the effects of wettability and surface...

Proceedings of The ASME 2017 Summer Heat Transfer ConferenceHT2017

July 10-14, 2017, Bellevue, WA, USA

HT2017-4847

THE EFFECTS OF WETTABILITY AND SURFACE MORPHOLOGY ON HEATTRANSFER FOR ZINC OXIDE NANOSTRUCTURED ALUMINUM SURFACES

Claire M. Kunkle, Jordan P. Mizerak, and Van P. CareyDepartment of Mechanical Engineering

University of California, BerkeleyBerkeley, California 94720

ABSTRACTThe development of hydrophilic surface coatings for en-

hanced wetting characteristics has led to improvement in heattransfer metrics like impinging droplet vaporization time andthe heat transfer coefficient. Hydrothermal synthesis, a methodof developing hydrophilic surfaces, has been previously shownto produce high performing heat transfer surfaces on coppersubstrates [1]. Our study applied this production method toaluminum substrates, which have the advantage of being cheaper,lighter, and a more widely used for heat sinks than copper.Previous experiments have shown that water droplets on ZnOnanostructure coated surfaces, at low superheats, evaporate viathin film evaporation rather than nucleate boiling. This leadsto heat transfer coefficients as much as three times higherthan nucleate boiling models for the same superheat. Ournanocoated aluminum surfaces exhibit superhydrophilicity withan average droplet liquid film thickness of 20-30 microns, whichcan produce heat transfer coefficients of over 25 kW/m2K.This study discusses characterization of ZnO nanostructuredaluminum surfaces to better understand the related mechanismswhich lead to such high heat transfer performance.

All ZnO nanostructured aluminum surfaces produced forthis study exhibited superhydrophilicity, with sessile dropletcontact angles of less than 5 degrees. The challenge of achievingaccuracy for such low contact angles led to the developmentof a new wetting metric related to the droplet's wetted areaon a surface rather than the contact angle. This new metricis predicated on the the fact that heat transfer performance isdirectly related to this wetted area, thickens, and shape of the

expanding droplet footprint. Shape irregularity of droplets onthese superhydrophilic surfaces is discussed in this study, wherethere appears to be advantages to irregular spreading comparedwith surfaces that produce symmetric radial spreading. One formof irregular spreading consists of liquid droplets spreading outboth on top of the surface and within the microstructure of thesurface coating. The liquid within the microstructure forms filmsless than 5 microns thick, making local heat transfer coefficientsof greater than 100 kW/m2K possible. SEM microscope imagingprovided additional insight to the underlying mechanisms whichcause these surfaces to produce such exceptional spreading aswell as irregular spreading, resulting in very good heat transferperformance.

Experimental work was coupled with computational anal-ysis to model the contact line of the droplet footprint. Imageprocessing of experimental photos helps to analyze spreadingcharacteristics, which can be directly related to heat transfer dueto film thickness at various points during spreading.

Approaches used to characterize these superhydrophilicsurfaces advance understanding of the connections betweennanoscale structural elements and macroscale performance char-acteristics in heat transfer. This understanding can reveal keyinsights for developing even better high performance surfaces fora broad range of applications.

contact e-mail: [email protected] 1 Copyright ©2017 by ASME

NOMENCLATURE

NW wetting numbertce complete evaporation timeAtot total wetted contact areahlv latent heat of vaporizationkl conductivity of waterρ water density

INTRODUCTION AND BACKGROUNDInterest in the field of surface coatings for heat transfer

enhancement has been growing along with the demand foreffective heat dissipation technologies. Liquid coolants arepreferred over air cooling for high heat flux applications becauseof liquid’s high latent heat and its ability to remove heat throughevaporative cooling. One such application which employseffective evaporative cooling is the spray cooling used for largescale power plants. The cooling of condensers is used toboost efficiency in the thermodynamic power cycle, howeverthe use of spray cooling for this process is very water intensivefrom an environmental standpoint. Motivated by this issue,this study explores how a Zinc Oxide (ZnO) nanostructuredsurface enhances hydrophilicity on aluminum. By using ascalable approach to surface production, it is feasible that thiscoating could be applied to condensers as a way to increase theeffectiveness of evaporative cooling on their surface.

Earlier studies have explored the effectiveness of nanostruc-tured surfaces on aluminum with the intention of developingspecialized surfaces, both hydrophilic and hydrophobic. Onesuch study manufactured micro grooves with 30µm width and10µm depth to create water repellant surfaces [2]. Thesetypes of surfaces are more challenging to scale, because ofthe intricacy of the manufacturing for the micro grooves. But,they offer very effective enhancement for hydrophobic surfaces.Other studies looked at specialized coatings to enhance thehydrophobicity of aluminum. Wang [3] applied his coatingsonto the aluminum metal substrate and found that hydrophobicitywas accomplished, but these coatings lack the rubustness tosurvive wear and tear over time. Study into surfaces thatare hydrophilic have resulted in patents for hydrophilic surfacecoatings on aluminum [4] previously, but these manufacturingprocesses involve highly chemical immersion processes and donot produce long lasting hydrophilicity, as was discovered byother researchers.

The interest in producing hydrophilic nanostructures sur-faces on aluminum marks a departure from standard hydrophiliccoatings on surfaces, usually the result of chemical immersion.Previous studies with ZnO coatings, like those in this study, weredone with copper substrates. These studies have shown that ZnOnanostructures result in hydrophilic characteristics and have the

potential to greatly enhance heat transfer performance [1,5]. Thisis a result of the specific wetting regimes that are caused bysurface hydrophilicity. As nanostructured surfaces move into thesuperhydrophilic regime, the wetting regime that results is oftennot a typical Wenzel wetting state as shown in Fig.1.

FIGURE 1. A typical Wenzel wetting regime where water permeatesinto the rough surface structure directly beneath the droplet [6]

In the Wenzel wetting regime, a deposited liquid dropleton the surface does not merely sit on top of any roughness orpillar-form microstructure, but rather fills in the spaces betweenthe rough structure, thus removing any air gap. This is adesirable wetting arrangement for heat transfer because liquidencounters increased surface area and thus more direct contactwith a heated surface for heat transfer. However, an evenmore desirable wetting regime known as the sombrero effecthas been observed for highly wetting surfaces, such as those inthis study. This wetting regime, previously observed on coppernanostructured surfaces [7], has been discussed in detail by otherauthors, referencing how the surface roughness can have directrelation to the wetting and spreading of water on the surface[8–10]. These coated copper surfaces have shown the potentialfor superhydrophilicity with a tendency to spread out past thecontact line of the droplet above the surface, and create an ultra-thin layer of water surrounding the droplet that is completelywithin the microstructure of a nanocoated surface. This wettingregime is shown in Fig.2 and represents the most desirablearrangement of a droplet on a surface for maximum heat transfer,and thus the highest potential for evaporative cooling.

FIGURE 2. Side graphic of the sobrero effect on a pillar-arraymicrostructured surface (left), and a top view of a Zinc Oxide coatedcopper surface with superhydrophilic characteristics that result in asombrero wetting regime. (right)

Knowing that use of copper as the substrate for these ZnOnanstructures produced such highly wetting characteristics, we

2 Copyright ©2017 by ASME

chose to focus this study on aluminum. Aluminum, as a lighterweight and cheaper metal is more often used in heat transfer andheat exchanger applications than other conductive metals likecopper. Developing surface coatings that are high performingand robust on aluminum surfaces is desirable precisely becauseof this wide usage and affordability, opening the door for manydiverse applications to increase heat transfer performance.

The hydrothermal synthesis method of surface enhanementhas never before been used on aluminum surfaces and isbeneficial both in its ability to be scaled up as well as it’sdurability as a surface enhancer. This opens the possibility forwidespread implementation in heat transfer and heat exchangerdevelopment in industry and energy sectors.

For this study, we focused on several different aluminumnanstructured surfaces, created using hydrotheral synthesis withZnO nanoparticles. The different iterations of this process weredesigned to produce different length scales for the nanostructureson these surface, thus affecting wetting characteristics observedon the macroscale as well as surface morphology on themicroscale.

Computational models were developed to replicate thespreading of droplets on these aluminum nanostructured sur-faces. This kind of modeling can be enhanced to predict averagefilm thickness, and ultimately heat transfer performance. It isthe goal of this study to approach these promising aluminumnanostructured surfaces from both experimental and computa-tional angles to gain valuable insight on the best techniques toimplement for a variety of heat transfer applications.

ZINC OXIDE SURFACES WITHSUPERHYDROPHILICITY

The surfaces developed for this study are made of 6061aluminum alloy substrate on which zinc oxide nanoparticleswere grown. A technique known as hydrothermal synthesis waspreviously used on copper surfaces for creation of hydrophilicwetting characteristics. This scalable manufacturing processinvolves the cleaning and polishing of the surface substrate priorto a simple room temperature deposition of ZnO nanoparticlessuspended in a diluted E4OH solution. The concentration of thissolution was one of the independent variables for the varioussurfaces made for this study (see Tab. 1) This ZnO and E4OHnanoparticle seeding solution quickly dries after deposition,creating an incredibly thin nanoparticle coating on the polishedsurface. After the surface is annealed at 120°C, it is submergedin a solution of Zinc Nitrate Hexahydrate and HexamethalineTetramine dissolved in water. This solution acts as a growthagent for the ZnO nanoparticles on the surface, which are grownin the solution for a period of four or eight hours. The length oftime for growth has a direct correlation to the scale and densityof the nanostructures on the surface once it is finished withthermal growth. The consistent superhydrophilicity of the ZnO

nanostructured aluminum surfaces produced was experimentallyobserved by sessile water droplet deposition.

Two independent variables were adjusted for creating thesesurfaces: the concentration of the deposited nanoparticle seedingsolution and the time that the surface was immersed in thegrowth solution. The four surfaces that will be discussed inthis paper, shown in Tab.1, include two aluminum substrate ZnOnanostructured surfaces, S1 and S2 that were coated with a 0.04molar solution of nanoparticles and placed in the growth solutionfor 4 and 8 hours respectively, as well as surfaces S3 and S4.These two were both grown in solution for 8 hours but S3 useda 0.02 molar solution of nanoparticles for deposition while S4used a 0.08 molar solution. These variations in growth time andnanoparticle concentration produced a differences in the wettingcharacteristics of the surfaces.

NANOSCALE FEATURES THAT ENHANCE WETTINGAND HEAT TRANSFER

In order to better understand the nature of these surfacesthat leads to their superhydrophilicity, a Scanning ElectronMicroscope (SEM) was used to image the surfaces of thedifferent nanostructured aluminum pieces produced for thisstudy. By comparing the different observed features to themacroscale wettability characteristics, it is possible to beginmaking correlations for the specific features that are mostbeneficial for optimized surface enhancement.

FIGURE 3. ZnO nanostructured aluminum surface (Surface S1 – SeeTable 1) grown for 4 hours with a 0.04 molar ZnO seeding solution.

The most commonly observed feature on all four of thesurfaces studied in this report was a porous base structure that


had grown out from the nanoparticles to create a base for thegrowth of pillarlike structures. This is most apparent in Fig.3on surface S1, which had the shortest growth time of the fourtypes of surfaces studied. The majority of the surface on thistest piece was covered with thin, tightly packed membranes onthe order of several nanometers thick. This surface served as abase for the growth of some pillar-like structures, shown in thecenter of Fig.3. On surface S1, the prevalence of these pillargrowths were few and far between. We suspect that the growthtime was a factor in the sparseness of the pillar growth. Previousstudies of this same nanostructure growth technique on coppersurfaces resulted in near complete coverage with nanostructurepillars [11].

This hypothesis is supported by the observations made onsurface S2 under the SEM Microscope. This surface wasdeposited with the same 0.04 molar concentration nanoparticlesolution as S1, but was baked for a total of 8 hours, or twiceas long as surface S1. The resulting predominance of pillarstructures showed a distinct difference between the shorter bakedof the two. In Fig.4, the pillars that grew on surface S2, on top ofthe porous base, were tightly packed and randomly angled. Thedensity of the pillars changed on various parts of the surface, butthe size of the pillars remained the same with an average lengthof 6-8µm and a diameter of less than 1µm.


These two surfaces with a ZnO molar seeding concentrationof 0.04 are contrasted with surfaces S3 and S4 which are coatedwith ZnO nanoparticle seeding concentrations of 0.02 and 0.08respectively. Both S3 and S4 were baked for a total of 8 hoursin the growth solution after the deposition of the nanoparticlesolution.


Surface S3, like surface S1 had a low prevalence of pillarstructures. The majority of the surface for S3 displayed a roughporous structure, with the occasional polyp, similar to that shownin the center of Fig.5. This was contrasted with surface S4,shown in Fig 6, which had an abundance of pillars. Notably,the pillars are much smaller than the pillars on surface S2 inFig.4. Where the pillars on Surface S2 formed with a 0.04molar solution of ZnO of nanoparticles are approximately 8-10µm long and have diameters of 0.5-1µm, surface S4 withdouble that molar concentration of ZnO nanoparticles producedpillars a whole order of magnitude smaller. These pillars in Fig.6are less than 2µm in length and have diameters on the order ofseveral nanometers.

These different characteristics observed on the microscaleresulted in differing wetting characteristics as well as heattransfer characteristics.

QUANTIFYING SURFACE WETTABILITYIn order to discover relations between surface morphology

and wettability, tests were run on surfaces S1, S2, S3, andS4 to determine droplet spreading and wicking rate for waterdroplets on room temperature surfaces. These sessile dropletobservations are frequently performed in the literature as a wayto assess wetting characteristics and most of these studies usecontact angles as the metric of choice for quantifying surfacewettability [12–14]. However, this metric became challengingto quantify as the water droplets on this study’s surfaces spreadout so significantly that measurement of the near-zero contactangles was either innacurate or impossible. Previous discussionregarding the challenges of using the contact angle as a wettingmetric for superhydrophilic surfaces was had in further detail in



previous papers by Kunkle [7]. Here, the author lays out themotivation for finding a better metric for highly wetting surfaces.The alternative choice of wetting metric became somethingknown as the wetting number. In this case, the total wetted areaof a sessile droplet is measured by taking an aerial photographof the droplet on the surface. This area measurement is thennondimensionalized with a calculated area for a droplet of equalvolume on a surface with a 90° contact angle. This wettingnumber, Nw, is defined in Eq. 1.

Nw =Awetted

A90(1)

The wetting number is able to accurately relate the spreadof the droplet to the surfaces wettability by showing muchmore wetting a surface is compared to a constant counterpartsurface that would produce a 90° contact angle. Any error inthese measurements is a function of camera accuracy, and palein comparison to inaccuracies in measuring near zero contactangles of a droplet on a superhydrophilic surface. Wettingnumbers for the 4 surfaces discussed in this paper are in Table1. Higher wetting numbers correlate to more wetting surfaceswhile the wetting numbers closer to one are less hydrophilic. Asdiscussed later in the paper, many of the surfaces exhibited non-homogeneous wetting characteristics. It is worth noting, then,that the wetting number values shown in Table 1 represent themost wetting parts of the surface.

As shown in Table 1, the surface S2 exhibited the highestwettability with a wetting number of 63.45, showing that it was63.45 times more wetting than a same volume droplet on a 90°contact angle surface. The wetting numbers for Surfaces S1and S2 represent equivalent contact angles of roughly 0.3 to 0.5

degrees, while the much lower wetting numbers of surfaces S3and S4 represent nearly 50° contact angles. The contact angleapproximations are made using geometric relations assumingspherical cap spreading. Assuming spherical cap spreading,however, is not always accurate for these hydrophilic surfacesand it is much more accurate to calculate the wetting numberby direct wetted area measurements. This is done by measuringexperimental wetted areas for a sessile water droplet depositionin room temperature conditions, after initial production of thesurfaces. The hydrophilicity can change over time, sometimes byas much as 50% but still maintain high heat transfer performance.Some of these differences between the four surfaces can, in part,be attributed to the non-homogeneity of the surface as well as thetendency for hydrophilic surfaces to adsorb particles onto theirsurface. The non-homogeneity appears to be a unique advantageafter studying the performance of the four surfaces.

While surface S2 had a consistent pillar formation acrossnearly 80% of the surface when viewed through the SEMmicroscope (see Fig.4), S3 and S4 had less than 40% coverageand this inconsistency affected the spreading of droplets. InTable 1, there is also a column noting the observation of asombrero effect. As discussed in the introduction, a sombreroeffect can act as a great enhancer of heat transfer due to thecreation of ultra thin liquid layers on the surface within thecoating microstructure. It was observed in experimental teststhat it was also this nonhomogeneous spreading that often wasrelated to the sombrero effect. Of the four surfaces discussed,it was S2 that had the most apparent sombrero effect as well asnon-symmetry in spreading. This is shown in Fig.9.

To further study these surfaces and thoroughly understandthe wetting characteristics they exhibit, a high speed camera wasused to provide visualization of the surface as droplets impingeand spread on them. With the capability to record over 10,000frames per second, we identified two distinct stages spreadingregimes for deposited droplets on the nanostructured surface.The first stage begins from the moment that the water droplettouches the surface and begins spreading, with the contact lineabove the nanostructured surface moving outwards. Shown inFig.7, this initial wetting phase consists of the droplet in aWenzel wetting state, where the liquid is both above the coatingas well as interspersed between the microstructures within thenanostructured surface, directly beneath the dome of the droplet.

FIGURE 7. Water droplet impinging on an 8 hour grown ZnOnanocoated surface. Time scale for this initial spread process is 0.05seconds for a 2µl droplet (Diameter = 1.56mm)


TABLE 1. Wetting Numbers for the four aluminum surfaces coated in ZnO nanoparticles, quantifying wettability for different bake times and differentZnO nanoparticle concentrations

Surface Description Droplet Volume [µl] Nw Sombrero Effect

S1 4 hour bake, 0.04 ZnO Nanoparticle Concentration 2 46.45 Yes

S2 8 hour bake, 0.04 ZnO Nanoparticle Concentration 2 63.45 Yes

S3 8 hour bake, 0.02 ZnO Nanoparticle Concentration 2 1.49 No

S4 8 hour bake, 0.08 ZnO Nanoparticle Concentration 2 1.92 No

For Figures 7, 8, and 9, the size of the deposited droplet was2µl, corresponding to a droplet diameter of 1.56mm. The threeframes shown in Fig.7 occur over a very short period of time thatis usually less than 0.05 seconds, reaching the classical Wenzelstate, looking like the droplet shown in Fig.8.

FIGURE 8. Water droplet at the end of the first phase of spreading on8 hour grown ZnO nanocoated surface, S2

The droplet spreading does not stop here, but rathercontinues outwards in a second phase of spreading, creating asombrero effect as described in the previous literature [7]. As itcontinues to spread, there is a departure from this strict contactline shown in Fig.8 as liquid spreads within the zinc oxidenanostructure, past the contact line from Fig.8. This sombreroeffect is shown in Fig.9 and was characterized by the continuedspread of water within the surface coating, while the contact lineof the above-surface droplet remained fixed.

Our high speed camera study found that the transition fromthis first phase of wetting, to this second somberero phaseoccurred very early on when the droplet was approximately 10%of the way to its final steady state wetted position. This earlydeparture into the second phase of wetting is beneficial for heattransfer because it also signals a transition to the growth of ultrathin films around the perimeter of the droplet. As the liquidspreads outward, pushing out the solid-liquid contact line, withina micro-layer structure, the liquid-solid-air contact line staysfixed. Knowing the volume of the droplet, we can calculate theheat transfer benefits of this by first imaging and measuring the

FIGURE 9. Water droplet in its final sessile state after impinging onthe 8 hour baked ZnO nanocoated surface, S2

dimensions of the microstructures created by growing zinc oxidenanoparticles on the aluminum substrate.

HEAT TRANSFER BENEFITS FORNANOSTRUCTURED ALUMINUM SURFACE

The surface with the highest wettability based on exper-imental testing was surface 2 (S2). Using this surface, theheat transfer performance of ZnO nanostructured aluminumsurfaces was quantified in two ways. First, evaporation timewas measured through a series of experimental tests. Theseresults were then used to calculate heat transfer coefficient usinga simple thermodynamic model. Evaporation of the droplets onthese new superhydrophilic aluminum surfaces was taken for avariety of superheats and compared to previously collected datafor droplet evaporation for nanocoated copper surfaces [1].

Drop volumes for the data shown in Fig.10 are all for 2µldroplets (1.56mm diameter) of water deposited on the differentheated surfaces from drop heights of less than 5mm. The generalobserved trend of decreasing evaporation time with increasedsuperheat is to be expected. Droplets on the ZnO coatedaluminum show equal or, in some cases, faster evaporationtime than the similarly ZnO coated copper counterparts. Forboth copper and aluminum, the nanostructured surfaces aresignificantly better at quickly evaporating droplets than the barealuminum and copper samples. Particularly with the higher


FIGURE 10. The time for complete evaporation for a 2µl dropleton ZnO nanostructure copper hydrophilic surfaces [Surface S2] [1]compared to the time for evaporation of the same size droplet on ZnOcoated aluminum surfaces.

superheats of 20°+, the coated surface also provided suppressionof nucleate boiling. While high speed video showed ripplingand disturbance on the surface of the droplet, no bubblesformed or broke on the surface even at these higher superheats.This was different than the bare copper and aluminum, whichdemonstrated full nucleate boiling for all experiments past 18°superheat.

Data shown in Fig.10 was further used to validate a com-putational thermodynamic model to calculate evaporation time.This calculation of time for complete evaporation, tce, discussedin detail in earlier literature for ZnO copper nanostructuredsurfaces [7], is a function of the wetting number from Eq. 1and is shown in Eq. 2.

tce =Atot

N3w· 2γ

3π· ρhlv

kl ·∆T(2)

In Eq.2, the Atot is the complete wetted area for a sessiledroplet in adiabatic conditions and γ is a constant that is set to 1,but can be adjusted based on matching experimental data. Thecorrelation between experimental results and evaporation timesfrom Eq.2 are shown in Fig.11.

The experimental data and the calculated data for time ofcomplete evaporation correlated linearly for γ = 1 in Eq.2. Forboth ZnO nanostructured copper and aluminum, the correlationproduces an estimate for evaporation that could reasonably beplugged into a heat transfer coefficient calculation to determineheat transfer performance for these superhydrophilic surfaces.The assumptions for this model are that there is no nucleationhappening on the surface. Because of the enhancement providedby the ZnO coating, nucleation is generally suppressed at tem-peratures where it would normally be occurring for a bare metal

FIGURE 11. Comparison of the experimental versus the calculatedcomplete evaporation time for ZnO nanostructured copper and alu-minum surfaces [Surface S2]

surface. This allows for enhanced, non-nucleating evaporativeheat transfer at the periphery of the droplet where the liquid isthinnest.

Enhanced Heat Transfer CoefficientsThe heat transfer coefficient, h, for these aluminum surfaces

was determined based on the experimental time of completeevaporation and a mean thickness, th, calculated from thewetted area and the volume of the droplet. Equation 3 is thesimple thermodynamic model for calculating this heat transfercoefficient.

h =ρ · th ·hlv

∆T · tce(3)

For Eq. 3, the density and latent heat values are for water asa liquid, since these tests were run without liquid nucleation inthe droplet. The resulting values of the heat transfer coefficienton the nanostructured aluminum surface were compared tocalculated values from nanostructured surface evaporation timesand are shown in Fig.12

The enhancement in the heat transfer coefficient, as a resultof decreased evaporation time compared to bare metal surfacesis a direct result of the reduced thickness of droplets on thesurface. An average liquid thickness of the droplets can beback-calculated from the volume and spread area of the dropletson nanostructured and bare metal surfaces. Figure 13 showshow dramatically this mean thickness changes for two differentsurface types.

The mean thickness of water droplets on the ZnO nanos-tructured surfaces are, in the case aluminum, one fifth of themean thickness of the bare aluminum droplet. Based on thecalculations in Eq. 3, this will result in a heat transfer coefficient


FIGURE 12. The calculated heat transfer coefficient for nanostruc-tured copper surfaces and nanostructured aluminum surfaces. Bothshow heat transfer coefficients well above a typical plain copper oraluminum surface.[Surface S2]

FIGURE 13. Calculated mean thickness of liquid layer for waterdroplet droplet on ZnO nanostructured and bare metal surfaces for bothcopper and aluminum

that is larger because as mean thickness decreases, the time ofevaporation also decreases at a much greater rate (shown by thewetting number in Eq. 2).

The accuracy and error for experimental measurementsof this kind is particularly important as small changes insurface wettability can lead to great changes in heat transferperformance. For this study, the accuracy of measurements forevaporation time and heat transfer coefficients are a function ofthe thermocouples used for temperature measurement. Thesewere accurate to 0.5°C. Therefore, error for the evaporation timeand the heat transfer coefficient ranges from 2.5% to 5% for 10°and 20° superheats respectively.

MODELING THE IMPORTANCE OF REGULAR VERSUSIRREGULAR SPREADING

For these advanced hydrophilic surfaces, it is useful toput together models that can better predict the performancecharacteristics of these surfaces. In the case of comparing copperand aluminum nanostructured surfaces, one of the definingdifferences in the ZnO growth patterns was the homogeneityof the surface morphology under the SEM microscope. Asdroplets spread on surfaces that have nonhomogeneous growthof pillar like nanostructures, the contact line deviated from aradially symmetric growth pattern. This increased the overalllength of the contact line and we believe this could enhance heattransfer. These different spreading patterns on the macroscalecan be seen in Fig.14. Observing in this figure the differencebetween the smooth contact line for the copper surface and themore jagged line for the aluminum, there is a clear difference intheir spreading symmetry. The aluminum surfaces, which tendedto have microscale morphology that was less homogeneous,displayed spreading patterns that are less symmetric, whilesymmetric radial spreading was observed for copper coatedsurfaces with more consistent surface morphology. The SEMimage of Surface S2 shown in Fig.14 differs from that of theS2 SEM image shown in Fig.4. This is an example of the non-homogeneity in the aluminum coated surface. When dropletswere deposited on certain sections of the sample surface, theyspread more symmetrically, while in some spots they spread innon-symmetric ways (shown in Fig.9). Droplets deposited onS2, in general, demonstrated more symmetric spreading which isbelieved to be a result of consistent pillar formationl, like thatshown in Fig.4. However, there were portions of the surfaceon S2 with nanoscale morphology like that shown in Fig.14. Itis parts of the surface like this that still demonstrated extremewettability, but had less symmetric spreading capabilities. Ingeneral, the less symmetric spreading areas had a higher instanceof the sombrero effect.

Looking at the copper surface shown in Fig.14, there wascomplete coverage of pillar-like structures visible in the SEMimagery. In contrast, the aluminum surfaces, specifically thelatter surfaces S3 and S4 displayed as low as 20% coveragewith pillar-like structures. This noticeable difference resultedin changes in wettability, but still maintained heat transferperformance equal to, and at times better than the copper coatedsurfaces. For Surface S2, there was approximately 80% coverageof the pillar formations. The other 20% demonstrated lesssymmetric wetting for deposited droplets. There is furtherinvestigation needed as to the effect of surface irregularity andthe resulting non-radial spreading of droplets on the surface.

The initial work focusing on this irregularity is done throughmodeling the wetting, spreading, and final spread area and profileof a droplet on a superhydrophilic nanostructured surface. In do-ing this, comparisons can be drawn as to the correlation between


FIGURE 14. Comparison of regular versus irregular spreading for a copper and an aluminum surface and the related SEM morphology that affectsthis spreading pattern. Contact line segmentation from python coding program also shown.

wetting characteristics and microscale surface morphology. Thespread areas shown in Fig.14 demonstrate the difference inspreading area for a droplet of 2µl on two different surfaces,which in turn greatly affects the thickness of the droplet. Thecopper droplet pictured in Fig.14 has a wetted footprint areaof 14mm2 while the droplet on the aluminum nanostructuredsurface is nearly 8 times that at 89mm2. This affects the heattransfer coefficient, noted in Fig.12. For low superheats, whenevaporation does not include nucleation, the superheat doesnot effect the heat transfer coefficient, and it can be seen thatseveral of the surfaces with nanostructures on aluminum haveheat transfer coefficients that rival the copper surfaces. Furtherstudy into the evaporative benefits of the irregular spread areas isbeing explored. It is expected that since the perimeter aroundthe droplet that is represented by the contact line is the areawhere the droplet is thinnest, it would likely be where the highestheat transfer can be achieved. Continued testing will be ableto reveal whether this tendency on nanostructured aluminumsurfaces towards irregular spreading is the variable that causesany heat transfer advantage.

CONCLUDING REMARKSThe Zinc Oxide nanostructures created in this study are

seen as the first attempt to reproduce the successful creationof superhydrophilic surfaces from ZnO nanoparticles on copper

substrates. Using aluminum as a substrate for these nanos-tructures promises an advantage over copper both in its manu-facturability and cheaper cost. Additionally, the hydrothermalsynthesis growth method of producing these superhydrophilicnanostructures is a scalable technique that offers the potentialfor coating large scale heat exchangers and greatly enhance heattransfer from their surfaces through liquid film evaporation.

The nanostructure coated aluminum surfaces in this studywere found to have less homogeneous surfaces when viewedunder SEM imagery than the copper surfaces with the sameZnO coating. The reason for this difference between copper andaluminum is hypothesized to be the result of aluminum oxidedevelopment on the surface, thus conflicting with the growthof the ZnO nano particles, as well as altering the chemicalmakeup of the surface. Further work with aluminum will includeefforts to avoid the pollution of aluminum oxide. Regardless,the result of this non-homogeneity on the surface was a non-radial spreading of droplets on the aluminum surfaces. This didnot have a negative consequence, however, on the heat transferperformance. The ZnO nanostructure coated aluminum surfacestill facilitated rapid droplet spread, sombrero effect, and veryhigh heat transfer coefficients of over 20kW/m2, up to 3 timeshigher than bare aluminum.

Further computation analysis can also help with the un-derstanding of heat transfer enhancement for superhydrophilicsurfaces, particularly modeling the outer thinnest regions of the


droplet contact area. This can be further studied to developrobust heat transfer modeling for these unique nanostructuredaluminum surfaces. The ultimate goal is to have robust,experimentally reinforced computational model for the heattransfer performance on superhydrophilic surfaces. This willallow for quicker manufacturing iterations and, ultimately, higherperforming surfaces.

ACKNOWLEDGEMENTSSupport for this research was provided by the US-China

Clean-Energy Research Center’s CERC-WET Grant. Researchassistance from the National Science Foundation GraduateResearch Fellowship and Energy and Multiphase TransportLaboratory at UC Berkeley is also gratefully acknowledged.

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the effects of wettability and surface...

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