Enhanced Evaporation of Sessile Water Droplet on Vertically Standing

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Enhanced Evaporation of Sessile Water Droplet on Vertically Standing Ag Nanorods Film Dhruv P. Singh and Jitendra P. Singh* Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India ABSTRACT: We report the effect of columnar morphology of Ag nanorods film on the evaporation process of sessile water droplets. Water droplets of 3 μL in volume were deposited over a conventional Ag thin film and columnar Ag nanorods film samples. The temperature of both the samples was maintained at 80 °C, and the water droplet profile was continuously monitored for a time interval of 10 s, immediately after the deposition. Interestingly, about 90% enhancement in the evaporation rate of water droplet was observed on Ag nanorods film compared to the conventional Ag thin film. The observed increase in the evaporation rate is explained by an enormous increment in the effective three phase contact line of water droplet for the columnar surface. The analysis shows a direct dependence of the observed enhancement in the evaporation rate on the size and distribution of the silver nanorods. This study shows a possibility to tune the evaporation rate with an optimized growth of silver nanorods over the solid surfaces.

1. INTRODUCTION The evaporation of a sessile liquid droplet from a solid surface is a common phenomenon in various industrial and scientific processes such as painting, inkjet printing, biological crystal growth, and nanoparticle deposition from the suspension. The evaporation rate of the sessile droplet plays a vital role in all such applications. The effect of temperature, humidity, and droplet surface area on evaporation rate of sessile droplet is well understood, but the effect of underlying surface features on the evaporation rate is not very clear.1
3 A significant role of surface features particularly nanorods and nanowires in modifying the wettability of surface is well-known and has been demonstrated experimentally as well as theoretically by many researchers.4
6 However the impact of these nanostructures on surface temperature driven wetting properties and dynamics of liquid at the interface is not properly understood. Recently in a remarkable study, Li et al. have shown that copper nanorods can enhance the boiling of water by boosting up the bubble formation rate at the interface.7 Their study shows the ability of nanorod film to modify the thermodynamics of liquid at the interface. In the present research work, we have investigated the effect of columnar morphology of Ag nanorods film on the evaporation process of a sessile water droplet. The silver nanorods films were found to influence the evaporation process of water droplet significantly, and an enhancement of about 90% in the evaporation rate was observed when compared with a conventional Ag thin film. The evaporation rate was found to depend on the size and distribution of the silver nanorods. r 2011 American Chemical Society

2. EXPERIMENTAL DETAILS Columnar silver nanorods films were grown over Si(100) substrates by thermal evaporation of silver powder (99.9%) using oblique angle deposition (OAD) method.8
16 For the growth of silver nanorods the substrates were inclined in the polar direction such that the substrate normal made a very high angle (R = 85°) with the direction of incident vapor flux as shown by the schematic in Figure 1. For a comparative study, two samples of silver film were also grown for the vapor incidence angles (R) of 0 and 75°. During deposition, the pressure in the OAD chamber was better than 2  10
6 Torr. The surface morphology and structural analysis were performed using scanning electron microscopy (SEM, ZEISS EVO 50) and glancing angle X-ray diffraction (GAXRD) (Phillips X’pert, PRO-PW 3040). To study the effect of surface morphology on the evaporation process of sessile liquid droplet, drops of deionized (DI) water of 3 μL in volume were put on the samples kept at three different surface temperatures (Ts = 25, 50, and 80 °C). The images of water droplet were captured for 10 s at the rate of 25 frames per second, and then change in the droplet shape and contact angle values were in situ monitored. Contact angle (θ) measurements are performed using sessile drop method (KRUSS, DSA100). To avoid the effect due to the presence of any surface adsorbed impurities the contact angle measurements were started immediately after taking out the samples from the high vacuum Received: January 26, 2011 Revised: May 4, 2011 Published: May 26, 2011 11914

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The Journal of Physical Chemistry C deposition chamber and the measurements were performed in a closed box with an opening at the top for inserting the water dispenser. The sample was heated by a thermoelectric heater attached to the sample holder assembly. The contact angle measurements were repeated five times at different positions of each sample while keeping the volume of droplets constant.

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3. RESULTS AND DISCUSSION In the case of OAD, the increase in atomic shadowing effect8,9,14 for higher oblique angle (R = 85°) results in the growth of silver nanorods inclined in the direction of incident vapor flux. The shadowing effect decreases sharply for the lower values of vapor incidence angle and hence for R = 75° the well separated nanorods were not observed. Figure 2 shows the SEM micrographs of conventional Ag thin film (Figure 2a), columnar Ag film (grown at R = 75°) (Figure 2b), and well-separated Ag nanorods (grown at R = 85°) (Figure 2c) grown at R = 0, 75, and 85°, respectively. The glancing angle X-ray diffraction analysis (not shown here) shows that all these samples have the same crystallographic structure with the presence of Ag (111) as the most intense peak. The nanorods were observed to have an average diameter of 126 nm and length of 390 nm. The thickness of Ag film deposited at normal incident (R = 0°) was measured to be 630 nm using a quartz crystal microbalance. The atomic force microscopy (AFM) analysis of the surface shows the Ag film to

be uniform and continuous having root-mean-square (rms) roughness value of 2.7 nm and the average grain size of 138 nm2. To investigate the effect of nanocolumnar surface on the evaporation process of the sessile water droplet, the Ag nanorods (grown at R = 85°), Ag columnar (grown at R = 75°), and conventional Ag thin film samples were first kept at a temperature Ts of 50 °C. The droplet images were captured immediately after depositing the drop over the sample surface. At this surface temperature (50 °C) a clear change in the initial contact angle values (as shown in Figure 3) with sample surface was observed, but for this value of surface temperature, the evaporation of water droplet was much slower, and hence, no significant change in the contact angle values with time could be observed. Therefore to observe a fast and significant change in droplet profile with time the sample surface temperature was raised to 80 °C. Figure 4 shows images of water droplets for Ag columnar nanorods (Figure 4a), Ag columnar film (grown at R = 75°) (Figure 4b), and conventional Ag thin film (Figure 4c) samples, captured at different time instants ranging from 0 to 9 s. A change in droplet shape with time can be observed from these images. It appears clearly from Figure 3 that the time-dependent change in droplet shape is different for all the three samples. To measure the change in droplet shape quantitatively, the volume of sessile droplet was calculated for both the samples at different time instants. Assuming that the droplets form a spherical shape over the sample surface, the volume (V) of a droplet can be

Figure 1. Schematic of an oblique angle deposition technique.

Figure 3. The variation of contact angle θ with time at surface temperature of 50 °C for Ag sample grown at vapor incidence angles R of 0° (filled square), 75° (filled circle), and 85° (filled triangle).

Figure 2. SEM image of Ag samples grown at vapor incidence angles R of (a) 0°, (b) 75°, (c) 85°. The scale bars correspond to 500 nm. 11915

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Figure 4. Images of sessile water droplets on preheated (80 °C) Ag samples. The base line of sessile droplet is marked by the dotted line.

Figure 5. The variation of sessile droplet’s (a) base radius r and (b) contact angle θ with time for pre heated (80 °C) Ag sample grown at vapor incidence angles R of 0° (filled square), 75° (filled circle), and 85° (filled triangle).

calculated using the geometrical relation17,18 V ¼

πr 3 ð2
3cos θ þ cos3 θÞ sin3 θ 3

ð1Þ

where r is the radius of droplet at base and θ the contact angle. The r and θ values of the sessile droplet were in situ calculated at different time instants for Ag columnar nanorods and conventional Ag thin film samples. The dependence of droplet base radius (r) and contact angle (θ) on time for Ag nanorods (grown at R = 85°), Ag columnar films (grown at R = 75°), and conventional Ag thin films are shown in parts a and b of Figure 5, respectively. For all three samples, initially the droplet radius (r) undergoes a small change and then becomes almost constant, which indicates that the water droplet does not spread over the sample surface with time (Figure 5a). Whereas, the contact angle (θ) of water droplet decreases linearly with time for all three

silver samples. However, it is interesting to notice that the decrement rate is different for the three samples. By substituting the observed values of these droplet parameters in the eq 1, we have calculated the volume of the sessile droplets over columnar Ag nanorods (grown at R = 85°), Ag columnar films (grown at R = 75°), and conventional Ag thin film samples as a function of time. The change in the sessile droplet volume (V) with time is shown in Figure 6 for these silver samples. The sessile droplet volume was found to decrease linearly with time with a different rate for all three samples. After linear fitting of the observed values, the value of droplet volume decrement rate (dV/dt) were determined as
0.098,
0.055, and
0.051 μL s
1 for nanorod (grown at R = 85°), columnar film (grown at R = 75°), and conventional thin films, respectively. The volume decrement rate was found to increase with the increase in value of oblique angle R. This temporal decrease in droplet volume reflects the evaporation of the sessile droplet from the sample surface. The 11916

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The Journal of Physical Chemistry C comparatively larger value of volume decrement rate (dV/dt) for the columnar film suggests a higher evaporation rate of the sessile water droplet over a columnar surface than on the conventional thin film surface. Quantitatively, an about 90% enhancement in the evaporation rate of sessile droplet on Ag nanorods film (grown at R = 85°) and 7% enhancement for Ag columnar film (grown at R = 75°) were observed when compared to the conventional Ag thin film sample. It shows that the surface morphology of Ag samples influence the evaporation process of sessile water droplet and boost up the evaporation rate with the highest effect observed for the Ag nanorods film. Evaporation is simply related to the escape of water molecules from the sessile droplet surface. This escape probability of water molecules depends on the position of the molecule in sessile droplet.19
22 The probability is lower for the center top position of the drop and increases toward the base with the highest value at contact line of sessile droplet. This behavior is well explained theoretically by Deegan et al.21 and Hu et al.22 in separate studies for the sessile droplet sitting over the plane surface. According to the Deegan et al.21 for a spherical shape sessile liquid droplet the local evaporation flux J varies with R (distance of the liquid molecule from the center of sessile droplet) as J(R) µ(r
R)
λ,

Figure 6. Plot of sessile water droplet’s volume vs time at 80 °C for Ag sample grown at vapor incidence angles R of 0° (filled square), 75° (filled circle), and 85° (filled triangle).

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where r is the base radius of droplet and λ is a positive value parameter that depends on the contact angle of sessile droplet. This relation clearly shows that as R approaches r that is at the edge or three phase contact line of droplet, the evaporation flux will increase significantly. Thus, for a sessile droplet the contact line which is the interface region of all three phases, namely, the solid sample surface, the liquid droplet, and the air, dominates the evaporation process with a higher magnitude of evaporation flux. For conventional thin film surface the contact line (l) of water droplet will be the circumference of circle formed by droplet at the base, i.e., at the plane of the solid
liquid interface and is found to be 6.97 mm. On the other hand, the vertically standing nanorods make the film surface very rough and porous so contact line of droplet cannot be calculated directly without knowing the exact wetting behavior of Ag nanorods film. To understand the behavior of the water droplet over the columnar film surface, the contact angles of similar water droplets over both the Ag columnar nanorods as well as conventional Ag thin film samples were measured at room temperature (Ts = 25 °C). The contact angle was found to increase from 95° for the conventional Ag thin film sample to 115.4° for columnar Ag nanorods sample. The observed increase in the contact angle suggests the increase in the hydrophobicity for the columnar Ag nanorods sample and can be explained using Cassie
Baxter model.23,24 In this model, the water droplet is considered to be sitting over a composite surface made up of air and solid. So, the replacement of solid surface by air reduces the availability of effective surface energies resulting in the less force acting to drag the water to spread over the surface and finally leads to the hydrophobic nature of surface with an increase in the contact angle value. For vertically standing columnar Ag nanorods film, air can exist in the vicinity of the silver nanorods to make it a composite (silver
air) surface. A schematic of a water droplet sitting over the composite surface of silver nanorods and air is shown in Figure 7b. With increase in the sample surface temperature, the surface energy increases which results in the decrease in contact angle value. This was observed when the water droplet was deposited on preheated columnar Ag nanorods and Ag thin film sample surfaces at 80 °C. The contact angles were found to decrease from 95 to 91° and from 115.4 to 105° for Ag thin films and Ag nanorods surfaces, respectively. It is important to notice that the contact angle decreases for both nanorods and thin film samples, but the θ value (105°) for

Figure 7. Schematic of a sessile droplet sitting over the nanorods surface. (a) The contact line formed around the nanorods lying under the droplet is shown by dashed yellow line. (b) The evaporation flux is shown by the red arrows. The enormous increase in the three phase contact line surrounding the nanorods enhances the evaporation flux considerably compared to the thin film surface. 11917

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The Ag columnar film (grown at R = 75°) was also found to influence the evaporation process of the sessile water droplet with a small increment (7%) in the evaporation rate from the value of conventional Ag thin film. The AFM analysis of these samples shows rms roughness value of 17 nm while for conventional thin film the value of rms roughness was 2.7 nm. The increased roughness is making the surface hydrophobic as suggested by the contact angle measurements and hence, it reduces the total effective surface area in contact to the water droplet. Thus, similar to the Ag nanorods film (grown at R = 85°) the three phase contact line increases when compared with the conventional thin film which is responsible for the observed enhancement in the evaporation rate. Figure 8. The schematic shows the wet area of nanorods having diameter d and inclined on surface with angle β. The water droplet forms an elliptical shape contact line surrounding each column with p and d as major and minor axis, respectively.

nanorods sample at 80 °C is still higher than the thin film (91°) sample. It suggests that the water droplet is still sits over the Ag
air composite surface; however the wet area of the nanorods lying under the droplet increases and the water droplet will form a contact line surrounding each of the underlying nanorods (Figure 7a). In this way, water droplet will form a number of small spherical troughs in between the nanorods (shown in the Figure 7b). In each of these spherical regions having the three phase contact line formed with the nanorods, the water molecules will observe the higher escape probability conditions as discussed earlier for the case of a sessile droplet on plane surface. Hence, all these individual contact lines formed along the underlying nanorods offer a prominent evaporation zone with a correspondingly higher value of the evaporation flux (Figure 7b). By analysis of the quantitative features of the nanorods surfaces, the average diameter (d) of nanorods is found to be 126 nm with a spatial density of 8  106 mm
2. Therefore, water droplet of 3 mm2 base area (for nanorods sample r = 0.97 mm) will cover 2  107 nanorods. Since, the nanorods are inclined over the substrate, so the water droplet will form an elliptical shape contact line (l) along the circumference of the nanorods. Following the simple geometry as shown in Figure 8, this elliptical shape contact line for a single nanorod can be calculated using Ramanujan’s first approximation for the circumference of an ellipse l¼

i πd h 3ð1 þ cos βÞ
fð1 þ 3cos βÞð3 þ cos βÞg1=2 cos β ð2Þ

where β is the inclination angle of nanorods with the substrate. The nanorod inclination angle can be calculated using the semiempirical relation β = R
sin
1((1
cos R)/2), where R is the vapor incidence angle (85°).25 By substituting the calculated value of β (58°) and diameter d in eq 2, the contact line l comes out to be 1164 nm for a single nanorod. Multiplying this with the number of nanorods lying under the droplet yields the effective contact line value as ∼2  104 mm, which is 4 orders of magnitude higher as compared to the value for the conventional thin film sample (6.97 mm). This enormous increase in the contact line on the columnar nanorods surface simply offers enhancement of region, which yields high evaporation flux compared to the conventional thin film surface. This results in the enhancement of evaporation rate of sessile droplet on the nanocolumnar surface.

’ CONCLUSION The present work shows the effect of columnar surface morphology of silver nanorods on the evaporation rate of a sessile water droplet. A significant enhancement of about 90% in the evaporation rate of the water droplet was observed for silver nanorods compared to the conventional silver thin film due to the enormous increment in effective contact line for the columnar nanorods surface. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT D.P.S. kindly acknowledges CSIR, India, for the senior research fellowship. We are thankful to S. Khurana (IIT Delhi) for his help in contact angle measurements. ’ REFERENCES (1) Guena, G.; Poulard, C.; Cazabat, A. M. J. Colloid Interface Sci. 2007, 312, 164. (2) Schonfeld, F.; Graf, K. H.; Hardt, S.; Butt, H. J. Int. J. Heat Mass Transfer 2008, 51, 3696. (3) Widjaja, E.; Harris, M. T. Comput. Chem. Eng. 2008, 32, 2169. (4) Guo, Z.; liu, W.; Su, B. L. J. Colloid Interface Sci. 2011, 353, 335. (5) Fan, J. G.; Tang, X. J.; Zhao, Y. P. Nanotechnol. 2004, 15, 501. (6) Martines, E.; Seunarine, K.; Morgan, H.; Gadegaard, N.; Wilkinson, C. D. W.; Riehle, M. O. Nano Lett. 2005, 5, 2097. (7) Li, C.; Wang, Z.; Wang, P. I.; Peles, Y.; Koratkar, N.; Peterson, G. P. Small 2008, 4, 1084. (8) Robbie, K.; Brett, M. J.; Lakhtakia, A. Nature 1996, 384, 616. (9) Abelmann, L.; Lodder, C. Thin Solid Films 1997, 305, 1. (10) Zhao, Y. P.; Ye, D. X.; Wang, G. C.; Lu, T. M. Nano Lett. 2002, 2, 351. (11) Ye, D. X.; Zhao, Y. P.; Yang, G. R.; Zhao, Y. G.; Wang, G. C.; Lu, T. M. Nanotechnology 2002, 13, 615. (12) Karabacak, T.; Wang, G. C.; Lu, T. M. J. Vac. Sci. Technol. A 2004, 22, 1778. (13) Singh, J. P.; Karabacak, T.; Ye, D. X.; Liu, D. L.; Picu, C.; Lu, T. M.; Wang, G. C. J. Vac. Sci. Technol. B 2005, 23, 2114. (14) Jensen, M. O.; Brett, M. J. IEEE Trans. Nanotechnol. 2005, 4, 269. (15) Zhou, C. M.; Gall, D. Appl. Phys. Lett. 2007, 90, 093103. (16) Singh, D. P.; Nagar, R.; Singh, J. P. J. Appl. Phys. 2010, 107, 074306. (17) Birdi, K. S.; Vu, D. T.; Winter, A. J. Phys. Chem. 1989, 93, 3702. (18) Schrader, M. E.; Weiss, G. H. J. Phys. Chem. 1987, 91, 353. (19) Parisse, F.; Allain, C. J. Phys. II 1996, 6, 1111. 11918

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