enhanced evaporation of sessile water droplet on silver nanorod arrays

8/12/2019 enhanced evaporation of sessile water droplet on silver nanorod arrays

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Published: May 26, 2011

r 2011 American Chemical Society 11914 dx.doi.org/10.1021/jp200819m |J. Phys. Chem. C2011, 115, 1191411919

ARTICLE

pubs.acs.org/JPCC

Enhanced Evaporation of Sessile Water Droplet on VerticallyStanding Ag Nanorods Film

Dhruv P. Singh and Jitendra P. Singh*

Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

1. INTRODUCTION

The evaporation of a sessile liquid droplet from a solid

surface is a common phenomenon in various industrial andscientic processes such as painting, inkjet printing, biologicalcrystal growth, and nanoparticle deposition from the suspen-sion. The evaporation rate of the sessile droplet plays a vital rolein all such applications. The effect of temperature, humidity,and droplet surface area on evaporation rate of sessile droplet iswell understood, but the effect of underlying surface features onthe evaporation rate is not very clear.13 A signicant role ofsurface features particularly nanorods and nanowires in mod-ifying the wettability of surface is well-known and has beendemonstrated experimentally as well as theoretically by manyresearchers.46 However the impact of these nanostructures onsurface temperature driven wetting properties and dynamics ofliquid at the interface is not properly understood. Recently in a

remarkable study, Li et al.have shown that copper nanorods canenhance the boiling of water by boosting up the bubbleformation rate at the interface.7 Their study shows the abilityof nanorod lm to modify the thermodynamics of liquid at theinterface.

In the present research work, we have investigated the effect ofcolumnar morphology of Ag nanorods lm on the evaporationprocess of a sessile water droplet. The silver nanorods lms werefound to inuence the evaporation process of water dropletsignicantly, and an enhancement of about 90% in the evapora-tion rate was observed when compared with a conventional Agthinlm. The evaporation rate was found to depend on the sizeand distribution of the silver nanorods.

2. EXPERIMENTAL DETAILS

Columnar silver nanorods lms were grown over Si(100)

substrates by thermal evaporation of silver powder (99.9%) usingoblique angle deposition (OAD) method.816 For the growth ofsilver nanorods thesubstrates were inclined in the polar directionsuch that the substrate normal made a very high angle (R = 85)with the direction of incident vapor ux as shown by theschematic in Figure 1. For a comparative study, two samples ofsilverlm were also grown for the vapor incidence angles (R) of0 and 75. During deposition, the pressure in the OAD chamberwas better than 2 106 Torr. The surface morphology andstructural analysis were performed using scanning electronmicroscopy (SEM, ZEISS EVO 50) and glancing angle X-raydiffraction (GAXRD) (PhillipsXpert, PRO-PW3040). To studythe effect of surface morphology on the evaporation process ofsessile liquid droplet, drops of deionized (DI) water of 3L in

volume were put on the samples kept at three diff

erent surfacetemperatures (Ts = 25, 50, and 80 C). The images of waterdroplet were captured for 10 s atthe rate of 25 frames per second,and then change in the droplet shape and contact angle valueswere in situ monitored. Contact angle () measurements areperformed using sessile drop method (KRUSS, DSA100). Toavoid the effect due to the presence of any surface adsorbedimpurities the contact angle measurements were started imme-diately after taking out the samples from the high vacuum

Received: January 26, 2011Revised: May 4, 2011

ABSTRACT:We report the effect of columnar morphology of Ag nanorodslm onthe evaporation process of sessile water droplets. Water droplets of 3 L in volumewere deposited over a conventional Ag thin lm and columnar Ag nanorods lmsamples. The temperature of both the samples was maintained at 80 C, and thewater droplet prole was continuously monitored for a time interval of 10 s,immediately after the deposition. Interestingly, about 90% enhancement in theevaporation rate of water droplet was observed on Ag nanorods lm compared to theconventional Ag thin lm. The observed increase in the evaporation rate is explainedby 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 observedenhancement in the evaporation rate on the size and distribution of the silvernanorods. This study shows a possibility to tune the evaporation rate with anoptimized growth of silver nanorods over the solid surfaces.


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11915 dx.doi.org/10.1021/jp200819m |J. Phys. Chem. C2011, 115, 1191411919

The Journal of Physical Chemistry C ARTICLE

deposition chamber and the measurements were performed in aclosed box with an opening at the top for inserting the waterdispenser. The sample was heated by a thermoelectric heaterattached to the sample holder assembly. The contact anglemeasurements were repeated ve times at different positions ofeach sample while keeping the volume of droplets constant.

3. RESULTS AND DISCUSSION

In the case of OAD, the increase in atomic shadowingeffect8,9,14 for higher oblique angle (R = 85) results in thegrowth of silver nanorods inclined in the direction of incidentvaporux. The shadowing effect decreases sharply for the lowervalues of vapor incidence angle and hence for R= 75the wellseparated nanorods were not observed. Figure 2 shows the SEMmicrographs of conventional Ag thin lm (Figure 2a), columnarAg lm (grown at R= 75) (Figure 2b), and well-separated Agnanorods (grown at R = 85) (Figure 2c) grown atR = 0, 75, and85, respectively. The glancing angle X-ray diffraction analysis(not shown here) shows that all these samples have the samecrystallographic structure with the presence of Ag (111) as the

most intense peak. The nanorods were observed to have anaverage diameter of 126 nm and length of 390 nm. The thicknessofAg lm depositedat normal incident (R = 0) was measured tobe 630 nm using a quartz crystal microbalance. The atomic forcemicroscopy (AFM) analysis of the surface shows the Ag lm to

be uniform and continuous having root-mean-square (rms)roughness value of 2.7 nm and the average grain size of138 nm2. To investigate the effect of nanocolumnar surface onthe evaporation process of the sessile water droplet, the Agnanorods (grown at R = 85), Ag columnar (grown at R = 75),and conventional Ag thin lm samples were rst kept at atemperature Ts of 50 C. The droplet images were captured

immediately after depositing thedrop over the sample surface. Atthis surface temperature (50 C) a clear change in the initialcontact angle values (as shown in Figure 3) with sample surfacewas observed, but for this value of surface temperature, theevaporation of water droplet was much slower, and hence, nosignicant change in the contact angle values with time could beobserved. Therefore to observe a fast and signicant change indroplet prole with time the sample surface temperature wasraised to 80 C. Figure 4 shows images of water droplets for Agcolumnar nanorods (Figure 4a), Ag columnarlm (grown at R =75) (Figure 4b), and conventional Ag thin lm (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 theseimages. It appears clearly from Figure 3 that the time-dependent

change in droplet shape is different for all the three samples. Tomeasure the change in droplet shape quantitatively, the volumeof sessile droplet was calculated for both the samples at differenttime instants. Assuming that the droplets form a spherical shapeover the sample surface, the volume (V) of a droplet can be

Figure 1. Schematic of an oblique angle deposition technique.

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.

Figure 3. The variation of contact angle with time at surfacetemperature of 50 C for Ag sample grown at vapor incidence anglesRof 0(lled square), 75(lled circle), and 85(lled triangle).


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calculated using the geometrical relation17,18

V r3

3

2 3cos cos3

sin3 1

whereris the radius of droplet at base and the contact angle.Therand values of the sessile droplet were in situ calculated atdifferent time instants for Ag columnar nanorods and conven-tional Ag thin lm samples. The dependence of droplet baseradius (r) and contact angle () on time for Ag nanorods (grownat R = 85), Ag columnar lms (grown at R = 75), andconventional Ag thinlms 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 thesample surface with time (Figure 5a). Whereas, the contact angle() of water droplet decreases linearly with time for all three

silver samples. However, it is interesting to notice that thedecrement rate is different for the three samples. By substitutingthe observed values of these droplet parameters in the eq 1, wehave calculated the volume of the sessile droplets over columnar

Ag nanorods (grown at R = 85), Ag columnar lms (grown atR = 75), and conventional Ag thinlm samples as a function oftime. The change in the sessile droplet volume (V) with time isshown in Figure 6 for these silver samples. The sessile dropletvolume was found to decrease linearly with time with a differentrate for all three samples. After linear tting of the observedvalues, the value of droplet volume decrement rate (dV/dt) weredetermined as0.098,0.055, and0.051L s1 for nanorod(grown at R = 85), columnar lm (grown at R = 75), andconventional thinlms, respectively. The volume decrement ratewas found to increase with the increase in value of oblique angle R.This temporal decrease in droplet volume reects the eva-poration of the sessile droplet from the sample surface. The

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.

Figure5. The variation of sessile droplets (a) baseradius rand(b) contact anglewith time for pre heated (80 C) Ag samplegrownat vapor incidenceangles Rof 0(lled square), 75(lled circle), and 85(lled triangle).


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comparatively larger value of volume decrement rate (dV/dt) forthe columnarlm suggestsa higher evaporation rate of the sessilewater droplet over a columnar surface than on the conventionalthin lm surface. Quantitatively, an about 90% enhancement inthe evaporation rate of sessile droplet on Ag nanorods lm(grown at R = 85) and 7% enhancement for Ag columnar lm(grown at R = 75) were observed when compared to the

conventional Ag thin lm sample. It shows that the surfacemorphology of Ag samples inuence the evaporation process ofsessile water droplet and boost up the evaporation rate with thehighest effect observed for the Ag nanorods lm.

Evaporation is simply related to the escape of water moleculesfrom the sessile droplet surface. This escape probability of watermolecules depends on the position of the molecule in sessiledroplet.1922 The probability is lower for the center top positionof the drop and increases toward the base with the highest valueat contact line of sessile droplet. This behavior is well explainedtheoretically by Deegan et al.21 and Hu et al.22 in separate studiesfor the sessile droplet sitting over the plane surface. According tothe Deegan et al.21 for a spherical shape sessile liquid droplet thelocal evaporation ux Jvaries with R(distance of the liquid

molecule from the center of sessile droplet) asJ(R)(rR)

,

where ris the base radius of droplet and is a positive valueparameter that depends on the contact angle of sessile droplet.This relation clearly shows that as R approachesrthat is at the edge or three phase contact line of droplet, theevaporation ux will increase signicantly. Thus, for a sessiledroplet the contact line which is the interface region of all threephases, namely, the solid sample surface, the liquid droplet, and

the air, dominates the evaporation process with a higher magni-tude of evaporation ux. For conventional thin lm surface thecontact line (l) of water droplet will be the circumference of circleformed by droplet at the base, i.e., at the plane of the solidliquidinterface and is found to be 6.97 mm.

On the other hand, the vertically standing nanorods make thelm surface very rough and porous so contact line of dropletcannot be calculated directly without knowing the exact wettingbehavior of Ag nanorodslm. To understand the behavior of thewater droplet over the columnarlm surface, the contact anglesof similar water droplets over both the Ag columnar nanorods aswell as conventional Ag thin lm samples were measured at roomtemperature (Ts = 25 C). The contact angle was found toincrease from 95 for the conventional Ag thin lm sample to

115.4for columnar Ag nanorods sample. The observed increasein the contact angle suggests the increase in the hydrophobicityfor the columnar Ag nanorods sample and can be explained usingCassieBaxter model.23,24 In this model, the water droplet isconsidered to be sitting over a composite surface made up of airand solid. So, the replacement of solid surface by air reduces theavailability of effective surface energies resulting in the less forceacting to drag the water to spread over the surface and nallyleads to the hydrophobic nature of surface with an increase in thecontact angle value. For vertically standing columnar Ag nano-rods lm, air can exist in the vicinity of the silver nanorods tomake it a composite (silverair) surface. A schematic of a waterdroplet sitting over the composite surface of silver nanorods andair is shown in Figure 7b. With increase in the sample surface

temperature, the surface energy increases which results in thedecrease in contact angle value. This was observed when thewater droplet was deposited on preheated columnar Ag nano-rods and Ag thin lm sample surfaces at 80 C. The contactangles were found to decrease from 95 to 91and from 115.4 to105 forAgthinlmsand Ag nanorods surfaces, respectively. It isimportant to notice that the contact angle decreases for bothnanorods and thin lm samples, but the value (105) for

Figure 6. Plot of sessile water droplets volume vs time at 80 C for Agsample grown at vapor incidence angles R of 0 (lled square), 75(lled circle), and 85(lled triangle).

Figure 7. Schematic of a sessile droplet sitting over the nanorods surface. (a) The contact line formed around the nanorods lying under the droplet isshown by dashedyellow line. (b) Theevaporation ux is shown by thered arrows. The enormous increase in thethree phase contact line surroundingthenanorods enhances the evaporation ux considerably compared to the thin lm surface.


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nanorods sample at 80 C is still higher than the thin lm (91)sample. It suggests that the water droplet is still sits over theAgair composite surface; however the wet area of the nanorods

lying under thedropletincreases andthe waterdroplet will form acontact line surrounding each of the underlying nanorods(Figure 7a). In this way, water droplet will form a number ofsmall spherical troughs in between the nanorods (shown in theFigure 7b). In each of these spherical regions having the threephase contact line formed with the nanorods, thewater moleculeswill observe the higher escape probability conditions as discussedearlier for the case of a sessile droplet on plane surface. Hence, allthese individual contact lines formed along the underlyingnanorods offer a prominent evaporation zone with a correspond-ingly higher value of the evaporation ux (Figure 7b).

By analysis of the quantitative features of the nanorodssurfaces, the average diameter (d) of nanorods is found to be126 nm with a spatial density of 8106 mm2. Therefore, water

droplet of 3 mm2

base area (for nanorods sampler= 0.97 mm)will cover 2 107 nanorods. Since, the nanorods are inclinedoverthe substrate, so the water droplet will form an elliptical shapecontact line (l) along the circumference of the nanorods. Follow-ing the simple geometry as shown in Figure 8, this elliptical shapecontact line for a single nanorod can be calculated using Ramanujansrst approximation for the circumference of an ellipse

l d

cos 31cos f13cos3 cosg1=2h i

2

where is the inclination angle of nanorods with the substrate.The nanorod inclination angle can be calculated using the

semiempirical relation = R sin1((1 cos R)/2), whereR is the vapor incidence angle (85).25 By substituting thecalculated value of (58) and diameterdin eq 2, the contactlinelcomes out to be 1164 nm for a single nanorod. Multiplyingthis with the number of nanorods lying under the droplet yieldstheeffective contact line value as2 104 mm,whichis4ordersof magnitude higher as compared to the value for the conven-tional thinlm sample(6.97 mm). This enormous increase in thecontact line on the columnar nanorods surface simply offersenhancement of region, which yields high evaporation uxcompared to the conventional thin lm surface. This results inthe enhancement of evaporation rate of sessile droplet on thenanocolumnar surface.

The Ag columnar lm (grown at R= 75) was also found toinuence the evaporation process of the sessile water dropletwith a small increment (7%) in the evaporation rate from thevalue of conventional Ag thin lm. The AFM analysis of thesesamples shows rms roughness value of 17 nm while for conven-tional thin lm the value of rms roughness was 2.7 nm. Theincreased roughness is making the surface hydrophobic as

suggested by the contact angle measurements and hence, itreduces the total effective surface area in contact to the waterdroplet. Thus,similar to the Ag nanorodslm(grownat R = 85)the three phase contact line increases when compared with theconventional thin lm which is responsible for the observedenhancement in the evaporation rate.

CONCLUSION

The present work shows the effect of columnar surfacemorphology of silver nanorods on the evaporation rate of asessile water droplet. A signicant enhancement of about 90% inthe evaporation rate of the water droplet was observed for silvernanorods compared to the conventional silver thin lm due tothe enormous increment in effective contact line for the colum-nar nanorods surface.

AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

ACKNOWLEDGMENT

D.P.S. kindly acknowledges CSIR, India, for the seniorresearch fellowship. We are thankful to S. Khurana (IIT Delhi)for his help in contact angle measurements.

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Figure 8. The schematic shows the wet area of nanorods havingdiameter dand inclined on surface with angle . The water dropletforms an elliptical shape contact line surrounding each column with

panddas major and minor axis, respectively.


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enhanced evaporation of sessile water droplet on silver nanorod arrays

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