Advanced Studies of Water Evaporation Kinetics over Teflon-Coated

ARTICLE pubs.acs.org/JPCC

Advanced Studies of Water Evaporation Kinetics over Teflon-Coated Tungsten Nanorod Surfaces with Variable Hydrophobicity and Morphology Khedir 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 ABSTRACT: Here, we present the process of water droplet evaporation over hydrophobic/superhydrophobic tungsten nanorod (WNRs) surfaces with various nanoscale morphologies and porosities. The WNR surfaces were fabricated by varying both Ar pressure and substrate tilting angle in radiofrequency magnetron sputtering by using the glancing angle deposition technique; their characteristics were analyzed by electron/atomic force microscopy and spectroscopy. The variation in the droplets’ contact angle, contact line diameter, and central height as a function of time showed that the evaporation process was highly influenced by the nanomorphology of the substrate. The surface roughness correlating with the wetting regime (Wenzel and/or Cassie) and the subsequent variation in the contact angle hysteresis (CAH) of the surfaces had a significant effect on the duration of each of the three evaporation modes that were identified. A strong agreement for the CAH determined by using two approaches—dynamic method (adding/withdrawing water to/from surfaces) and natural evaporation process—was observed. In addition, these nanoscale rough surfaces have shown no abrupt transition from dewetting (Cassie) regime to wetting (Wenzel) regime, and the surfaces are less vulnerable to the transition in the case of very small-sized water droplets. Such studies could be the foundation for the development of highly tunable surface platform technologies with applications in water or possibly ice mitigation, biology, aerospace.

1. INTRODUCTION The nature of water interaction with a solid surface under thermodynamic stability was studied by Young two centuries ago.1 The interaction is mainly governed by the competition between the cohesion forces among the water molecules in the water droplet and their adhesion forces on the solid surface. Therefore, materials respond differently to the presence of water at the interface, depending on their corresponding surface energy. Surfaces with high free excess energy tend to overcome the cohesion forces of water molecules resulting in the spread of the water droplet, while surfaces with low free excess energy cause the water droplet to bead up. The water droplet contact angle (CA) at the point of interface of the three phases (air/ liquid/solid) is the measure of solid surface wettability. An increase in the CA of water droplets enhances the water repellency of a solid surface. On the basis of Young’s model, Wenzel2 and Cassie3 further increased the fundamental understanding of the water repellency of surfaces by developing mathematical models that take into consideration the surface morphology. By consideration of the r 2011 American Chemical Society

roughness factor, Wenzel developed a classic model to describe the homogeneous wetting between the rough solid surface and the liquid in contact. A high degree of roughness with specific geometrical design would promote composite wetting (the combination of both solid and vapor under the contact area of the water droplet), which is described by the Cassie model. Mathematically, 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 solidliquid and airliquid interfaces, respectively, and γs is the roughness factor over the top of the solid fraction which is in contact with the liquid, fa = 1 fs and θa = 180° as the contact angle at the airliquid interface. In the case of homogeneous wetting Received: April 7, 2011 Revised: June 10, 2011 Published: June 13, 2011 13804

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

The Journal of Physical Chemistry C

ARTICLE

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

(no air trapped) fs = 1, the Wenzel model will be obtained cosθ ¼ γs cos θY

ð2Þ

Both physical and chemical heterogeneity of the surfaces result in the different local contact angles, which hinders the motion of the three phases’ contact line. Therefore, the study of water repellency of surfaces could not be fully understood in terms of only static water contact angle measurements. Furthermore, the investigation of contact angle hysteresis (the difference between advancing and receding contact angles) as the measure of contact line “stickiness” to the surface was presented.46 Recently, monitoring the evolution of water droplet evaporation on the superhydrophobic surface accompanying the decrease in the size of water droplet has revealed more information about the kinetics of interfacial interactions.712 The modifications in the contact angle (CA) and contact line (CL) of water droplets deposited on a solid substrate in calm air due to the diffusion and/or convection of water molecules in the environment and the consequent decrease in the size of the water droplets can be explained by three modes of evaporation.1315 The first one is the constant contact line mode (CCL): the line of contact remains constant, but a decrease in the contact angle can be observed. The second stage is the constant contact angle mode (CCA): the contact angle remains constant, but the contact line decreases. The third is the mixed mode with variation in both the contact line and contact angle. Schematic representations of the three modes of evaporation of water droplets on a solid substrate are illustrated in Figure 1. The presence of a specific mode of evaporation on the solid surface is directly associated with the surface geometry and surface chemistry of the sample in addition to the type of associated wetting regimes.16 Despite the vital information that can be obtained from the characterization of water evaporation on hydrophobic surfaces regarding the dynamics of wetting, very few studies have been conducted on nanoscale rough hydrophobic surfaces.17,18 In this research, the evaporation kinetics of microliter water droplets on the surfaces of hydrophobic tungsten nanorods (WNRs), coated with Teflon AF 2400, with various nanoscale morphologies was investigated. Such structures represent ideal systems for the in depth analysis of the behavior of water droplets deposited on nanostructured surfaces. The ability to control the morphology and the solid fractions of the surfaces, while keeping the chemistry identical, is a unique model for such studies, given their tunable hydrophobic properties. Glancing angle magnetron sputtering deposition technique was used to fabricate WNRs with various morphologies and porosities and contact angles in the range of 122160°. The surface nanoscale roughness and solid fraction of the WNRs strongly influence the water wetting properties 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 models for the droplet shape. These models were used to explain the analytically obtained results.

2. EXPERIMENTAL PROCEDURE 2.1. Hydrophobic Tungsten Nanorod Fabrication. Glancingangle radiofrequency (RF) magnetron sputtering deposition technique was used to fabricate WNRs. The variation in Ar pressure during the deposition, along with variation in the substrate tilting angle, generated a wide range of nanoscale roughness with different morphologies and porosities. A thin layer (6 nm) of Teflon AF 2400, purchased from DuPont, was coated by thermal evaporation out of a crucible at a sublimation temperature of around 260 °C, using an effusion cell. More details regarding the fabrication of WNRs as well as the influence of Ar pressure and substrate tilting angle on the morphology of WNRs and water repellency after chemical modification of their surfaces, can be found in our previous reports.1921 2.2. Topography and Surface Chemistry Analysis. The surface topography of the samples was characterized using both scanning electronic microscopy (SEM, JEOL SEM7000FE) and atomic force microscopy (AFM, VEECO AFM, Nanoscope3100). The SEM images were taken from three random locations on the sample; considerable uniformity in the WNR surfaces was observed. In the AFM characterization, tapping mode with a slow frequency of 0.5 μm/s (to give the tip enough time to effectively invade the voids between nanorods) was utilized to measure the surface roughness of WNRs. The values of three different scanned locations were averaged after obtaining consistent results. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to characterize the surface chemistry of the hydrophobized WNRs after coating with a thin layer of Teflon AF 2400. By use of the Thermo Scientific K-Alpha X-ray photoelectron spectrometer, the XPS data were obtained at a background pressure of 1 109 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 survey scans of (01350 eV) on each sample at a pass energy (CAE) of 200 and 1 eV step size. The obtained data were referenced to the C1s’ peak at 284.6 eV. For higher resolution analysis of the peaks, narrow scans (2540 eV width) of the peaks of interest (C1s, O1s, F1s, Ti2p, and W4f) were taken at a pass energy of 50 and 0.1 eV step size. In addition, curve fitting was performed on the narrow 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 the prepared surfaces using a computer-controlled automated syringe associated with EasyDrop (DSA20) contact angle tool (Kruss Company, Germany). The evaporation of water droplets on the surfaces of hydrophobic WNRs as a function of time was 13805

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


ARTICLE

Figure 2. Top-view SEM images of WNRs deposited under (a) different Ar pressures and (b) with a substrate tilt angle of 89° with various thicknesses at 1 mTorr. Insets are the AFM images of the corresponding WNR surfaces with scanned area of 1.0 μm2.

video recorded, using a CCD camera with the capacity for recording 60 fps at 780 580 resolution, until the entire water droplet had evaporated. This procedure was repeated for all of the samples under the same ambient condition. The temperature of the room during all of these experiments was about 23 °C, and the relative humidity was 3560%. Eventually, the captured videos were reloaded and characterization of the corresponding CA, CL, surface area, and central height of the digitalized images of water droplets at the rate of 1 fps was carried out. To ensure accuracy of the results, the entire procedure was repeated three times, and similar results of evaporation were observed. 2.4. Contact Angle Hysteresis (CAH) Measurements. The CAH measurements of the hydrophobic surfaces were carried out by using the dynamic method of adding and withdrawing water from the surface while the needle was kept inside the water droplet. First, the water was dispensed onto the surfaces in a small amount of 0.01 μL with a rate of 10 μL/min. The slow rate of addition gave CL enough time to reach the thermodynamic equilibrium and also minimized the influence of viscous forces inside the water droplet. The adding of water, increasing the size of water droplet, was proceeding until a steady CA was obtained, which is considered as the advancing CA. Then, the water was withdrawn from the surface with the amount and speed of dispensing. In the case of relatively small water droplets, the CL starts receding which keeps the CA unchanged. This measured CA value was considered as the receding CA. Finally, by subtracting both advancing and receding CAs, the CAHs for the prepared samples were obtained. This procedure was repeated three times for each sample to enhance the accuracy of the data.

3. RESULTS AND DISCUSSIONS In this study, nanoscaled thin films of WNRs were generated in one step by using the glancing angle deposition (GLAD) technique and by varying both the Ar pressure and the substrate tilting angle. These films were found to have various overall morphologies and porosities due to the nature of the resulting nanorods, as depicted in Figure 2. An increase in the Ar pressure of deposition from 1.0 to 10 mTorr caused an increase in the density and a decrease in the lateral size of the WNRs. A further increase in the Ar pressure above 10 mTorr resulted in an agglomeration of the tungsten nanorods, and a continuous normal thin film was obtained at 20 mTorr. The individual isolation of the nanorods that presented well-defined and unique pyramidal tips was significant at low Ar pressures of 1.0 mTorr. The isolation of the nanorods was minimal at higher pressures of above 5 mTorr with nanorods showing more mushroomlike head-type tips rather than pyramidal.19 It can be seen in Figure 3a that both surface roughness and the top solid fraction of WNRs have shown an opposite trend with increases in the deposition pressure. On the other hand, WNRs deposited at an extreme substrate tilting angle of 89°, but different thicknesses have shown various morphologies and higher porosities, as well. The more pronounced shadowing effects at such high tilting angles and the failure of smaller WNRs to reach the surface as the deposition time increased have resulted in the generation of well-isolated nanorods with significantly sharper pyramidal tips. This fact can be observed from the top-view SEM images shown in Figure 2b, and their surface roughness and top solid fraction values are 13806



Figure 3. Solid fraction and surface roughness determined by image analysis techniques and AFM, respectively, against (a) the deposition pressure and (b) the height of WNRs deposited at constant Ar pressure of 1.0 mtorr and substrate tilting angle of 89°.

ARTICLE

plotted in Figure 3b. An image analysis technique was applied to the top view SEM images to determine the solid fraction of the as-prepared samples. The details of this approach have been presented elsewhere.21 The chemical modification of the WNR film surfaces was performed by depositing a thin layer of Teflon AF2400. The XPS analysis as demonstrated in Figure 4 has shown the elemental composition of Teflon AF 2400 on the surface of chemically modified WNRs. The heights of the elemental peaks for both the predeposited and postdeposited Teflon AF 2400 are comparable with slight variations for the O peak. The increase in the elemental concentration of O is due to the existence of TiO2 on the surface of the WNRs. The target that was used for WNR deposition had a 10 wt % of Ti for enhancing the adhesion of W material to the glass substrate. This accounts for the appearance of a significant Ti peak among the Teflon AF 2400 elemental peaks, in addition to the W peak from the WNRs. The layer of Teflon AF 2400 (of less than 10 nm thickness) permitted the X-ray beam to reach the surface of deposited WNRs, and consequently, peaks of both W and Ti appear in the spectrum of Teflon AF 2400, as well. The prepared samples demonstrated highly tunable hydrophobicity with apparent water contact angles (AWCAs) ranging from 122 to 160°, as shown in Figure 3. The wide range of water repellency properties of these surfaces can be attributed to their various surface roughness and solid fraction values in contact with water droplets. In addition, the unique pyramidal tips of WNRs fabricated at relatively low Ar pressures along with the increase in their tip sharpness with height had significant impact on enhancing the water repellency due to lowering of the effective solid fraction. The adoption of a particular solid surface water interaction regime (Wenzel and/or Cassie) at the interfaces of the WNR surfaces can be studied in terms of both the films’ surface roughness and the solid fraction of the samples. Such an investigation has shown that the surfaces formed of isolated

Figure 4. XPS analysis of WNR surfaces coated with a nanolayer of Teflon AF 2400. 13807



ARTICLE

Figure 5. Evolution of CA, CL, and central height (h) of 2 μL water droplet at 23 °C on the surfaces of hydrophobic/superhydrophobic WNRs prepared under (a) different Ar pressures and (b) substrate tilt angle of 89° with various thicknesses and constant deposition pressure of 1.0 mtorr.

WNRs with significant empty spaces in between (WNRs deposited under Ar pressure of 1.0, 5.0 mTorr, and the extreme substrate tilting angle of 89°) and with AWCA of around 150

to 160°, can largely be accounted for by the Cassie model. The samples generated at 10 mTorr and with relatively lower roughness but higher solid fraction are more likely to induce the 13808



Figure 6. Percentages of the time periods for the three modes of water droplet evaporation placed over hydrophobic/superhydrophobic WNR interfaces fabricated at (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 at 20 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 our previous work.21 A small deionized (DI) water droplet of 2 μL was gently dispensed on the as-produced surfaces, and its morphology and stability were monitored as a function of time. The diffusion of water molecules into the ambient environment results in the size reduction of the droplet and consequently a variation of the CA and CL parameters along with a modification in its central height (h). Figure 5 shows the evolution of CA, CL, and h as a result of the evaporation process of small water droplets placed over hydrophobic WNR surfaces. It can be clearly observed that the evaporation of water droplets undergoes three different modes of evaporation. The first stage begins with the constant CL (CCL) mode of evaporation and a nonlinear decrease in CA but linear decrease in h. After the droplet reached a significantly smaller size compared to the initial value, because of the water molecules’ diffusion into the surrounding environment, the CL began to recede linearly accompanied by a quasiconstant CA along with a nonlinear reduction in h. While the evaporation was proceeding in the CCA mode, the CAs fluctuated, exhibiting local peaks in that stage, which was more pronounced for surfaces with higher values of solid fraction. This phenomenon, as can be noticed from Figure 5, is most probably due to the swift receding of the CL in the stickslip 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,

ARTICLE

reduction in CA, CL, and h. However, the evaporation process in the MM regime seems to be much faster than in the two previous modes—a fact clearly observed from both the CL and h magnitudes at the end of the two curves showing a very fast decrease slope.22 A similar observation was reported previously by Zhang et al.23 when studying water droplet evaporation over a Lotus leaf (hierarchical rough surface that showed a droplet CA of more than 160°). It was shown that the evaporation process primarily occurred in the CCL mode. However, the reduction in CA was very slow in the first 15 min—followed by a sharp decrease in its value (occurring at CA of 140°) until the droplet entirely vanished after 20 min. Figure 6 shows the time-percentage period of each mode during the entire evaporation process related to each sample. For the samples prepared under various Ar pressure, Figure 6a shows that the CCL mode is the dominant mode of evaporation for all of the samples. However, the corresponding lengths of time decreased substantially from 75 to 45% of the total evaporation time as the deposition pressure, during the WNR generation, increased from 1.0 mTorr to 20 mTorr, respectively. This means that the decrease in surface roughness eased the CL motion as the droplet reached a critical size. On the other hand, the porosity induces an opposite effect as long as the CL is bridging over the pillars (Cassie regime). Therefore, a lesser solid fraction at the solidliquid interface generates lower resistance to the CL motion. This fact can be clearly seen in the case of WNRs fabricated at extreme substrate tilting angles of 89° and with height profiles of 200, 400, and 600 nm as shown in Figure 5b. Despite a substantial increase in the surface roughness, the water droplets stay in the CCA mode for almost 30% of the total evaporation time. The WNRs fabricated under various Ar pressures and, more specifically, the samples that were deposited at relatively high Ar pressures (10 and 20 mtorr) with a low degree of roughness have adopted the CCA mode of evaporation for 26 and 45% of the evaporation time, respectively. It has been shown that the transition between the various modes of evaporation at the interfaces with solid surfaces is controlled by the onset of CAH, induced by both physical and chemical heterogeneities over the surfaces.23,24 To investigate the range of applicability of this hypothesis for nanoscale roughness surfaces, CAH measurements were conducted by using the dynamic CA method (adding and withdrawing water from the surface). A typical example of these measurements for the two WNR surfaces with minimum and maximum CAs is demonstrated in Figure 7. In the comparison of the CAHs that were measured by both evaporation and dynamic (adding/withdrawing of water) methods, Figure 8 shows a consistent agreement between the two types of CAH measurements for the two sets of hydrophobic/superhydrophobic WNRs. This linear relationship was also previously reported by Xinping et al.11 while conducting time dependence CA on polymeric surfaces. The values of CAH, plotted in Figure 8, show that the surfaces with higher CAH stayed in the CCL evaporation mode for longer periods of time. Kulinich et al.9 have reported the same observation by examining water droplet evaporation on two different superhydrophobic surfaces with CAs of ∼152° but different CAH (one sample with high CAH of around 70° and a second one with very low CAH of 5°). Their investigation showed that the sample with high CAH was evaporating mostly in the CCL mode due to the pinned CL in the surface texture, while the sample with low CAH followed the CCA evaporation mode as a result of the very low solid fraction and consequently lower 13809



ARTICLE

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

Figure 8. CAH determined by both dynamic and evaporation processes for fabricated WNRs for the following conditions: (a) various Ar pressures; (b) substrate tilt angle of 89° and different nanorod heights but constant Ar pressure of 1.0 mtorr.

resistance of the solid surface to the motion of CL. Thus, the easy motion of the CL and consequent quasiconstant CA over the solid surface is in response to the low CAH of the surface. Theoretically, for smooth solid substrates with ideally vanishing CAH, the water droplets should retain the initial CA during the entire evaporation process. Experimentally, it has been reported that the CCL mode is the dominant characteristic of the water droplet evaporation process over smooth hydrophilic

surfaces, while the CCA mode is dominant for smooth hydrophobic surfaces.12,14,25 However, water droplet evaporation on rough surfaces undergoes various modes with different time durations, depending on the CAH.710,26 In this study, because of the various morphologies and porosities of the WNR surfaces— different wetting regimes—and consequent variation in their CAH, the three modes of evaporation with different time periods have been observed and recorded. This is primarily due to the variation in the CAH of the surfaces that ranged from 25° to almost 60°. Therefore, the surface roughness in the case of Wenzel state and solid fraction in the case of Cassie state, along with the chemical homogeneity of the surface, are the most important characteristics controlling a specific mode of water droplet evaporation over a specific surface. Interestingly, it was observed (Figure 5) that there was no sudden increase in the CL or sudden decrease in the height of water droplets for the samples that are believed to have adopted the Cassie regime of wetting. However, the dramatic decrease in CA during the last part of the evaporation process could be an indication of a smooth transition from Cassie to Wenzel states.27 This fact can also be realized from the evolution of the central height profile of the water droplet (Figure 5) and digital images of the water droplet which flatten smoothly as a function of time, as depicted in Figure 9. While in the case of micropatterned surfaces even with very low CAH, the transition from Cassie state to Wenzel state due to the small size of water droplets takes place abruptly with a significant increase in the CL and sudden decrease in the height of the water-droplet cap.7,8 Tsai et al.28 have observed experimentally the critical CL at which the transition from Cassie to Wenzel regime occurs. Using the global energy at the interface, they were able to predict the critical CL for such transition—showing that there is a profound relation between the size of water droplets and the geometric arrangement of micropillars. To prevent the microstructures from triggering such transitions in the wetting regimes, Reyssat et al.8 have suggested that the pillars must be tall enough to prevent the pressure on the curvature to touch the base of the pillars and consequently decrease the energy of the system by wetting the nanostructural texture. In this study, theoretical investigation of the results was also performed. The two geometrical cap models (spherical cap model and two-parameter ellipsoidal cap model)—derived for hydrophobic surfaces by Erbil and Meric29 and extended for superhydrophobic surfaces by Zhang et al.23—were examined. 13810



ARTICLE

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) hydrophobic WNRs deposited under Ar pressure of 1.0 mtorr and substrate tilt angle of 85° and (b) superhydrophobic WNRs fabricated under Ar pressure of 1.0 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 water droplet’s central height vs evaporation time for WNRs deposited at (a) 10 mTorr and (b) 1.0 mTorr, substrate tilt-angle (89°), and thickness of 600 nm.

In the two-parameter ellipsoidal cap geometry, the height of the water droplet in terms of both CA and CL diameter can be represented as follows d pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð3Þ ½ RðR + tan2 θÞ ( R hðθ, dÞ ¼ 2 tan θ where R = 1 e2, where e is the eccentricity of the ellipse with values 1 > e g 0 for the oblate ellipsoidal cap in the case of sessile water droplet. Both θ and d are the CA and CL diameter, respectively. For the spherical cap model (e = 0), the above equation of height reduces in the following formula d tanðθ=2Þ ð4Þ 2 The simulation results using both geometrical models showed that the experimental data are in better agreement when e f 0 is in the limits of the spherical cap model. As shown in Figure 10, for CCL stage, which was the first portion of the evaporation process for all of the samples, the experimental data would fit better with the ellipsoidal cap model. While the match between the experimental results and simulation using the spherical cap model is more pronounced in the CCA stage, the CL shrinkage from both sides during the evaporation process retains the spherical shape of the droplet. The natural evaporation of water droplets over the hydrophobic WNR surfaces of various morphologies followed a similar scenario in terms of the sequence of the evaporation modes. The dynamics of water drop size reduction began in the hðθ, dÞ ¼

CCL mode; then, after some time, the evaporation switched to a CCA mode. Finally, at the very end of evaporation, the MM process dominated. Nevertheless, the primary difference was the time period for each mode governing the evaporation process, which was mainly associated with the CAH values. In addition, the analyses of small water droplet evaporation over nanoscale films showed the robustness of these hydrophobic surfaces against a sudden collapse of the water droplets into the gaps between the nanorods. Even if there was a transition from a composite state (Cassie state) to a homogeneous state (Wenzel state), this would have been characteristic of a smooth process without a steplike discontinuity and would probably occur at the very end of the evaporation process (very small-sized water droplets). On the other hand, the as-produced surfaces exhibited relatively high CAH (as determined from the dynamic CAH measurements), which resulted in CCL becoming the dominant mode of evaporation. Therefore, robust superhydrophobic metallic materials can be produced by using nanoscale pillarlike surface geometries that, in addition to exhibiting low CAH (slippery surfaces), also prevent the pinning of the CL and consequent wetting of the surface texture of small diameter water droplets. The growth of such nanomaterials with tunable surface properties and morphologies but identical chemistries represents a powerful model for the study of water droplet evaporation kinetics. We presented a simple GLAD-based, one-step process for the growth of tungsten nanorods with variable lengths, interspacing, and top geometries that were found to have tunable surface properties 13811


The Journal of Physical Chemistry C with a CA ranging from 122 to 160°, with important applications in a large number of areas: materials science, biology, water and ice mitigation, self-cleaning surfaces, and the aerospace industry. Moreover, as mentioned above, one potential application of superhydrophobic surfaces, which promotes the CCA mode of water droplet evaporation, is ice and water mitigation. Recent studies have shown a strong correlation between the water repellency of surfaces and their anti-icing properties in low humidity conditions.3032 Superhydrophobic surfaces with the water droplet evaporation taking place mostly in the CCA mode, with easy mobility of the CL and low CAH, tend to repel supercooled water droplets when the temperature of the surface is as low as 25 °C and relative humidity is less than 10%.31 However, in more humid conditions, and in the presence of frost formation, superhydrophobic surfaces have failed to prevent ice accumulation on their surfaces.33,34 Interestingly, a very recent study by Antonini et al.35 demonstrated that Teflon-coated superhydrophobic airplane wing structures with CAH of around 6° required 80% less energy to remove the accumulated ice as compared to the uncoated aluminum surface, using embedded heaters on the wings, under the same icing conditions. Therefore, superhydrophobic surfaces with low CAH can still be considered the most promising candidates for icemitigation applications even at relatively high humidity.

4. CONCLUSIONS In this work, water droplet evaporation processes over nanoscale WNR films with tunable hydrophobic/superhydrophobic properties were studied by monitoring the dimensions of the water droplets as a function of time. The WNRs with various surface morphologies and porosities were fabricated using RF magnetron sputtering (GLAD technique) by varying the Ar pressure and substrate tilting angle. The analyses have shown that the evaporation process occurred in three modes but of various durations explained by the films’ nanoscale surface roughness and corresponding wetting regime. The pinned CL mode was the first and relatively dominant mode of evaporation. However, no sign of droplet collapse or abrupt transition from dewetting regime to wetting regime over the water repellent WNR surfaces was observed. A comparison between the dynamic CAH measured by adding/withdrawing water from the surfaces with the CAH due to evaporation was performed, and a consistent agreement was observed. Theoretical simulation of the decrease in the central height of the water droplet during the evaporation process showed that, during the CCL stage, the experimental data is best fitted by the ellipsoidal cap geometry model, whereas, during the CCA stage, the spherical cap model is the most accurate. Finally, it can be concluded that, for small droplets, the superhydrophobic surfaces with nanoscale roughness are less prone to trigger the transition from Cassie to Wenzel regimes than surfaces with 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 Cooperative Agreement Number: W912HZ-09-02-0008), the Arkansas

ARTICLE

Science & Technology Authority (Grant No. 08-CAT-03), the Department of Energy (DE-FG36-06GO86072), and National Science Foundation (NSF/EPS-1003970) is greatly appreciated. The editorial assistance of Dr. Marinelle Ringer is also acknowledged.

’ REFERENCES (1) Young, T. Phil. Trans. R. Soc. London 1805, 95, 65. (2) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (3) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (4) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (5) Patankar, N. A. Langmuir 2003, 19, 1249. (6) Xiu, Y.; Zhu, L.; Hess, D. W.; Wong, C. P. J. Phys. Chem. C 2008, 112, 11403. (7) McHale, G.; Aqil, S.; Shirtcliffe, N. J.; Newton, M. I.; Erbil, H. Y. Langmuir 2005, 21, 11053. (8) Reyssat, M.; Yeomans, J. M.; Quere, D. Europhys. Lett. 2008, 81, 26006. (9) Kulinich, S. A.; Farzaneh, M. Appl. Surf. Sci. 2009, 255, 4056. (10) Shin, D. H.; Lee, S. H.; Choi, C. K.; Retterer, S. J. Micromech. Microeng. 2010, 20, 055021. (11) Wang, X.; Chen, Z.; Shen, Z. Sci. China, Ser. B: Chem. 2005, 48, 553. (12) Erbil, H. Y.; McHale, G.; Rowan, S. M.; Newton, M. I. Langmuir 1999, 15, 7378. (13) Kim, J.; Ahn, S. I.; Kim, J. H.; Zin, W. Langmuir 2007, 23, 6163. (14) McHale, G.; Rowan, S. M.; Newton, M. I.; Banerjee, M. K. J. Phys. Chem. B 1998, 102, 1964. (15) Li, G.; Flores, S. M.; Vavilala, C.; Schmittel, M.; Graf, K. Langmuir 2009, 25, 13438. (16) Shin, D. H.; Jin, S. Y.; Lee, S. H.; Yoo, J. Y. TFEC 2008, B213Japan. (17) Furuta, T.; Nakajima, A.; Sakai, M.; Isobe, T.; Kameshima, Y.; Okada, K. Langmuir 2009, 25, 5417. (18) Shin, D. H.; Lee, S. H.; Retterer, S.; Choi, C. K. J. Heat Tran. 2010, 132, 080905. (19) Khedir, K. R.; Kannarpady, G. K.; Ishihara, H.; Woo, J.; Ryerson, C.; Biris, A. S. Phys. Lett. A 2010, 374, 4430. (20) Kannarpady, G. K.; Sharma, R.; Liu, B.; Trigwell, S.; Ryerson, C.; Biris, A. S. Surf. Sci. 2010, 256, 1679. (21) Khedir, K. R.; Kannarpady, G. K.; Ishihara, H.; Woo, J.; Ryerson, C.; Biris, A. S. Langmuir 2011, 27, 4661. (22) Shin, D. H.; Lee, S. H.; Jung, J.; Yoo, J. Y. Microelec. Eng. 2009, 86, 1350. (23) Zhang, X.; Tan, S.; Zhao, N.; Guo, X.; Zhang, X.; Zhang, Y.; Xu, J. ChemPhysChem 2006, 7, 2067. (24) Poulard, C.; Benichou, O.; Cazabat, A. M. Langmuir 2003, 19, 8828. (25) Hu, H.; Larson, R. G. J. Phys. Chem. B 2002, 106, 1334. (26) Shuai-xia, Tan; Xiao-yan, Zhang; Zhao, N.; Xu, J. Chin. J. Chem. Phys. 2007, 20, 140. . G.; Nair, H.; van Houselt, A.; (27) Gelderblom, H.; Marin, A Lefferts, L.; Snoeijer, J. H.; Lohse, D. Phys. Rev. E 2011, 83, 26306. (28) Tsai, P.; Lammertink, R. G. H.; Wessling, M.; Lohse, D. Phys. Rev. Lett. 2010, 104. (29) Erbil, H. Y.; Meric, R. A. J. Phys. Chem. B 1997, 101, 6867. (30) Meuler, A. J.; Smith, J. D.; Varanasi, K. K.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E. ACS Appl. Mater. Interfaces 2010, 2, 3100. (31) Mishchenko, L.; Hatton, B.; Bahadur, V.; Taylor, J. A.; Krupenkin, T.; Aizenberg, J. ACS Nano 2010, 4, 7699. (32) Menini, R.; Farzaneh, M. J. Adhes. Sci. Technol. 2011, 25, 971. (33) Varanasi, K. K.; Deng, T.; Smith, J. D.; Hsu, M.; Bhate, N. Appl. Phys. Lett. 2010, 97, 234102. (34) Kulinich, S. A.; Farhadi, S.; Nose, K.; Du, X. W. Langmuir 2011, 27, 25. (35) Antonini, C.; Innocenti, M.; Horn, T.; Marengo, M.; Amirfazli, A. Cold Reg. Sci. Technol. 2011, 67, 58. 13812


Advanced Studies of Water Evaporation Kinetics over Teflon-Coated

Recommend Documents