advanced studies of water evaporation kinetics over teflon-coated tungsten nanorod surfaces with...

Published: June 13, 2011

r 2011 American Chemical Society 13804 dx.doi.org/10.1021/jp203238v | J. Phys. Chem. C 2011, 115, 13804–13812

ARTICLE

pubs.acs.org/JPCC

Advanced Studies of Water Evaporation Kinetics over Teflon-CoatedTungsten Nanorod Surfaces with Variable Hydrophobicity andMorphologyKhedir R. Khedir,† Ganesh K. Kannarpady,*,† Hidetaka Ishihara,† Justin Woo,† Steve Trigwell,‡

Charles Ryerson,§ and Alexandru S. Biris*,†

†Nanotechnology Center, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, Arkansas 72204,United States‡Applied Science and Technology, ASRC Aerospace, ASRC-24 Kennedy Space Center, Orlando, Florida 32899, 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

The nature of water interaction with a solid surface underthermodynamic stability was studied by Young two centuriesago.1 The interaction is mainly governed by the competitionbetween the cohesion forces among the water molecules in thewater droplet and their adhesion forces on the solid surface.Therefore, materials respond differently to the presence of waterat the interface, depending on their corresponding surfaceenergy. Surfaces with high free excess energy tend to overcomethe cohesion forces of water molecules resulting in the spreadof the water droplet, while surfaces with low free excess energycause the water droplet to bead up. The water droplet contactangle (CA) at the point of interface of the three phases (air/liquid/solid) is the measure of solid surface wettability. Anincrease in the CA of water droplets enhances the waterrepellency of a solid surface.

On the basis of Young’s model, Wenzel2 and Cassie3 furtherincreased the fundamental understanding of the water repellencyof surfaces by developing mathematical models that take intoconsideration the surface morphology. By consideration of the

roughness factor, Wenzel developed a classic model to describethe homogeneous wetting between the rough solid surface andthe liquid in contact. A high degree of roughness with specificgeometrical design would promote composite wetting (the com-bination of both solid and vapor under the contact area of thewater droplet), which is described by the Cassie model. Math-ematically, the two models can be represented as follows

cosθ� ¼ γs fs cos θY + fa cos θa ð1Þ

where θ* and θY are the apparent and intrinsic contact angles,respectively, while fs and fa are the fraction of solid�liquidand air�liquid interfaces, respectively, and γs is the roughnessfactor over the top of the solid fraction which is in contact withthe liquid, fa = 1 � fs and θa = 180� as the contact angle atthe air�liquid interface. In the case of homogeneous wetting

Received: April 7, 2011Revised: June 10, 2011

ABSTRACT: Here, we present the process of water dropletevaporation over hydrophobic/superhydrophobic tungstennanorod (WNRs) surfaces with various nanoscale morpholo-gies and porosities. The WNR surfaces were fabricated byvarying both Ar pressure and substrate tilting angle in radio-frequency magnetron sputtering by using the glancing angledeposition technique; their characteristics were analyzed byelectron/atomic force microscopy and spectroscopy. The var-iation in the droplets’ contact angle, contact line diameter, andcentral height as a function of time showed that the evaporationprocess was highly influenced by the nanomorphology of thesubstrate. The surface roughness correlating with the wetting regime (Wenzel and/or Cassie) and the subsequent variation in thecontact angle hysteresis (CAH) of the surfaces had a significant effect on the duration of each of the three evaporation modes thatwere identified. A strong agreement for the CAH determined by using two approaches—dynamic method (adding/withdrawingwater to/from surfaces) and natural evaporation process—was observed. In addition, these nanoscale rough surfaces have shown noabrupt transition from dewetting (Cassie) regime to wetting (Wenzel) regime, and the surfaces are less vulnerable to the transitionin the case of very small-sized water droplets. Such studies could be the foundation for the development of highly tunable surfaceplatform technologies with applications in water or possibly ice mitigation, biology, aerospace.

13805 dx.doi.org/10.1021/jp203238v |J. Phys. Chem. C 2011, 115, 13804–13812

The Journal of Physical Chemistry C ARTICLE

(no air trapped) fs = 1, the Wenzel model will be obtained

cosθ� ¼ γs cos θY ð2ÞBoth physical and chemical heterogeneity of the surfaces result

in the different local contact angles, which hinders the motion ofthe three phases’ contact line. Therefore, the study of waterrepellency of surfaces could not be fully understood in terms ofonly static water contact angle measurements. Furthermore, theinvestigation of contact angle hysteresis (the difference betweenadvancing and receding contact angles) as the measure of contactline “stickiness” to the surface was presented.4�6 Recently,monitoring the evolution of water droplet evaporation on thesuperhydrophobic surface accompanying the decrease in the sizeof water droplet has revealedmore information about the kineticsof interfacial interactions.7�12

The modifications in the contact angle (CA) and contact line(CL) of water droplets deposited on a solid substrate in calm airdue to the diffusion and/or convection of water molecules in theenvironment and the consequent decrease in the size of the waterdroplets can be explained by three modes of evaporation.13�15

The first one is the constant contact line mode (CCL): the line ofcontact remains constant, but a decrease in the contact angle canbe observed. The second stage is the constant contact anglemode (CCA): the contact angle remains constant, but the con-tact line decreases. The third is the mixed mode with variation inboth the contact line and contact angle. Schematic representa-tions of the three modes of evaporation of water droplets on asolid substrate are illustrated in Figure 1. The presence of aspecific mode of evaporation on the solid surface is directlyassociated with the surface geometry and surface chemistry of thesample in addition to the type of associated wetting regimes.16

Despite the vital information that can be obtained from thecharacterization of water evaporation on hydrophobic surfacesregarding the dynamics of wetting, very few studies have beenconducted on nanoscale rough hydrophobic surfaces.17,18

In this research, the evaporation kinetics of microliter waterdroplets on the surfaces of hydrophobic tungsten nanorods(WNRs), coated with Teflon AF 2400, with various nanoscalemorphologies was investigated. Such structures represent idealsystems for the in depth analysis of the behavior of water dropletsdeposited on nanostructured surfaces. The ability to control themorphology and the solid fractions of the surfaces, while keepingthe chemistry identical, is a unique model for such studies, giventheir tunable hydrophobic properties. Glancing angle magnetronsputtering deposition technique was used to fabricate WNRswith various morphologies and porosities and contact angles inthe range of 122�160�. The surface nanoscale roughness andsolid fraction of the WNRs strongly influence the water wettingproperties and result in dramatic changes in evaporation kinetics.The experimental results were compared with simulated results

based on spherical cap and two-parameter ellipsoidal cap modelsfor the droplet shape. These models were used to explain theanalytically obtained results.

2. EXPERIMENTAL PROCEDURE

2.1. Hydrophobic TungstenNanorod Fabrication.Glancing-angle radiofrequency (RF) magnetron sputtering depositiontechnique was used to fabricate WNRs. The variation in Arpressure during the deposition, along with variation in the sub-strate tilting angle, generated a wide range of nanoscale rough-ness with different morphologies and porosities. A thin layer(6 nm) of Teflon AF 2400, purchased from DuPont, was coatedby thermal evaporation out of a crucible at a sublimation tem-perature of around 260 �C, using an effusion cell. More detailsregarding the fabrication of WNRs as well as the influence of Arpressure and substrate tilting angle on the morphology of WNRsand water repellency after chemical modification of their sur-faces, can be found in our previous reports.19�21

2.2. Topography and Surface Chemistry Analysis. Thesurface topography of the samples was characterized using bothscanning electronic microscopy (SEM, JEOL SEM7000FE) andatomic forcemicroscopy (AFM,VEECOAFM,Nanoscope3100).The SEM images were taken from three random locations on thesample; considerable uniformity in the WNR surfaces was ob-served. In the AFM characterization, tapping mode with a slowfrequency of 0.5 μm/s (to give the tip enough time to effectivelyinvade the voids between nanorods) was utilized to measure thesurface roughness ofWNRs. The values of three different scannedlocations were averaged after obtaining consistent results. Further-more, X-ray photoelectron spectroscopy (XPS) was used tocharacterize the surface chemistry of the hydrophobized WNRsafter coating with a thin layer of Teflon AF 2400. By use of theThermo Scientific K-Alpha X-ray photoelectron spectrometer, theXPS data were obtained at a background pressure of 1 � 10�9

Torr via a monochromated Al KR (hυ = 1436.6 eV) X-ray source.A 100-W, 400-μm diameter X-ray beam was used with the surveyscans of (0�1350 eV) on each sample at a pass energy (CAE) of200 and 1 eV step size. The obtained data were referenced to theC1s’ peak at 284.6 eV. For higher resolution analysis of the peaks,narrow scans (25�40 eV width) of the peaks of interest (C1s,O1s, F1s, Ti2p, and W4f) were taken at a pass energy of 50 and0.1 eV step size. In addition, curve fitting was performed on thenarrow scans using the Avantage V. 4.38 software.2.3. Monitoring Natural Evaporation of Water Droplets.

Deioinized water droplets of 2 μL were gently dispensed on theprepared surfaces using a computer-controlled automated syr-inge associated with EasyDrop (DSA20) contact angle tool(Kruss Company, Germany). The evaporation of water dropletson the surfaces of hydrophobic WNRs as a function of time was

Figure 1. Schematic illustration of the three modes of evaporation that were observed for water droplets placed on hydrophobic substrates.



video recorded, using a CCD camera with the capacity for re-cording 60 fps at 780 � 580 resolution, until the entire waterdroplet had evaporated. This procedure was repeated for all ofthe samples under the same ambient condition. The temperatureof the room during all of these experiments was about 23 �C, andthe relative humidity was 35�60%. Eventually, the capturedvideos were reloaded and characterization of the correspondingCA, CL, surface area, and central height of the digitalized imagesof water droplets at the rate of 1 fps was carried out. To ensureaccuracy of the results, the entire procedure was repeated threetimes, and similar results of evaporation were observed.2.4. Contact Angle Hysteresis (CAH) Measurements. The

CAH measurements of the hydrophobic surfaces were carriedout by using the dynamic method of adding and withdrawingwater from the surface while the needle was kept inside the waterdroplet. First, the water was dispensed onto the surfaces in asmall amount of 0.01 μL with a rate of 10 μL/min. The slow rateof addition gave CL enough time to reach the thermodynamicequilibrium and also minimized the influence of viscous forcesinside the water droplet. The adding of water, increasing the sizeof water droplet, was proceeding until a steady CA was obtained,which is considered as the advancing CA. Then, the water waswithdrawn from the surface with the amount and speed of dis-pensing. In the case of relatively small water droplets, the CLstarts receding which keeps the CA unchanged. This measuredCA value was considered as the receding CA. Finally, by sub-tracting both advancing and receding CAs, the CAHs for theprepared samples were obtained. This procedure was repeatedthree times for each sample to enhance the accuracy of the data.

3. RESULTS AND DISCUSSIONS

In this study, nanoscaled thin films of WNRs were generatedin one step by using the glancing angle deposition (GLAD)technique and by varying both the Ar pressure and the substratetilting angle. These films were found to have various overall mor-phologies and porosities due to the nature of the resulting nano-rods, as depicted in Figure 2. An increase in the Ar pressure ofdeposition from 1.0 to 10 mTorr caused an increase in thedensity and a decrease in the lateral size of the WNRs. A furtherincrease in the Ar pressure above 10 mTorr resulted in an ag-glomeration of the tungsten nanorods, and a continuous normalthin film was obtained at 20 mTorr. The individual isolation ofthe nanorods that presented well-defined and unique pyramidaltips was significant at low Ar pressures of 1.0 mTorr. The iso-lation of the nanorods was minimal at higher pressures of above5 mTorr with nanorods showing more mushroomlike head-typetips rather than pyramidal.19 It can be seen in Figure 3a that bothsurface roughness and the top solid fraction ofWNRs have shownan opposite trend with increases in the deposition pressure.

On the other hand, WNRs deposited at an extreme substratetilting angle of 89�, but different thicknesses have shown variousmorphologies and higher porosities, as well. The more pro-nounced shadowing effects at such high tilting angles and thefailure of smaller WNRs to reach the surface as the depositiontime increased have resulted in the generation of well-isolatednanorods with significantly sharper pyramidal tips. This fact canbe observed from the top-view SEM images shown in Figure 2b,and their surface roughness and top solid fraction values are

Figure 2. Top-view SEM images ofWNRs deposited under (a) different Ar pressures and (b) with a substrate tilt angle of 89�with various thicknesses at1 mTorr. Insets are the AFM images of the corresponding WNR surfaces with scanned area of 1.0 μm2.



plotted in Figure 3b. An image analysis technique was appliedto the top view SEM images to determine the solid fraction ofthe as-prepared samples. The details of this approach have beenpresented elsewhere.21

The chemical modification of the WNR film surfaces wasperformed by depositing a thin layer of Teflon AF2400. The XPSanalysis as demonstrated in Figure 4 has shown the elementalcomposition of Teflon AF 2400 on the surface of chemicallymodifiedWNRs. The heights of the elemental peaks for both thepredeposited and postdeposited Teflon AF 2400 are comparablewith slight variations for the O peak. The increase in the ele-mental concentration of O is due to the existence of TiO2 on thesurface of the WNRs. The target that was used for WNRdeposition had a 10 wt % of Ti for enhancing the adhesion ofW material to the glass substrate. This accounts for the appear-ance of a significant Ti peak among the Teflon AF 2400 ele-mental peaks, in addition to the W peak from the WNRs. Thelayer of Teflon AF 2400 (of less than 10 nm thickness) permittedthe X-ray beam to reach the surface of deposited WNRs, andconsequently, peaks of both W and Ti appear in the spectrum ofTeflon AF 2400, as well.

The prepared samples demonstrated highly tunable hydro-phobicity with apparent water contact angles (AWCAs) rangingfrom 122 to 160�, as shown in Figure 3. The wide range of waterrepellency properties of these surfaces can be attributed to theirvarious surface roughness and solid fraction values in contactwith water droplets. In addition, the unique pyramidal tips ofWNRs fabricated at relatively low Ar pressures along with theincrease in their tip sharpness with height had significant impacton enhancing the water repellency due to lowering of the effec-tive solid fraction. The adoption of a particular solid surface waterinteraction regime (Wenzel and/or Cassie) at the interfaces ofthe WNR surfaces can be studied in terms of both the films’surface roughness and the solid fraction of the samples. Suchan investigation has shown that the surfaces formed of isolated

Figure 3. Solid fraction and surface roughness determined by imageanalysis techniques and AFM, respectively, against (a) the depositionpressure and (b) the height of WNRs deposited at constant Ar pressureof 1.0 mtorr and substrate tilting angle of 89�.

Figure 4. XPS analysis of WNR surfaces coated with a nanolayer of Teflon AF 2400.



WNRs with significant empty spaces in between (WNRs depos-ited under Ar pressure of 1.0, 5.0 mTorr, and the extremesubstrate tilting angle of 89�) and with AWCA of around 150

to 160�, can largely be accounted for by the Cassie model. Thesamples generated at 10 mTorr and with relatively lower rough-ness but higher solid fraction are more likely to induce the

Figure 5. Evolution of CA, CL, and central height (h) of 2μLwater droplet at 23 �Con the surfaces of hydrophobic/superhydrophobicWNRs preparedunder (a) different Ar pressures and (b) substrate tilt angle of 89� with various thicknesses and constant deposition pressure of 1.0 mtorr.



coexistence of both regimes. Finally, the thin films prepared at20 mTorr, which present relatively smooth surfaces (rs = 1.05),promote a homogeneous wetting process in the Wenzel regime.The details of these analyses were presented in great detail in ourprevious work.21

A small deionized (DI) water droplet of 2 μL was gentlydispensed on the as-produced surfaces, and its morphology andstability were monitored as a function of time. The diffusion ofwater molecules into the ambient environment results in the sizereduction of the droplet and consequently a variation of the CAand CL parameters along with a modification in its central height(h). Figure 5 shows the evolution of CA, CL, and h as a result ofthe evaporation process of small water droplets placed overhydrophobic WNR surfaces. It can be clearly observed that theevaporation of water droplets undergoes three different modes ofevaporation. The first stage begins with the constant CL (CCL)mode of evaporation and a nonlinear decrease in CA but lineardecrease in h. After the droplet reached a significantly smaller sizecompared to the initial value, because of the water molecules’diffusion into the surrounding environment, the CL began torecede linearly accompanied by a quasiconstant CA along with anonlinear reduction in h. While the evaporation was proceedingin the CCA mode, the CAs fluctuated, exhibiting local peaks inthat stage, which was more pronounced for surfaces with highervalues of solid fraction. This phenomenon, as can be noticedfrom Figure 5, is most probably due to the swift receding of theCL in the stick�slip fashion, as suggested by Erbil et al.12

Eventually, at very small droplet sizes, the mixed mode (MM)governs the evaporation process by a simultaneous, almost linear,

reduction in CA, CL, and h. However, the evaporation process inthe MM regime seems to be much faster than in the two previousmodes—a fact clearly observed from both the CL and h mag-nitudes at the end of the two curves showing a very fast decreaseslope.22 A similar observation was reported previously by Zhanget al.23 when studying water droplet evaporation over a Lotus leaf(hierarchical rough surface that showed a droplet CA of morethan 160�). It was shown that the evaporation process primarilyoccurred in the CCL mode. However, the reduction in CA wasvery slow in the first 15 min—followed by a sharp decrease inits value (occurring at CA of 140�) until the droplet entirelyvanished after 20 min.

Figure 6 shows the time-percentage period of each modeduring the entire evaporation process related to each sample. Forthe samples prepared under various Ar pressure, Figure 6ashows that the CCL mode is the dominant mode of evaporationfor all of the samples. However, the corresponding lengths oftime decreased substantially from 75 to 45% of the total evap-oration time as the deposition pressure, during the WNR gen-eration, increased from 1.0 mTorr to 20 mTorr, respectively.This means that the decrease in surface roughness eased the CLmotion as the droplet reached a critical size. On the other hand,the porosity induces an opposite effect as long as the CL isbridging over the pillars (Cassie regime). Therefore, a lesser solidfraction at the solid�liquid interface generates lower resistanceto the CL motion. This fact can be clearly seen in the case ofWNRs fabricated at extreme substrate tilting angles of 89� andwith height profiles of 200, 400, and 600 nm as shown in Figure 5b.Despite a substantial increase in the surface roughness, the waterdroplets stay in the CCA mode for almost 30% of the totalevaporation time. The WNRs fabricated under various Ar pres-sures and, more specifically, the samples that were deposited atrelatively high Ar pressures (10 and 20 mtorr) with a low degreeof roughness have adopted the CCA mode of evaporation for26 and 45% of the evaporation time, respectively.

It has been shown that the transition between the variousmodes of evaporation at the interfaces with solid surfaces iscontrolled by the onset of CAH, induced by both physical andchemical heterogeneities over the surfaces.23,24 To investigatethe range of applicability of this hypothesis for nanoscale rough-ness surfaces, CAH measurements were conducted by using thedynamic CA method (adding and withdrawing water from thesurface). A typical example of these measurements for the twoWNR surfaces with minimum and maximum CAs is demon-strated in Figure 7. In the comparison of the CAHs that weremeasured by both evaporation and dynamic (adding/withdraw-ing of water) methods, Figure 8 shows a consistent agreementbetween the two types of CAHmeasurements for the two sets ofhydrophobic/superhydrophobic WNRs. This linear relationshipwas also previously reported by Xinping et al.11 while conductingtime dependence CA on polymeric surfaces.

The values of CAH, plotted in Figure 8, show that the surfaceswith higher CAH stayed in the CCL evaporationmode for longerperiods of time. Kulinich et al.9 have reported the same observa-tion by examining water droplet evaporation on two differentsuperhydrophobic surfaces with CAs of ∼152� but differentCAH (one sample with high CAH of around 70� and a secondone with very low CAH of 5�). Their investigation showed thatthe sample with high CAH was evaporating mostly in the CCLmode due to the pinned CL in the surface texture, while thesample with low CAH followed the CCA evaporation mode asa result of the very low solid fraction and consequently lower

Figure 6. Percentages of the time periods for the three modes of waterdroplet evaporation placed over hydrophobic/superhydrophobic WNRinterfaces fabricated at (a) different Ar pressures and (b) substrate tiltangle of 89�with various thicknesses and constant deposition pressure of1.0 mtorr.



resistance of the solid surface to the motion of CL. Thus, the easymotion of the CL and consequent quasiconstant CA over thesolid surface is in response to the low CAH of the surface.

Theoretically, for smooth solid substrates with ideally vanish-ing CAH, the water droplets should retain the initial CA duringthe entire evaporation process. Experimentally, it has beenreported that the CCL mode is the dominant characteristic ofthe water droplet evaporation process over smooth hydrophilic

surfaces, while the CCA mode is dominant for smooth hydro-phobic surfaces.12,14,25 However, water droplet evaporation onrough surfaces undergoes various modes with different time du-rations, depending on the CAH.7�10,26 In this study, because ofthe various morphologies and porosities of the WNR surfaces—different wetting regimes—and consequent variation in theirCAH, the three modes of evaporation with different time periodshave been observed and recorded. This is primarily due to thevariation in the CAH of the surfaces that ranged from 25� toalmost 60�. Therefore, the surface roughness in the case ofWenzel state and solid fraction in the case of Cassie state, alongwith the chemical homogeneity of the surface, are the most im-portant characteristics controlling a specific mode of waterdroplet evaporation over a specific surface.

Interestingly, it was observed (Figure 5) that there was nosudden increase in the CL or sudden decrease in the height ofwater droplets for the samples that are believed to have adoptedthe Cassie regime of wetting. However, the dramatic decrease inCA during the last part of the evaporation process could be anindication of a smooth transition from Cassie to Wenzel states.27

This fact can also be realized from the evolution of the centralheight profile of the water droplet (Figure 5) and digital imagesof the water droplet which flatten smoothly as a function of time,as depicted in Figure 9. While in the case of micropatternedsurfaces even with very lowCAH, the transition fromCassie stateto Wenzel state due to the small size of water droplets takes placeabruptly with a significant increase in the CL and sudden de-crease in the height of the water-droplet cap.7,8 Tsai et al.28 haveobserved experimentally the critical CL at which the transitionfrom Cassie to Wenzel regime occurs. Using the global energy atthe interface, they were able to predict the critical CL for suchtransition—showing that there is a profound relation between thesize of water droplets and the geometric arrangement of micro-pillars. To prevent the microstructures from triggering such transi-tions in the wetting regimes, Reyssat et al.8 have suggested that thepillarsmust be tall enough to prevent the pressure on the curvatureto touch the base of the pillars and consequently decrease theenergy of the system by wetting the nanostructural texture.

In this study, theoretical investigation of the results was alsoperformed. The two geometrical cap models (spherical capmodel and two-parameter ellipsoidal cap model)—derived forhydrophobic surfaces by Erbil and Meric29 and extended forsuperhydrophobic surfaces by Zhang et al.23—were examined.

Figure 7. A typical example of CAH analysis performed by increasing and then decreasing the water droplet sizes that were placed over the surfaces ofhydrophobic WNRs obtained under the following experimental conditions: (a) 20 mtorr Ar pressure and substrate tilt angle of 85�; (b) 1.0 mtorr Arpressure, substrate tilt angle of 89�, and height of 600 nm.

Figure 8. CAHdetermined by both dynamic and evaporation processesfor fabricated WNRs for the following conditions: (a) various Arpressures; (b) substrate tilt angle of 89� and different nanorod heightsbut constant Ar pressure of 1.0 mtorr.



In the two-parameter ellipsoidal cap geometry, the height of thewater droplet in terms of both CA and CL diameter can berepresented as follows

hðθ, dÞ ¼ d2 tan θ

½ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiRðR + tan2 θÞ

p( R� ð3Þ

where R = 1 � e2, where e is the eccentricity of the ellipse withvalues 1 > eg 0 for the oblate ellipsoidal cap in the case of sessilewater droplet. Both θ and d are the CA and CL diameter,respectively. For the spherical cap model (e = 0), the aboveequation of height reduces in the following formula

hðθ, dÞ ¼ d2tanðθ=2Þ ð4Þ

The simulation results using both geometrical models showedthat the experimental data are in better agreement when ef 0 isin the limits of the spherical capmodel. As shown in Figure 10, forCCL stage, which was the first portion of the evaporation processfor all of the samples, the experimental data would fit better withthe ellipsoidal cap model. While the match between the experi-mental results and simulation using the spherical cap model ismore pronounced in the CCA stage, the CL shrinkage from bothsides during the evaporation process retains the spherical shapeof the droplet. The natural evaporation of water droplets over thehydrophobic WNR surfaces of various morphologies followed asimilar scenario in terms of the sequence of the evaporationmodes. The dynamics of water drop size reduction began in the

CCL mode; then, after some time, the evaporation switched to aCCA mode. Finally, at the very end of evaporation, the MMprocess dominated. Nevertheless, the primary difference was thetime period for each mode governing the evaporation process,whichwasmainly associatedwith theCAHvalues. In addition, theanalyses of small water droplet evaporation over nanoscale filmsshowed the robustness of these hydrophobic surfaces against asudden collapse of the water droplets into the gaps between thenanorods. Even if there was a transition from a composite state(Cassie state) to a homogeneous state (Wenzel state), this wouldhave been characteristic of a smooth process without a steplikediscontinuity and would probably occur at the very end of theevaporation process (very small-sized water droplets). On theother hand, the as-produced surfaces exhibited relatively highCAH (as determined from the dynamic CAH measurements),which resulted in CCL becoming the dominant mode of evap-oration. Therefore, robust superhydrophobic metallic materialscan be produced by using nanoscale pillarlike surface geometriesthat, in addition to exhibiting low CAH (slippery surfaces), alsoprevent the pinning of the CL and consequent wetting of thesurface texture of small diameter water droplets. The growth ofsuch nanomaterials with tunable surface properties and mor-phologies but identical chemistries represents a powerful modelfor the study of water droplet evaporation kinetics. We presenteda simple GLAD-based, one-step process for the growth oftungsten nanorods with variable lengths, interspacing, and topgeometries that were found to have tunable surface properties

Figure 9. A typical example of digital images of 2 μL water droplets as a function of evaporation time when placed over the surfaces of (a) hydrophobicWNRs deposited under Ar pressure of 1.0 mtorr and substrate tilt angle of 85� and (b) superhydrophobic WNRs fabricated under Ar pressure of1.0 mtorr, substrate tilt angle of 89�, and height of 600 nm.

Figure 10. Comparison between simulation [using both spherical and ellipsoidal cap geometry (e = 0.5) models] and experimental data of waterdroplet’s central height vs evaporation time forWNRs deposited at (a) 10 mTorr and (b) 1.0 mTorr, substrate tilt-angle (89�), and thickness of 600 nm.



with a CA ranging from 122 to 160�, with important applicationsin a large number of areas: materials science, biology, water andice mitigation, self-cleaning surfaces, and the aerospace industry.

Moreover, as mentioned above, one potential application ofsuperhydrophobic surfaces, which promotes the CCA mode ofwater droplet evaporation, is ice and water mitigation. Recentstudies have shown a strong correlation between the water repel-lency of surfaces and their anti-icing properties in low humidityconditions.30�32 Superhydrophobic surfaces with the water dropletevaporation taking place mostly in the CCA mode, with easymobility of the CL and low CAH, tend to repel supercooled waterdroplets when the temperature of the surface is as low as �25 �Cand relative humidity is less than 10%.31 However, in more humidconditions, and in the presence of frost formation, superhydro-phobic surfaces have failed to prevent ice accumulation on theirsurfaces.33,34 Interestingly, a very recent study by Antonini et al.35

demonstrated that Teflon-coated superhydrophobic airplane wingstructures with CAH of around 6� required 80% less energy toremove the accumulated ice as compared to the uncoated alumi-num surface, using embedded heaters on thewings, under the sameicing conditions. Therefore, superhydrophobic surfaces with lowCAH can still be considered the most promising candidates for ice-mitigation applications even at relatively high humidity.

4. CONCLUSIONS

In this work, water droplet evaporation processes over nanoscaleWNR films with tunable hydrophobic/superhydrophobic proper-ties were studied by monitoring the dimensions of the water drop-lets as a function of time. The WNRs with various surface morpho-logies and porosities were fabricated using RFmagnetron sputtering(GLAD technique) by varying the Ar pressure and substrate tiltingangle. The analyses have shown that the evaporation process oc-curred in three modes but of various durations explained by thefilms’ nanoscale surface roughness and corresponding wettingregime. The pinned CL mode was the first and relatively dominantmode of evaporation.However, no sign of droplet collapse or abrupttransition from dewetting regime to wetting regime over the waterrepellent WNR surfaces was observed. A comparison between thedynamic CAH measured by adding/withdrawing water from thesurfaces with the CAH due to evaporation was performed, and aconsistent agreement was observed. Theoretical simulation of thedecrease in the central height of the water droplet during the evap-oration process showed that, during the CCL stage, the experi-mental data is best fitted by the ellipsoidal cap geometry model,whereas, during the CCA stage, the spherical cap model is the mostaccurate. Finally, it can be concluded that, for small droplets, thesuperhydrophobic surfaces with nanoscale roughness are less proneto trigger the transition fromCassie toWenzel regimes than surfaceswith microscaled roughness.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected] (G.K.K.); [email protected](A.S.B.). Phone: 501-569-8067 (G.K.K.); 501-551-9067 (A.S.B.).Fax: 501-683-7601 (G.K.K.); 501-683-7601 (A.S.B.).

’ACKNOWLEDGMENT

Financial support from the U.S. Army (ERDC CooperativeAgreement Number: W912HZ-09-02-0008), the Arkansas

Science & Technology Authority (Grant No. 08-CAT-03), theDepartment of Energy (DE-FG36-06GO86072), and NationalScience Foundation (NSF/EPS-1003970) is greatly appre-ciated. The editorial assistance of Dr. Marinelle Ringer is alsoacknowledged.

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