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Nanouids Alter the Surface Wettability of Solids Sangwook Lim, Hiroki Horiuchi, Alex D. Nikolov, and Darsh Wasan* Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, United States * S Supporting Information ABSTRACT: We report the results of our studies on the changes in the contact angle and interfacial tension using a nanouid composed of silica nanoparticles dispersed in water on three dierent solid substrates: gold (partially hydrophobic), glass (hydrophilic), and a silicon wafer (hydrophilic). We used both the goniometric method and drop-shape analysis to make the measurements. On the basis of the results of the drop-shape analysis using the Laplace equation, we evaluated the contributions of the interfacial tension change to the equilibrium contact angle and the presence of nanoparticles near the solid substrate, thereby elucidating the change in the wettability of the solid substrate. We found that the nanoparticles decrease the contact angle of the substrate with the increase in the nanoparticle concentration. To rationalize our experimental observations on the decrease in the contact angle of the solid substrate in the presence of nanoparticles, we calculated the surface volume fraction of the nanoparticles in the layer near the solid substrate using the particle layering model (based on the nanoparticlesexcluded volume eect). We found that the volume fraction of the nanoparticles in the layer close to the substrate increased with an increase in the nanoparticle volume fraction in the bulk and correlated qualitatively with the change in the substrate wettability. The extent of the wettability alteration depends on the volume fraction of the nanoparticles, their size, and the type of substrate. We found a strong correlation between the change in the substrate wettability and the nanoparticle volume fraction in the layer closer to the substrate surface. 1. INTRODUCTION Nanouids, which are composed of a liquid suspension of nanoparticles such as nanosized particles, polymer latexes, globular proteins, and surfactant micelles, have garnered attention because of their many practical and industrial applications in areas such as coating, cooling, and heavy engine transportation; this is largely due to their desirable optical and thermal properties. 14 Corresponding to this trend, the fundamental behavior of nanouids on a solid substrate (such as wetting and spreading) has been investigated experimentally and theoretically. 2,58 However, the classical concepts of simple liquid wetting do not properly explain the behavior of nano- and complex uids. 911 The mechanisms of nanouid wetting behavior on solid substrates are still not well established. Previous experimental observations showed that there are several factors associated with nanouids wetting solids. For instance, one such experiment clearly showed that the wetting of nanouids on solids is enhanced by changing the concentration of the nanoparticles (i.e., silicon oxide). 12,13 Also, several studies reported that the complex nature of the interactions between the particles in the nanouid and the solid substrate greatly alters the wettability of the solid sub- strate. 2,68,1217 Seane et al. 12 investigated the dynamic spreading of a nanouid solution composed of ethanol and aluminum nanoparticles on a hydrophobic (Teon-AF) surface. These authors observed that the spreading and wetting of the nanouid on the Teon-coated solid surface are enhanced by increasing the nanoparticle concentration (wt %) through the expulsion or withdrawal of the nanouid at a xed volumetric ow rate below a concentration of 1 wt %. However, for concentrations higher than 1 wt %, the contact line velocity slowed down slightly or was constant. The authors suggested two possible mechanisms to explain the experimental results. They believed that the spreading of the nanouid could be driven by the structural disjoining pressure, as originally proposed by Wasan et al. 6 At concentrations above 1 wt %, however, they hypothesized that the viscous force was dominant, rather than the structural disjoining pressure. Vafaei et al. 7,13 studied the inuence of the nanoparticle size and concentration on the surface wettability using bismuth telluride particles (Bi 2 Te 3 ) as the nanoparticles and glass and silicon wafers as the solid surfaces. They reported that the lower nanoparticle size plays a role in more eciently changing the contact angle of the nanoparticle suspension on the solid substrate as well as the liquidgas and solid surface tension. They hypothesized that the repulsive forces between the solid and the nanouid could consequently change the three-phase contact angle; these forces could result in complex wetting behavior that increases with the concentration of the nanoparticles. Unfortunately, they used modied nanoparticles (functionalized with thiol groups). Modied nanoparticles in a Received: March 6, 2015 Revised: April 27, 2015 Published: April 28, 2015 Article pubs.acs.org/Langmuir © 2015 American Chemical Society 5827 DOI: 10.1021/acs.langmuir.5b00799 Langmuir 2015, 31, 58275835
Transcript

Nanofluids Alter the Surface Wettability of SolidsSangwook Lim, Hiroki Horiuchi, Alex D. Nikolov, and Darsh Wasan*

Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, United States

*S Supporting Information

ABSTRACT: We report the results of our studies on the changes in thecontact angle and interfacial tension using a nanofluid composed of silicananoparticles dispersed in water on three different solid substrates: gold(partially hydrophobic), glass (hydrophilic), and a silicon wafer(hydrophilic). We used both the goniometric method and drop-shapeanalysis to make the measurements. On the basis of the results of thedrop-shape analysis using the Laplace equation, we evaluated thecontributions of the interfacial tension change to the equilibrium contactangle and the presence of nanoparticles near the solid substrate, therebyelucidating the change in the wettability of the solid substrate. We foundthat the nanoparticles decrease the contact angle of the substrate withthe increase in the nanoparticle concentration. To rationalize ourexperimental observations on the decrease in the contact angle of thesolid substrate in the presence of nanoparticles, we calculated the surface volume fraction of the nanoparticles in the layer nearthe solid substrate using the particle layering model (based on the nanoparticles’ excluded volume effect). We found that thevolume fraction of the nanoparticles in the layer close to the substrate increased with an increase in the nanoparticle volumefraction in the bulk and correlated qualitatively with the change in the substrate wettability. The extent of the wettabilityalteration depends on the volume fraction of the nanoparticles, their size, and the type of substrate. We found a strong correlationbetween the change in the substrate wettability and the nanoparticle volume fraction in the layer closer to the substrate surface.

1. INTRODUCTION

Nanofluids, which are composed of a liquid suspension ofnanoparticles such as nanosized particles, polymer latexes,globular proteins, and surfactant micelles, have garneredattention because of their many practical and industrialapplications in areas such as coating, cooling, and heavy enginetransportation; this is largely due to their desirable optical andthermal properties.1−4 Corresponding to this trend, thefundamental behavior of nanofluids on a solid substrate (suchas wetting and spreading) has been investigated experimentallyand theoretically.2,5−8 However, the classical concepts of simpleliquid wetting do not properly explain the behavior of nano-and complex fluids.9−11 The mechanisms of nanofluid wettingbehavior on solid substrates are still not well established.Previous experimental observations showed that there are

several factors associated with nanofluids wetting solids. Forinstance, one such experiment clearly showed that the wettingof nanofluids on solids is enhanced by changing theconcentration of the nanoparticles (i.e., silicon oxide).12,13

Also, several studies reported that the complex nature of theinteractions between the particles in the nanofluid and the solidsubstrate greatly alters the wettability of the solid sub-strate.2,6−8,12−17

Sefiane et al.12 investigated the dynamic spreading of ananofluid solution composed of ethanol and aluminumnanoparticles on a hydrophobic (Teflon-AF) surface. Theseauthors observed that the spreading and wetting of thenanofluid on the Teflon-coated solid surface are enhanced by

increasing the nanoparticle concentration (wt %) through theexpulsion or withdrawal of the nanofluid at a fixed volumetricflow rate below a concentration of 1 wt %. However, forconcentrations higher than 1 wt %, the contact line velocityslowed down slightly or was constant. The authors suggestedtwo possible mechanisms to explain the experimental results.They believed that the spreading of the nanofluid could bedriven by the structural disjoining pressure, as originallyproposed by Wasan et al.6 At concentrations above 1 wt %,however, they hypothesized that the viscous force wasdominant, rather than the structural disjoining pressure.Vafaei et al.7,13 studied the influence of the nanoparticle size

and concentration on the surface wettability using bismuthtelluride particles (Bi2Te3) as the nanoparticles and glass andsilicon wafers as the solid surfaces. They reported that the lowernanoparticle size plays a role in more efficiently changing thecontact angle of the nanoparticle suspension on the solidsubstrate as well as the liquid−gas and solid surface tension.They hypothesized that the repulsive forces between the solidand the nanofluid could consequently change the three-phasecontact angle; these forces could result in complex wettingbehavior that increases with the concentration of thenanoparticles. Unfortunately, they used modified nanoparticles(functionalized with thiol groups). Modified nanoparticles in a

Received: March 6, 2015Revised: April 27, 2015Published: April 28, 2015

Article

pubs.acs.org/Langmuir

© 2015 American Chemical Society 5827 DOI: 10.1021/acs.langmuir.5b00799Langmuir 2015, 31, 5827−5835

suspension act like a surfactant and dramatically affect thechange in the interfacial tension or surface tension. This changein the interfacial tension or surface tension can result in asubstantial change in the contact angle of the nanofluid on thesolid. Vafaei et al.’s study did not exclude the effects of thenanoparticles from the interfacial tension decrease on thecontact angle (and thereby the energy of the solid substrate).Karimi et al.17 experimentally studied the wettability

alteration with a zirconium oxide (ZrO2) nanofluid incarbonate reservoir rock. They used a composition of ZrO2nanoparticles and mixtures of nonionic surfactants as thenanofluid; they found a significant change in the wettability ofthe rock from the oil-wet to water-wet condition. Theysuggested that the adsorption of the nanoparticles results in theformation of the nanoparticle structuring, what they called“nanotextured surfaces” on the rock. They concluded that thewettability alteration is mainly due to the formation ofcomposite nanotextured surfaces. Because they used a complexmixture of nanofluids and surfactants on a porous substrate, it isdifficult to analyze the mechanism of the wettability alterationfrom their experimental data.Kim et al.2 observed the enhancement in the critical heat flux

(CHF) due to the enhancement in the wettability of thenanofluid on the solid surface. The pool boiling characteristic ofnanofluids as a method of heat transfer is highly dependent onboth the particle concentration and nanofluid/surface wett-ability.18

Nikolov et al.19 studied the complex solid−nanofluid−oilinteractions in our laboratory and directly observed the self-structuring of the nanoparticles (i.e., stratification) using thecombined differential and common light interferometricmethod. This self-structuring phenomenon is a consequenceof the fact that the ordering increases the entropy of the overalldispersion by allowing greater freedom for the nanoparticles inthe bulk liquid and results in excess pressure (i.e., the disjoiningpressure) in the film relative to that in the bulk solution.Wasan et al.14 reported the role of the structural disjoining

pressure on the wetting and spreading phenomena of thenanofluid solution composed of silica nanoparticles. In theirexperiment, a drop of a silica nanoparticle suspension (with anominal diameter of 19 nm) was placed on a hydrophilic glasssurface at a low contact angle. This was achieved by adding 2.5mM sodium dodecyl sulfate, which was below the criticalmicellar concentration in the nanofluid. When a drop of thenanofluid was placed on a glass surface immersed inhexadecane, it displaced the oil phase and the nanofluid dropcontact diameter increased with time.Chaudhuri et al.20 recently studied the influence of

hydrophilic titanium oxide (TiO2) and hydrophobic sulfur(S) nanofluids on the surface wettability and surface tension ofglass and PTFE. They investigated the effects of the particlesize, the material properties, and the concentration of particleson the surface tension and the effect of the contact angle of ananofluid droplet on a solid. They found that the surfacetension of the nanofluids decreased with decreasing particlesize. For the PTFE solid substrate, the change in the contactangle was more pronounced with the TiO2 nanofluids ratherthan with the sulfur nanofluids. The reduction in the surfacetension was also more pronounced for the TiO2 nanofluids. Onthe other hand, the sulfur nanofluids showed a morepronounced increase in the contact angle for the glass substrate.They concluded that the surface tension, the deposition of aparticle on a solid, and the structural disjoining pressure due to

the presence of nanoparticles in the liquid film are the keyfactors that change the contact angle of nanofluids on solidsubstrates. However, the nanofluids in their experiment had abroad range of nanoparticles (polydisperse nanoparticles) andmainly consisted of large nanoparticles. With a polydispersenanoparticle suspension and large nanoparticles, it is difficult toexpect a strong structural disjoining pressure resulting from theordering of the nanoparticles in the liquid film on the substrate.Furthermore, their experiments allowed the evaporation of thenanofluids. Nanofluid evaporation provokes the evaporative fluxon a solid substrate, making it difficult to analyze theexperimental data or to elucidate the mechanism of nanofluidwetting on a solid substrate.In addition to numerous experimental reports asserting the

potential of nanofluids in enhancing and altering the wettabilityof a solid substrate, there are few papers that actually report themodeling mechanism that alters a solid’s wettability. Theliterature data is mainly based on complex systems containingnanoparticles modified with functional groups or cappingagents and polydisperse nanoparticles, which may haveprevented the authors from establishing a proper model forthe effect of nanoparticles on solid substrate wettability.13,15,20

In the present study, we experimentally investigate the effectsof the nanoparticle concentration, particle size, the interfacialtension between the nanofluids and hexadecane and the type ofsolid substrate on the surface wettability for glass (hydrophilic),gold (partially hydrophobic), and silicon wafer (hydrophilic)substrates. We monitored the variations in the equilibriumcontact angle and interfacial tension between the nanofluidsand hexadecane using drop-profile analysis augmented with theLaplace equation; we also used the goniometric method byvarying the concentration of the nanofluids, the size of thenanoparticles, and the type of solid substrate. The simultaneousmeasurement of the contact angle and interfacial tension wasachieved by using drop-shape analysis. We successfullyseparated the effect of the interfacial tension change fromthat of the equilibrium contact angle using drop-shape analysis.By quantifying the effect of the interfacial tension on theequilibrium contact angle, we found that the interfacial tensioncan contribute to the change in the solid substrate wettability,but it is not sufficient to explain the total change in the contactangle.We also examined the bulk properties of the nanoparticle

suspension, such as the second virial coefficient and osmoticpressure which are strongly related to the interactions amongthe nanoparticles in the suspension and thereby the interactionbetween the nanoparticles and the solid substrate. This is anaspect of the issue that other researchers have only rarelyconsidered.We used the particle-layering model based on the excluded

volume effect to verify the role of the nanoparticles on the solidwettability change. We now introduce a new mechanism basedon the nanoparticle layering driven by the attractive depletionforce, which is based in the work done by Trokhymchuk etal.:21−23 the layering model of nanoparticles near the solidsubstrate can predict the change in the solid substratewettability.Our study is the first to introduce the nanoparticle layering

concept that explains the wettability alteration near the solidsurface.

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2. EXPERIMENTSMaterials. We used 8 nm silica nanoparticle suspensions (silica

nanofluids) of Nalco 1130 (30 wt %, Nalco Chemical Company, USA)in water and 20 nm silica nanoparticle suspensions (40 wt %, AlfaAesar, USA) in water. The gold-coated microscope slides (25 × 75 × 1mm3, Platypus, USA) and glass microscope slides (25 × 75 × 1 mm3,Fisher Scientific, USA) were used as the hydrophilic and partiallyhydrophobic solid substrates, respectively, and were cut into smallplates (20 × 20 × 1 mm3). The hexadecane was supplied by Acros(USA).Methods. Nanofluid Preparation. To remove the surface-active

materials from the commercial silica nanofluids, we put the nanofluidsin a 60 mL glass vial and closed the cap. We shook the bottle to makebubbles and then removed these bubbles (which contain surface-activematerials) using a glass capillary connected to a suction pump. Wethen repeated the same procedure 10 times. We monitored theinterfacial tension of the silica nanofluid in the hexadecane and foundthat there is no change in the surface activity. Different concentrationsof nanofluids were obtained by dilution with an aqueous solution madefrom deionized water (a Milli-Q water deionizer system was suppliedby Millipore Corp., Bedford, MA) and sodium hydroxide to keep thepH constant (i.e., pH 9.9 for the 8 nm silica nanofluids and pH 9.5 forthe 20 nm silica nanofluids). The pH was measured with a pH paperindicator (Whatman pH paper indicator; pH range 0−14; no. 2613-991). The silica nanofluids composed of 0.3, 0.5, 1.0, 1.5, 2.0, and 3.0vol % silica suspensions of particles 8 and 20 nm in nominal diameterwere pre-equilibrated with hexadecane for 1 day before theexperiments.The effective diameter of the silica nanoparticles for every

concentration was determined by the dynamic light scattering method(Malvern). The average effective diameters of the silica nanoparticlesin different concentrations of the suspension were found to be about19 ± 1.0 nm for the nominal 8-nm-diameter silica nanoparticlesuspension and 39 ± 2.0 nm for the nominal 20-nm-diameter silicananoparticle suspension. We found that the polydispersity is around0.12 for the 19 nm silica nanofluid and 0.15 for the 39 nm silicananofluid. Nikolov et al.19,24 measured the effective size of thenanoparticles by using the capillary force balance apparatus inconjunction with the reflected light common interferometrictechnique; our measurement agreed with their experimental results.The effective volume fraction (effective concentration) of nano-

particles in the bulk, ϕe, is calculated in the following relation

ϕ ϕ=⎛⎝⎜

⎞⎠⎟

dde N

N

3

(1)

where ϕN is the nominal volume fraction of the nanoparticles(concentration of the nanofluids), d is the effective diameter of thenanoparticles, and dN is the nominal diameter of the nanoparticles.Solid Substrate Preparation. The glass slides and all of the

glassware were precleaned by immersion in a mixture of sulfuric acidand chromic acid for 1 day. Afterward, they were rinsed with deionizedwater. The glass slides were then dried in a vacuum oven at 25 °C forabout 3 days.The gold plates were cleaned by immersion in a sequence of toluene

for 6 h, acetone for 6 h, and deionized water for 1 day. After that, theywere dried in a vacuum oven at 25 °C for about 2 days.The silicon wafer sample (supplied by Cabot Microelectronics,

USA) was made of 7.5 mm silicon and covered with an 800 nmthickness of silica deposited by the plasma-enhanced chemical vapordeposition method. The sample is a 2.5 in. disk that has not receivedany surface treatment. The silicon side of the wafer is silver-colored,and the silica side is flawless and mirrorlike. The silica wafer was cut bya laser beam into 20 mm × 20 mm squares.Interfacial Tension Measurement. To measure the interfacial

tension between the silica nanofluids and hexadecane, the classicalmethod of drop-shape analysis based on the Laplace equation wasused.25,26 The droplet profile of the liquid/hexadecane interface isgoverned by the following interface profile equation

γ θ γθ + = + Δρ⎜ ⎟⎛⎝

⎞⎠s x b

z gdd

sin 2LO

LO

(2)

where γLO is the liquid/hexadecane interfacial tension, b is the apexradius of curvature, Δρ is the difference in density between thenanofluids and hexadecane, g is the gravity constant, θ is the runningangle, s is the arc length, x is the radial distance, and z is the verticaldistance.

Figure 1 shows the side-view image of the 0.27 effectiveconcentration of the 8 nm (effective diameter 19 nm) silica nanofluid

solution on a gold substrate immersed in hexadecane and thecorresponding experimental drop profile fitted by the drop profileequation. A good fit between the experimental drop profile andtheoretical drop profile using the profile equation is observed,indicating that the drop profile of the silica nanofluids in theexperiment is in an equilibrium shape.

The detailed procedure is explained in Appendix A (SupportingInformation).

Contact Angle Measurement. The glass and gold substrates wereplaced in a transparent glass cuvette filled with hexadecane. The wafersample was placed in a plastic box under atmospheric conditions andcovered with a plastic cap (30 × 30 × 20 mm3) with a wet sponge inorder to create a water-vapor-saturated condition inside the box.

A sessile droplet of pre-equilibrated silica nanofluid was placed onthe glass and gold substrates immersed in the hexadecane with aHamilton microsyringe (B-D needle, 25G 5/8). The droplet volume ofthe 5.5 ± 0.5 μL silica nanofluid was used on the glass substrate, and10 ± 1.0 μL was used on the gold substrate. For the droplet on thegold substrate, we used a larger-volume droplet to obtain consistentinformation about the three-phase contact line and angle.27

The sessile droplet (5 ± 1.0 μL) of the silica nanofluid wasintroduced onto the wafer surface via a tiny hole in the top of the capwith a microsyringe. We employed the sessile drop techniques tomeasure the three-phase contact angle in the experiment with agoniometer. The image of the droplet on the solid was captured with adigital camera (Canon A720 IS) 3 h after placing the droplet on thesolid substrate. Each experiment was conducted at room temperature(25 ± 1 °C). The three-phase contact angle was calculated by imageanalysis (Image Pro, version 6). The effect of the contact anglehysteresis on the equilibrium value of the contact angle was alsoanalyzed. It was found that in the case of small angles (e.g., <25°) thehysteresis value was ±1° and for large contact angle values (e.g., >90°)it was ±5°. The contact angle measurement from the goniometer wascompared to the contact angle from the drop-shape analysis.

Turbidity, Refractive Index, and Osmotic Pressure Measurements.To understand the behavior of the changes in the solid substratewettability with nanofluids, we studied the interaction of the silicananoparticles in the bulk suspension with the turbidity, second virialcoefficient, and osmotic pressure. With the second virial coefficient,the osmotic pressure of the silica nanofluid suspension was calculatedusing the following equation25,28

≈ +PcRT M

Bc1

(3)

Figure 1. (a) Side view of a sessile droplet for the 0.27 effectiveconcentration of the 8 nm (effective diameter 19 nm) silica nanofluidsolution on a gold solid substrate. (b) Corresponding drop profilefitted with the profile equation.

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where P is the osmotic pressure, R is the gas constant (= 8.315 J/molk), T is the temperature, c is the concentration (g/cm3), and M is themolecular weight of one particle (g/mol). The detailed procedure isavailable in Appendix B.

3. RESULTS AND DISCUSSIONContact Angle of Nanofluids on Glass Substrates.

Figure 2 shows the three-phase contact angles of the 8 nm

(effective diameter 19 nm) silica nanofluids using thegoniometer and drop-shape analysis on the glass substrate(hydrophilic) immersed in hexadecane with varying effectiveconcentrations of nanoparticles.The contact angle decreases with the increase in the

nanoparticle concentration. The contact angles determinedfrom the goniometer measurement and drop-shape analysisusing the Laplace equation show a similar trend. When thecontact angle is less than 30°, the contact angle from thegoniometric method introduces errors of about ±10%. Itshould be noted that evaluating the exact equilibrium contactangle is difficult using only the goniometric method. However,the drop-shape analysis based on the Laplace equation cansimultaneously give a more precise equilibrium contact angleand interfacial tension between the hexadecane and thenanofluid. Therefore, we decided to use the contact anglefrom the drop-shape analysis as the equilibrium contact angle,θeq, in our modeling study. Other calculations will be discussedin later sections.Interfacial Tension of the Nanofluid/Hexadecane. The

interfacial tension between the silica nanofluid and hexadecaneon the glass and gold substrates for the 8 and 20 nm (effectivediameters of 19 and 39 nm) nanoparticles was measured usingdrop-shape analysis.Figure 3 shows the interfacial tension between the silica

nanofluids (8 and 20 nm; effective diameters of 19 and 39 nm)and hexadecane on the glass and gold substrates vs the effectiveconcentration of the nanoparticles; the trend line is also shown.In Figure 3, the interfacial tension between the pH 9.9 and

9.5 aqueous solutions and hexadecane for the 8 and 20 nm(effective diameters of 19 and 39 nm) nanofluids is 53 ± 0.5and 53.0 ± 1.0 dyn/cm, respectively; these are the same values

that exist between pure water and hexadecane at 25 °C, asreported in the literature.29,30

The interfacial tension between the silica nanofluid andhexadecane, γOL, gradually decreases with the increasingconcentration of nanoparticles and becomes constant for the8 nm (effective diameter 19 nm) silica nanofluid. Because thecommercially available silica nanoparticle suspension may havea small amount of surface-active materials (such as a suspensionstabilizer), those surface-active materials could decrease theinterfacial tension. Metin et al.15 reported that the interfacialtension between decane and an unmodified 5 nm silicananofluid was slightly smaller (∼43 dyn/cm) than the valuebetween decane and pure water (45 dyn/cm) at roomtemperature.The interfacial tension between hexadecane and the silica

nanofluid shows a similar value (error <5%) on both the glassand gold substrates, indicating that the interfacial tension of thehexadecane/nanofluid is the same regardless of the type of solidsubstrate used.

Separating the Interfacial Tension Contribution fromthe Contact Angle Contribution. To evaluate the effect ofthe presence of nanoparticles near the solid substrate on theequilibrium contact angle, we need to separate the contributionof the interfacial tension to the change in the contact angle withthe proper method. Because we measured the equilibriumcontact angle and the nanofluids/hexadecane interfacial tensionsimultaneously using drop-shape analysis, we can successfullyseparate the contribution of the interfacial tension to theequilibrium contact angle by using the procedure that follows.Equation 4 is Young’s equation31 for an oil−liquid−solid

system

γ γ θ γ= +cosSO OL eq LS (4)

where γSO represents the interfacial tension between the solidand oil, γOL is the interfacial tension of the liquid/oil interface,θeq is the equilibrium contact angle, and γLS is the interfacialenergy between the solid and liquid.By assuming the interfacial tension of the nanofluids/

hexadecane, γOL varies only with the concentration of thenanofluids and other terms; γSO and γOL are constant in Young’sequation, so we can derive eq 5

Figure 2. Variation in the three-phase contact angle of the 8 nm(effective diameter 19 nm) silica nanofluids from the goniometer, θ,and drop-shape analysis, θeq, with the effective concentration ofnanoparticles on a solid glass substrate immersed in hexadecane.

Figure 3. Variation in the interfacial tension between the silicananofluid and hexadecane with the effective concentration for the 8and 20 nm (effective diameters of 19 and 39 nm) silica nanofluids onglass and gold solid substrates.

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γ θ γ θ=cos cosOL0 0 OL I (5)

where γOL0 and θ0 are the interfacial tension and contact angleat zero concentration of nanoparticles. Because γOL0, θ0, andγOL are known values measured in the experiment, we cancalculate the contact angle, θI, which is truly dependent uponthe change in the nanofluids/hexadecane interfacial tension.To calculate the effect of the presence of nanoparticles near

the substrate on the equilibrium contact angle, we define thedifference between the estimated contact angle from theinterfacial tension, θI, and the contact angle from the drop-shape analysis, θeq, as Δθ = θI − θeq, which is the contributionof the presence of nanoparticles near the solid substrate to theequilibrium contact angle. Then, we calculate the contact angledue to the effect of the nanoparticles alone near the substratewith a different effective concentration, which is defined as θL =θ0 − Δθ. This procedure, separating the interfacial tension’scontribution to the equilibrium contact angle, can be achievedonly if the interfacial tension and equilibrium contact angle aremeasured simultaneously. To the best of our knowledge, thiscalculation has never been performed before our present study.Figure 4 shows the calculated contact angle, θI, from eq 5, the

experimentally measured equilibrium contact angle, θeq, based

on the drop-shape analysis, and the decrease in the contactangle due to the presence of nanoparticles near the glasssubstrate, θL, immersed in hexadecane with an increasingconcentration of the 8 nm (effective diameter 19 nm) silicananofluid.We observe a substantial difference between the contact

angle calculated from the change in the interfacial tension, θI,and the experimentally measured equilibrium contact angle, θeq.This difference clearly demonstrates that the effect of theinterfacial tension alone, θI, cannot fully explain theexperimental data, θeq, of the substrate.As the concentration of the nanofluid increases, the contact

angle change due to the nanoparticles near the glass substratedecreases with the increasing effective concentration. Thiscalculation implies that the change in the contact angle due tothe nanoparticles near the glass substrate becomes pronouncedwith the increase in the effective concentration of thenanoparticles and becomes independent of the concentration

after a certain concentration. On the basis of the results inFigure 4, we can conclude that the interfacial tension and thepresence of nanoparticles near the glass substrate contribute tothe decrease in the equilibrium contact angle.For the gold substrate, we can also observe a phenomenon

similar to that of the glass substrate: the contact angle decreasedue to the nanoparticles near the substrate is more pronouncedwith the increasing effective concentration and the contributionof the interfacial tension is not sufficient to explain theexperimental data (Figure 5).

We also analyzed the data reported in the literature,15 whichgive the contact angle of decane on a quartz substrate immersedin a nanoparticle suspension of varying sizes of unmodifiedsilica nanoparticles (Figure 6). We found that the differencebetween the decrease in the contact angle observed in theexperiment and the decrease in the contact angle calculated onthe basis of the interfacial tension alone demonstrates that theinterfacial tension alone cannot explain the previouslypublished experimental data.

Figure 4. Contact angle of the 8 nm (effective diameter 19 nm) silicananofluid on glass immersed in hexadecane from the drop-shapeanalysis, θeq, the estimated value of the interfacial tension, θI, using eq5, and the presence of nanoparticles near the substrate, θL.

Figure 5. Contact angle of the 8 nm (effective diameter 19 nm) silicananofluid on gold immersed in hexadecane from the drop-shapeanalysis, θeq, the estimated value of the interfacial tension, θI, using eq5, and the presence of nanoparticles near the substrate, θL.

Figure 6. Comparison of the decrease in the contact angle in theexperiment (black spheres) with the decrease in the equilibriumcontact angle calculated from the interfacial tension (blue triangles)with a decane droplet on a quartz substrate immersed in differentlysized silica nanoparticle suspensions.15

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Effect of Nanoparticle Size on the Contact Angle. Thecomparison of θL for the 8 and 20 nm (effective diameters of 19and 39 nm) silica nanofluids on a glass substrate is described inFigure 7.

From Figure 7, we can confirm that the decrease in thecontact angle due to the nanoparticles near the glass substrate ismore pronounced with the 8 nm (effective diameter 19 nm)nanoparticle suspension. Specifically, the decrease in thecontact angle for the 8 nm (effective diameter 19 nm) silicananofluid on glass is around 18°, whereas it is 4° for the 20 nm(effective diameter 39 nm) silica nanofluid. Recent studies havereported nanoparticle size effects on a solid’s energy, wetting,and spreading.7,13,14 They reported that nanoparticle suspen-sions with smaller particles are more efficient with respect tochanges in the contact angle and therefore affect the wetting.The contact angles of a 20 nm (effective diameter 39 nm)

silica nanofluid on a gold substrate immersed in hexadecaneand a silicon wafer in air were also measured. The decrease inthe equilibrium contact angle of the 20 nm (effective diameter39 nm) nanofluid on the gold substrate is less pronounced thanthat of the 8 nm (effective diameter 19 nm) nanofluid(Appendix C). The contact angle on the silicon wafer substratedecreases as a function of the effective nanoparticleconcentration. These observations concerning the gold andsilicon wafer substrates support our observation that thepresence of nanoparticles near the solid substrate can alter thewettability of a solid substrate.Modeling of Nanoparticle Layering near the Solid

Substrate. Previously, Trokhymchuk et al.21,23 theoreticallystudied the ordering of a binary hard-sphere system composedof nanoparticles and solvent molecules near a planar wall byusing the integral equation theory. They found that the inducedexcluded volume interaction (attractive depletion) between thelarge particles and the flat wall provokes the localization of largeparticles near the wall, forming a two-dimensional monolayer ofparticles; the nanoparticles in a suspension have a tendency tocreate a layer in the vicinity of the solid substrate. They alsoobserved that the ordering of these nanoparticles becomes wellpacked with the increasing concentration of nanofluids ordecreasing particle size. Recently, the development oftheoretical modeling using the depletion force for a colloidal

system was reported.32,33 Kralchevsky et al.33 reported thetheoretical expression considering the depletion attraction dueto the hard spheres to estimate the contact angle of a planeparallel foam film and the micelle aggregation number. Itshowed good agreement with other experimental data. Theyalso developed the theoretical model for charged particles topredict the equilibrium thickness of the parallel films, andexcellent agreement was observed with the experiment. Thedepletion effects for the particle−particle and particle−wallinteractions become indispensable to the theoretical modelingof the colloidal system in explaining the experimental values.In this work, we adopted the concept of the layering of

nanoparticles near a solid substrate to rationalize the observedtrends in the relative change in the solid substrate wettability.We used a scanning electron microscope (SEM) to observe theparticles on the substrate before and after the experiment andfound that the particles are not deposited on the substrate.However, from atomic force microscope (AFM) images, wefound that the particles form a layered structure near thesurface during the experiment.We hypothesized that the layering of the nanoparticles near

the solid substrate results in a change in the solid wettabilityand therefore a change in the solid substrate energy. Accordingto the layering phenomenon, well-constructed nanoparticlelayering is expected with the increase in the concentration andthe decrease in the size of the nanoparticles. With enoughnanoparticles near the solid, the saturation of the nanoparticlesin the first layer is expected, and even second and third layerscan be formed near the solid substrate. However, thecontribution of the second and third layers to the substrateenergy is negligible because they are not well organized.14,22

Once the first layer of nanoparticles is formed near the solidsubstrate, we can expect no change in the contact angle and nochange in the substrate energy, even with the increasingconcentration of nanoparticles in the bulk. Interestingly, ourexperimental data (such as the change in the contact angle)exactly follow the concept of the nanoparticle layeringphenomenon near the wall.

Estimating the Nanoparticle Concentration in theLayer Close to the Solid Substrate. First, we consider thedepletion interaction between two small colloidal hard sphereswith diameter d (Figure 8a).The excluded volume vex is equal to the shaded volume (light

blue), and it is calculated according to the following

π= =vd

v4

38ex

3

d (6)

Figure 7. Decrease in the contact angle due to the presence ofnanoparticles, θL, near the glass substrate immersed in hexadecane forthe 8 and 20 nm (effective diameters of 19 and 39 nm) silicananofluids.

Figure 8. Excluded volume interaction (a) between a hard sphere anda hard sphere and (b) between a hard sphere and a wall.

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where vd is the volume of the hard sphere. Here, the total freevolume, Vf, in the system can be described as Vf = V(1 − 8vdn),and its elementary volume is dVf = V(1 − 8vdn), where nrepresents the number of neighbors in the bulk and is definedas n = N/V (N is the total number of the particles in the bulk).As a result, the probability of finding particles is given by

= =WV

VV

Vd

d df

f (7)

Second, we examined the depletion interaction between a hardsphere with diameter d and a hard wall (Figure 8b). In contrastto the depletion interaction between two particles, the excludedvolume near the wall, v′ex, is half the volume.

π′ = =⎛⎝⎜

⎞⎠⎟v

dv

12

43

4ex

3

d(8)

In the case of the sphere−wall interaction, the total free volumenear the wall is calculated to be Vs = V(1 − 4vdns) and theelementary volume is dVs = dV(1 − 4vdns), where ns is thenumber of neighborhoods near the wall. The probability offinding particles near the wall is calculated as

= =−−

WV

Vv nv n

VV

dd (1 4 )

(1 8 )ds

f

d s

d (9)

By comparing eq 7 with eq 8, we obtain the followingrelation for the number of particles in the bulk and near thewall:34

=−

nn

v n(1 4 )sd (10)

Finally, by multiplying vd on both sides of eq 10, we can obtainthe relation between the effective volume fraction in the bulkand the surface

ϕϕ

ϕ=

−(1 4 )sB

B (11)

where ϕB = nvd is the effective volume fraction of thenanoparticles in the bulk and ϕs = nsvd is the surface volumefraction of the nanoparticles near the substrate.We also calculated the relative change in the glass substrate

wettability due to the nanoparticle layer near the solidsubstrate, cos θL, as a function of the effective volume fractionof the nanoparticles and compared it to the surface volumefraction based on eq 11. In Figure 9, we plot both the change inthe glass substrate wettability with the 8 nm (effective diameter19 nm) silica nanofluid on glass (the blue squares) and thesurface volume fraction in the layer next to the substrate (blackline) with the effective volume fraction of the nanoparticles inthe bulk. We can confirm that the change in the glass substratewettability follows the trend in the theoretically calculatedsurface volume fraction using eq 11.The wettability of the glass substrate becomes more

hydrophilic due to the increase in the silica nanoparticlesnear the solid substrate as the volume fraction of the nanofluidincreases.The change in the gold substrate wettability with the surface

volume fraction is also plotted in Figure 10. The wettabilitychange and surface volume fraction calculated using eq 11follow similar trends.The correlation between the substrate wettability and the

surface volume fraction is strong at the low effective volume

fraction of the nanoparticles but shows a deviation at the higheffective volume fraction because of the strong interparticleinteractions at high particle concentration as indicated by theosmotic pressure data given in Appendix B.

4. CONCLUSIONSIn this work, we explored the effect of nanofluids on alteringthe solid wettability. We used silica nanoparticle suspensionswith two different sizes of silica nanoparticles, 8 and 20 nm(effective diameters of 19 and 39 nm) and three different solidsubstrates (glass, gold, and a silicon wafer). We examined theeffect of different parameters such as concentration, size ofnanoparticles, and type of substrate on the interfacial tensionand equilibrium contact angle. The results of the present studyare summarized below.The interfacial tension between the silica nanofluid and

hexadecane and the equilibrium contact angle were measuredsimultaneously using drop-shape analysis with the Laplaceequation. Increasing the nanofluid concentration caused adecrease in the interfacial tension between the nanofluid andhexadecane; it then became almost constant. The interfacialtension between the silica nanofluid and hexadecane is similarfor the 8 and 20 nm nanoparticles, regardless of the type ofsubstrate. This indicates that the interfacial tension is

Figure 9. Change in the wettability of the glass substrate for the 8 nm(effective diameter 19 nm) silica nanofluid (blue squares) and thesurface volume fraction of the layer (black line) with the effectivevolume fraction of the nanoparticles in the bulk.

Figure 10. Change in the wettability of the gold substrate for the 8 nm(effective diameter 19 nm) silica nanofluid (red squares) and thesurface volume fraction of one layer (black line) with the effectivevolume fraction of the nanoparticles in the bulk.

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independent of the size of the particle and the type of solidsubstrate.The contact angle from the drop-shape analysis and

goniometer measurements on the gold and glass substratesshowed good agreement except when the contact angle was lessthan 30°; this indicates that the drop-shape analysis is a goodmethod for measuring the exact equilibrium contact angle.We separated the contribution of the interfacial tension on

the equilibrium contact angle by using Young’s equation, andwe calculated the decrease in the contact angle purely due tothe presence of nanoparticles near the solid substrate. Wefound that the interfacial tension contributes to the change inthe equilibrium contact angle, but this change is not sufficientto explain the experimental data.As the concentration of the silica nanofluids increased, we

observed a decrease in the contact angle of the nanofluids onthe glass, gold, and silicon substrates, indicating that the solidsubstrates become more hydrophilic.The decrease in the contact angle was more pronounced

when the 8 nm (effective diameter 19 nm) silica nanoparticlesuspension was used for both the glass and gold substrates,indicating that the smaller size of the nanoparticles affects thewettability of the solid substrate.To rationalize the change in the solid substrate wettability

due to the nanoparticles near the substrate, we used ananoparticle layering model. We estimated the surface volumefraction of the nanoparticles for the 8 nm (effective 19 nm)silica suspension in the particle layer next to the solid substratesurface due to the excluded volume effect. We found that thesurface volume fraction from our theoretical work followed thesame trend as the change in the solid wettability. In otherwords, the proposed layering modeling was able to explainqualitatively the trend in the change in the solid substratewettability; this indicates the nanoparticle layering near thesolid substrate contributes to the alteration in the wettability ofthe solid substrate. Therefore, we concluded that the layeringmodel is potentially a good framework to verify the mechanismof solid wettability alteration caused by nanofluids, especially atthe low nanoparticle concentration.However, further studies to explain the quantitative change

in the solid substrate wettability are necessary for highnanoparticle concentration. Additional studies are warrantedto evaluate the effects of several parameters on the wettabilityalteration, such as the size of the sessile droplets and thepolydispersity of the nanoparticles. A systematic study needs tobe carried out to develop a more comprehensive understandingof how nanofluids alter solid wettability. A study using a purenanofluid system (without any surface-active impurities) needsto be conducted in order to conclusively establish the role ofthe nanoparticle layering phenomenon in altering the surfacewettability.

■ ASSOCIATED CONTENT

*S Supporting InformationDrop-shape analysis, osmotic pressure, and contact angle ofnanofluids on the silicon wafer substrate. The SupportingInformation is available free of charge on the ACS Publicationswebsite at DOI: 10.1021/acs.langmuir.5b00799.

■ AUTHOR INFORMATION

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

NotesThe authors declare no competing financial interest.

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