique angle,the naturally sharp pyramidal tip of WNRs with the height of 400and 600 nm has contributed significantly in decreasing theeffective solid fraction and consequently increasing the AWCAto as high as160�. For WNRs deposited at high Ar pressures ofmore than 1mTorr, the pyramidal tip has started to diminish andalmost disappears at 20 mTorr. It is also noticeable that theAWCAs predicted by both the Wenzel and Cassie models are

comparable, which means that the solid fractions determined byimage analysis techniques are very reasonable.3.4. Effect of theWNRs Pyramidal Tips on the AWCAs.The

AWCAs predicted by the Cassie model, assuming the top ofnanopillars as flat triangular areas, are still lower than themeasured AWCAs over the surfaces of prepared samples. There-fore, we believe that the natural pyramidal tip of WNRs isresponsible for the higher AWCAs by changing the effectivesolid fraction in contact with water droplets. In a geometricalstudy, Patankar25 has suggested the design of periodic pillars withinclined side walls to lower the top solid fraction of the pillars. Asignificant decrease in the solid fraction of the proposed geome-trical design can be noticed from the model that has beenproposed to predict their solid fraction, compared to the samepillars with vertical sides. The effect of solid fraction due to thetop geometrical design has been reported in some other studies,as well. Oner and McCarthy26 have shown that the pillars witha star cross-section pin more efficiently in the liquid meniscusthan pillars with circular cross sections. Gao and McCarthy27

have reported perfectly hydrophobic surfaces with both advan-cing and receding contact angles of 180� on the surface of a loosenetwork of thin fibers. They argue that the elasticity of thefabricated network might have contributed to the unexpectedlyhigh AWCA.To estimate the solid fraction of the WNRs with a pyramidal

tip, we used the geometrical approach that has been followed inother studies for various geometries, such as square pillars28 andcircular pillars.14 For simplicity, we assumed that the pyramid hasa simple tetrahedral shape with four equilateral triangles withdimensions very close to the diameter d of the nanorod, asillustrated in Figure 7a. Therefore, the solid fraction for pillarswith triangular base tips and spaces S in between can berepresented as follows:

fs ¼

ffiffiffi3

p4

d2

Sþ 23d

� �Sþ

ffiffiffi3

p2d

! ð3Þ

The surface area of the tetrahedral pyramidal tip is 3 times thesurface area of its base, so the solid fraction determined for theWNRs, generated at different Ar pressures, by using an imageanalysis technique is for nanopillars with flat triangular tips.Therefore, less than one-third of the pyramidal tip would beoccupied by the water droplet to predict the observed AWCAs onthe surface of prepared samples. To determine the reduction inthe solid fraction due to the pyramidal tip of WNRs, which mayrepresent the effective solid fraction occupied by the waterdroplet and promoting higher AWCA, the eq 3 can be rewrittenas follows:

fs ¼

ffiffiffi3

p4

d� 2htan R

� �2

Sþ 23d

� �Sþ

ffiffiffi3

p2d

! ð4Þ

The most practical way to predict the effective solid fractionoccupied by the water droplet on the surface area of thepyramidal tip is to gradually decrease the water droplet occupa-tion on its surface area by adjusting the parameter h, which

Figure 6. (a) The observed AWCAs on the surfaces of chemicallymodifiedWNRs with different solid fraction factors generated at variousAr pressures of 1.0, 5.0, 10, and 20 mTorr versus AWCAs predicted byboth the Cassie model and modified Cassie model. (b) The observedAWCAs on the surfaces of chemically modified WNRs with differentsolid fraction factors generated at constant Ar pressure of 1.0 mTorr andsubstrate tilting angle of 89� with thicknesses of 200, 400, and 600 nmversus AWCAs predicted by both the Cassie model and modifiedCassie model.


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represents the height of a triangular face of the tetrahedralpyramid, so we start from the base of the triangle at h = 0, whichindicates that only one-third of the surface area of the pyramid(tetrahedral pyramid has three equilateral triangles on the surfacearea) has been occupied by the water droplet (Figure 7b), andadjust to the top of the pyramid at h = 31/2/2d, which representsalmost zero effective solid fraction (Figure 7c). Eventually, themodified Cassie�Baxter model due to the proposed solidfraction formula can be represented:

cos θCB ¼

ffiffiffi3

p4

d� 2htan R

� �2

Sþ 23d

� �Sþ

ffiffiffi3

p2d

!ð1þ cos θYÞ � 1 ð5Þ

After examining the modified Cassie model for differentvalues of h, we learned that, with a 30% decrease in the solidfraction of the triangular base, which represents a 77% decreasein the surface area of pyramidal tip, the predicted AWCAswere in good agreement with observed AWCAs for the tworough films prepared under deposition pressures of 1.0 and 5.0mTorr, as demonstrated in Figure 6A. Therefore, only 23%of the pyramidal tip was occupied by the water droplet, whichmeans suspending the water droplet at the very top of thepyramidal tip. However, the model cannot be applied for thesamples generated at 10 mTorr and above, where there are noisolated nanorods with significant pyramidal tips and relativelywell-defined space/diameter ratios. Meanwhile, the highOAWCA on the sample generated at 10 mTorr may be due tothe large size of the voids among relatively large bunches ofWNRs, which generally promotes the coexistence of the twostates.29

With the second variable used to control the growth of thenanostructures, the extreme tiling angle of 89�, the increase in thethickness of the films facilitated the growth of taller nanorods atthe expense of shorter ones. This increased the porosity of thefilms with the decrease in the density of the nanorods thatreached the surface of the film. However, the solid fractions of theWNR surfaces with heights of 200, 400, and 600 nm determinedby image analysis technique were much larger, as indicated bypredicted Cassie contact angles that were lower than theobserved contact angle (Figure 6b). As shown in the figure, thepredicted AWCAs with the modified Cassie model were inexcellent agreement due to well-isolated WNRs and their notice-able pyramidal tip that was promoted with the increase in theheight of prepared surfaces. It turned out that the effective solidfractions, the average pyramidal surface area occupied with water,

were around 23%, 20%, and 18% of the surface area of geome-trical pyramidal tip for WNR surfaces with height profiles of 200,400, and 600 nm, respectively.For the samples deposited with different thicknesses but at a

constant deposition pressure of 1.0 mTorr and a substrate tiltingangle of 85�, a very slight difference in the OAWCAs of150� �151� was observed. This fact can be understood simplyby looking at the SEM images shown in Figure 1b, where the firstrow of images show no significant decrease in solid fraction(decrease in porosity) or increase in the sharpness of thenanorods’ pyramidal tips. The quasiconstant OAWCAs on thesesurfaces—despite a significant increase in their surface roughnessdue to the increase in the height of WNRs from 200 to 600 nm—is another concrete piece of evidence that these samples haveadopted the composite Cassie state without being affected by theincrease in their roughness. As we have demonstrated in thisstudy, the nanoscale WNRs with pyramidal tips exhibitedOAWCAs as high as 160o by utilizing the two factors of Arpressure and substrate tilting angle. Also, the proposed modelwas able to predict the reduction in solid fraction due to thepyramidal tips of WNRs; consequently, the predicted AWCAswere very consistent with the measured values. Therefore, thepossibility of water droplets touching the sides of WNRs, whichmay not be coated with Teflon, was very low. Otherwise, therewould be a change in the liquid/metal interface that wouldreduce the AWCAs and increase the possibility for the transitionfrom composites to the homogeneous state. Therefore, thegeometric design of the tops of pillars would have a significantimpact on the reduction of effective solid fraction and conse-quent robust hydrophobic surfaces.

4. CONCLUSIONS

In this work, nanoscale WNRs generated under various Arpressures and substrate tilting angles by using GLAD techniquehave shown tunable hydrophobic properties after their surfaceswere modified with a thin layer of Teflon. The natural pyramidaltips and variation in theWNRs’ surface morphology and porositycontributed to the enhancement of water repellency for thesemetallic surfaces with AWCAs ranging from 122.7� to 160�. Aproposed theoretical model was able to predict the high AWCAsmeasured for the WNRs generated particularly at low Arpressures and extreme substrate tilting angles due to theirsignificant pyramidal tip and isolation. It is concluded thatcontrolling both the space/diameter ratio of nanorods andlowering their solid fractions, along with the presence of pyr-amidal tips, would have a significant impact on the fabrication ofrobust superhydrophobic metallic surfaces.

Figure 7. Schematic illustration of (a) the unit area of flat triangular nanopillars, with the two situations (b and c) of water occupation on thepyramidal tip.


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’AUTHOR INFORMATION

Corresponding Author*G.K.K.: e-mail, [email protected]; tel, 501-569-8067; fax,501-683-7601. A.S.B.: e-mail, [email protected]; tel, 501-551-9067; fax, 501-683-7601.

’ACKNOWLEDGMENT

Financial support from the U.S. Army (ERDC CooperativeAgreementNumber:W912HZ-09-02-0008), theArkansas Science& Technology Authority (Grant # 08-CAT-03), and the Depart-ment of Energy (DE-FG36-06GO86072) and National ScienceFoundation (NSF/EPS-1003970) is greatly appreciated. Theeditorial assistance of Dr. Marinelle Ringer is also acknowledged.

’REFERENCES

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