Journal of Colloid and Interface Science 434 (2014) 141–151

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The wettability of PTFE and glass surfaces by nanofluids Rajib Ghosh Chaudhuri, Santanu Paria ⇑ Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela 769008, Orissa, India

a r t i c l e

i n f o

Article history: Received 22 April 2014 Accepted 29 July 2014 Available online 9 August 2014 Keywords: Nanofluids Wetting Disjoining pressure Contact angle Adhesion tension

a b s t r a c t Wetting of solid surfaces by surfactant solutions is well focused in the literature compared that of nanofluids. Similar to the surfactant solutions nanofluids are also able to reduce the surface tension as well as influence on contact angle at the solid, liquid and gas interface. The surface tension and wettability of two different nanofluids containing hydrophilic (TiO2) and hydrophobic (S) particles have been experimentally studied here. The surface tension reduction of nanofluids strongly depends on material property, particle size and as well as concentration. These parameters also influence the change in contact angle on both hydrophilic (glass) and hydrophobic (PTFE) surfaces. Three important factors such as surface tension, surface hydrophobicity after deposition of particles on a solid surface, and the disjoining pressure influence the final contact angle of nanofluids on a solid surface. Sulfur nanofluids show maximum enhancement in contact angle (30.6°) on the glass surface; on the other hand TiO2 nanofluids show maximum reductions in surface tension (25.4 mN/m) and contact angle on the PTFE surface (17.7°) with respect to pure water. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Nanofluids are suspensions (colloidal state) of nanomaterials in a base liquid. Nanofluids have drawn attention of researchers initially because of their enhanced thermo-physical properties (thermal conductivity, thermal diffusivity). In addition to their thermal applications in industrial and nuclear reactor cooling, cooling in electronics, heavy engines transportation [1,2], the nanofluids have also potential applications in biomedical (magnetic or ferrofluids in drug delivery, MRI contrast), magneto-optical wavelength filter, antibacterial activity, optical modulators, nonlinear optical materials, ink jet printing, soil remediation, oily soil removal, lubrication and enhanced oil recovery, surface coating, wetting and surface cleaning, energy storage and so on [1–6]. During the past few years, the researchers’ attention as well as the publications on nanofluids increasing exponentially because of these exciting applications. Similarly, the wetting of solid surfaces by liquids is also of immense interest towards a broad research community from the past few decades surely because of the practical, and scientific importance [7–9]. Wettability of the flat solid surfaces by pure liquids or surfactant solutions is a complex phenomenon; it depends on the movement of the triple line, where the three phases are in mutual contact; which in turn depends on physical

⇑ Corresponding author. Fax: +91 661 246 2999. E-mail addresses: [email protected], [email protected] (S. Paria). http://dx.doi.org/10.1016/j.jcis.2014.07.044 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

properties of a solid (homogeneity, roughness, surface energy) as well as liquid (surface tension, polarity, viscosity). But the wettability of solid surfaces by nanofluids is a more complex process because of the presence of particles, where the well-established theories of wetting by pure liquids or solutions are insufficient to explain the observations [10,11]. In this case several additional factors such as particle size, concentration at the triple line, particles solid, particles fluid, particles particles interactions are also equally important in addition to the common factors for pure liquids or solutions. The detail explanations of individual factors are mentioned in result and discussion section. There are numerous studies available on the wetting behavior of surfactant solutions on solid surfaces [8,9,12,13], however, limited on nanofluids [14–16]. Liquid surface tension plays an important role in the wetting process, in general, lower surface tension liquid favors the wetting of low surface energy or hydrophobic surfaces. Similar to surfactants, addition of nanoparticles also can reduce surface tension as well as influenced the movement of the triple line because of strong attachment of the particles at fluid–fluid interface, whether the particles are hydrophobic or hydrophilic [17,18]. But these interfacial properties of nanoparticles suspension or nanofluids greatly depend on the material property, size, shape, and the concentration of the particles [16]. The literature available on wetting of nanofluids can be broadly classified into three categories: (i) bubble growth or dynamics on solid surface inside the liquid [19,20], (ii) removal of oil droplets from a solid surface [10], (iii)

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wetting of solid surface [15,16,21,22]. Vafaei and Wen [19,20,23] reported the extent of surface wettability of a solid–liquid–gas system which is dependent on the material property, size and concentration of the particles. More specifically, a unique pinning behavior of the triple line was observed when bubbles formed on a metal surface inside the gold nanofluids compared to that of pure water, whereas a spreading of triple line was observed for bubbles forming inside alumina nanofluids for a constant bubble volume [20]. The nanofluids containing alumina, zirconia, and silica have shown a significant enhancement in critical heat flux (CHF) because of improvement in the wettability of the fluid on the solid surface [24,25]. The pool boiling characteristic of nanofluids as a heat transfer fluid, highly dependent on both particle concentration and fluid/surface wetting characteristics [20]. The removal of oil drops from the solid surface by nanofluids is another important application. Where the nanoparticles are deposited near the triple line and generate an excess structural disjoining pressure which favors removal of oil droplets from the surface. Most of the studies in this field are theoretical based but some experimental papers are also available by different research groups [10,14,26]. Wasan and coworkers are the pioneer in this field [10,14,26–29], however gradually some other research groups have also started working in this area [14–16,30]. Finally, the studies on wettability of solid flat surfaces by nanofluids are also limited [15,16,21,31]. Vafaei et al. [15,16] studied the wetting of a solid surface by the nanofluids which containing Bi2Te3 particles on different hydrophilic surfaces (glass and silicon wafer) and their results show the contact angle depends on both particle size and concentration. They used 2.5–10.4 nm particles functionalized with thioglycolic acid and studied the contact angle and surface tension of the suspensions. According to their results, for a particular particle size, the contact angle initially increases with increasing particle concentration and attains a maximum for both surfaces. Further, at a constant particle concentration lower size particle is more efficient to increase the contact angle. It can be seen from the literature that until now most of the studies on the wetting properties of nanofluids are theoretical based and only limited experimental studies are available [15,17]. The available experimental studies are mostly in the presence of capping agents or functionalized the particles with some molecules. Herein, we studied the effect of possible different parameters of nanofluids on the wetting properties of the solid surfaces and surface tension. A detailed knowledge about the mechanism of increase or decrease the wetting of hydrophobic or hydrophilic surfaces by these nanofluids could be useful in various applications such as paint, antimicrobial or antifungal agents, energy storage, heat transfer fluids, or any other thermal applications. Based on the applications nanofluids of different materials are used. Especially TiO2 nanofluis are widely used with other nanofluids (Al2O3, Fe2O3, graphene or Fe) to enhanced the heat transfer and used as nanorefrigerant [32], used in enhanced oil recovery [33]. Similarly, sulfur (S) based nanofluid is used and antimicrobial or anti fungal agents, in a recent study it has been reported that S nanofluids can be as used a green pesticide for agricultural applications [34], where wetting is also an important issue. So, aiming to these applications this study reports the wetting behavior of nanofluids containing hydrophobic (S) and hydrophilic (TiO2) particles in the absence of any capping agents on both hydrophobic (PTFE) and hydrophilic (Glass) surfaces. The particles were synthesized in situ in aqueous media without any dispersing agent. Effects of various parameters on wettability and surface tension, such as particle size, particle concentration, and material property were studied which may have lots of practical importance as well as academic interest. To the best of our knowledge similar studies have not been reported.

2. Experimental Section 2.1. Materials The required chemicals used for this study were taken from the following companies: sodium thiosulphate (Na2S2O35H2O) from Rankem (99.5% assay), nitric acid (HNO3) from Merck (69% assay), and Tetrabutyl ortho titanate (TBOT) from Sigma Aldrich (97% assay). All the chemicals were used as those were received without any further purification. Ultrapure water of 18.2 MX cm resistivity 71.5 mN/m surface tension, and 6.4–6.5 pH was used for all the experiments. The constant temperature 28 ± 0.5 °C was maintained throughout the experiments. 2.2. Methods 2.2.1. Particles synthesis Sulfur nanoparticles was synthesized from HNO3 catalyzed reaction of sodium thiosulphate in aqueous media according to our previous study [35]. Both precursors sodium thiosulphate and HNO3 were filtered through 0.2 lm nylon 6, 6 membrane filter paper (from Pall Corporation, USA). The stock Ti(OC4H9)4 solution was prepared in anhydrous ethanol. TiO2 particles were synthesized by the acid catalyzed (HNO3) sol–gel method. In this reaction first Ti(OC4H9)4 was hydrolyzed to Ti(OH)4 in the presence of acid, then Ti(OH)4 was polymerized and condensed to TiO2 according to the following reactions.

nTiðOC4 H9 Þ4 þ 4nH2 O ! ½TiðOHÞ4 n þ 4nC4 H9 OH

ð1Þ

½TiðOHÞ4 n ! nTiO2 þ 2nH2 O

ð2Þ

In both cases, reactants were added under continuous mixing, and the solution was kept for 1 h to complete the reactions and particle sizes were measured immediately by dynamic light scattering (DLS) method after 10 min sonication in a bath sonicatior. 2.2.2. Particle and solid surface characterization Particle size and zeta potential measurement were carried out by DLS using a Malvern Zeta Size analyzer, (Nano ZS) where size was measured with the help of cumulant fitting model and intensity based size distribution within the media; whereas, zeta potential was measured by using Smoluchowski model. The size, shape, and phase of the particles were characterized by the help of scanning electron microscope (JEOL, JSM-6480LV) and X-ray diffraction (XRD) (Philips, PW 1830 HT). Roughness of both solid surfaces (glass and PTFE) was characterized using atomic force microscopy (Veeco). 2.2.3. Surface tension and contact angle measurements The particles were synthesized with the increasing reactants (thiosulphate and TBOT) concentrations and after the particle formation each suspension was diluted to the desired concentration with water. Then the suspension was sonicated for 10 min in a water bath sonicator and surface tension and contact angle were measured immediately by using surface tensiometer, (Data Physics, DCAT-11EC) and video based contact angle meter (Data Physics, OCA-20) respectively. The surface tension was measured by the Wilhelmy plate technique. The contact angle was measured by the sessile drop technique with 4 ll drop volume. Before each measurement both platinum plate and solid surfaces (glass and PTFE) were dipped in an ultrasonic cleaning bath for 15 min, then washed thoroughly using water and acetone and finally dried blowing hot air. To get better repeatability quality of solid surfaces were also cheeked in terms of contact angle by the pure water time to time and if required plates were changed after few measurements.

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3. Results and discussion 3.1. Synthesis and characterization of sulfur and TiO2 nanoparticles Before studying the wetting properties of nanofluids containing sulfur and TiO2 particles on the glass and PTFE surfaces, the particles were characterized by DLS, SEM, and XRD to confirm the particle size, shape, and phases respectively. Fig. 1 shows the effect of reactant concentration on the sizes of sulfur and TiO2 nanoparticles. The figure depicts for both the cases particle size increases sharply with the increasing reactant concentration. The sulfur particles used for this study were in the range of 158–860 nm synthesized using 0.5–5 mM sodium thiosulphate concentrations, respectively, beyond that concentration the size was too big for a stable suspension, which was separated out from the media within a short time. Sulfur particles are hydrophobic in nature having very low zeta potential value (2.17 mV) in aqueous reaction media as reported before [35]. The TiO2 particles were synthesized by acid catalyzed (HNO3) sol–gel reaction of TBOT, where TBOT reacts with water. Since the reaction was carried out in aqueous media by the addition of the alcoholic TBOT solution, therefore, the particle size totally depends on the TBOT concentration, which increases with increasing TBOT concentration. The particle size increases from 135.9 ± 10.6 nm (at 0.01 mM TBOT concentration) to a plateau value of 380 ± 16.4 nm (at 0.5 mM or above TBOT). While comparing both particles, it is found that the size increasing trend for the TiO2 particle is different from the sulfur particles. The particle size increases with increasing concentration of both particles, however at a low reactant concentration the rate of increase is very fast for TiO2 (slope of linear portion = 767.5) than the sulfur (slope of linear portion = 459.3) particles and finally attain a plateau level; on the other hand sulfur particle size increases gradually with the increasing reactant concentration. The plateau level particle size for TiO2 is lower compared to sulfur at higher reactant concentrations because of the higher zeta potential value (30.4 mV) in aqueous media. The final particle size depends on the growth rate, which again depends on the diffusion of newly born nuclei or small sized particles from the bulk to the particle surface as the reaction rate is very fast for both cases. The growth process favors energetically because of the lowering of total surface energy, because of that particle size increases. For the hydrophilic particles, this will continue till the electrical repulsive force between two particles is less than the gain in surface energy. However, in general, the final particle size depends on several factors including the material type. Since sulfur is hydrophobic material, at low reactant concentration diffusion rate would be lower than the hydrophilic material, ultimately leads to lower particle size till a certain concentration.

Fig. 2 shows the SEM images of sulfur and TiO2 particles. Fig. 2(a) indicates the sulfur particles are mostly spherical in shape and the sizes are close to that with the average size obtained by DLS measurements (387.5 ± 18.4 nm). Fig. 2(c) shows the micrograph of TiO2 nanoparticles, the particles are in a spherical shape with an average size of 135 ± 25.7 nm confirmed by DLS measurements. The EDAX analysis of the particles are shown in Fig. 2(b) and (d) respectively, for sulfur and TiO2 particles and these results show 46.56 and 16.59 atomic% of sulfur and Ti are presented in individual samples. The polydispersity index data from the DLS analysis are 0.736 and 0.517 for sulfur and TiO2 respectively, which indicate the TiO2 particles are quite monodisperse compared to that of sulfur particles. The particle size distribution plot (Fig. 2e) of sulfur and TiO2 particles obtained from the DLS data at the same concentration as in SEM analysis (1 mM thiosulphate concentration for sulfur and 0.01 mM TBOT concentration for TiO2) shows the size distribution of sulfur particles is comparatively much wider than TiO2 particles (also confirmed from the polydispersity values), that mainly because of the lower zeta potential value of sulfur in aqueous media. In fact, in both cases agglomeration is an important factor to control the particle size, as the zeta potential of the S particles is comparatively lower than that of TiO2 particles, the agglomeration after growth process is also more for S particles. Fig. 2(e) also shows wide distribution peak at higher particle size for sulfur particles. Fig. 3 shows the XRD patterns of sulfur and TiO2 particles. The position and intensities of the diffraction peak for both materials match with the literature data of orthorhombic or a-phase sulfur with S8 structure (JCPDS PDF Number: 74-1465) and anatase phase TiO2 (JCPDS PDF Number: 84-1286). In case of sulfur particles, sharp intensity peaks at 23.16°, 27.81°, and 28.76° angle (2h) clearly indicate the particles are highly crystalline in nature. However, in case of TiO2 particles peaks are not very sharp, which indicate the particles are less crystalline. The peaks at 25.33°, 48.04° are well matched with anatase TiO2 and an extra peak at 30.36° with the brookite phase TiO2 (JCPDS PDF Number: 76-1937). 3.2. Characterization of glass and PTFE surfaces

800

The solid surfaces were characterized by atomic force microscopy to see the surface irregularity or roughness and the results are shown in Fig. 4. Fig. 4(a) shows the surface topography of the glass plate used for this study. The surface roughness of the glass plate werecalculated from the AFM images using the SpmLab analysis software and it was found to be 1.02 nm. The Fig. 4(a) also shows the height profile along the z-axis through the lines and the position of the lines are shown in the insert figure. Similarly, Fig. 4(b) shows the surface irregularity of the PTFE sheet which was used for the study. It has been found that the roughness were comparatively more for PTFE sheet (22.7 nm) than the glass surface. Fig. 4(b) also shows the height profile along the line and the position of the lines are shown in the insert figure.

700

3.3. Surface tension of nanofluids

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Particle Size (nm)

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600 500

S TiO

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2

300 200 100

0

1

2

3

4

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6

Reactant Concentration (mM) Fig. 1. Variation of particles size of sulfur and TiO2 with the increasing respective reactant concentrations.

The interfacial property of aqueous nanofluids containing both particles in terms of surface tension was studied. The effect of particle size on the surface tension at a particular particle concentration (0.8 mg/l for TiO2, 0.32 and 16 mg/l for sulfur) is shown in Fig. 5(a). From the figure it is clear that in the presence of both particles surface tension decreases significantly compare to pure water. To test the interference of reaction mixture present in the particle suspension, surface tension of supernatant solutions after separating the particles were measured and found it was very close to pure water for both cases. The surface tension of sulfur nanofluids gradually decreases with the increasing particle size for

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Fig. 2. (a) SEM image of sulfur particles from 1 mM thiosulphate concentration. (b) EDAX analysis of sulfur particles. (c) SEM image of TiO2 particles from 0.01 mM TBOT concentration. (d) EDAX analysis of TiO2 particles. (e) Particles size distribution of sulfur and TiO2 particles under same conditions using DLS.

Counts Rate (cps)

200

TiO2

150 100 50

^

8000

S

6000

^ ^ ^ ^ ^ ^ ^^

4000 2000 0

20

25

30

35

40

2θ/

o

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50

55

60

Fig. 3. XRD pattern of sulfur and TiO2 particles. Inserted symbol represents the following phase Anatase TiO2 (⁄), Brookite TiO2 (#), Orthorhombic sulfur (^).

16 mg/l (0.5 mM) sulfur concentration, and finally decreases to a value 50.3 mN/m in the presence of 600 nm particles. In contrast, in the presence 0.32 mg/l (0.01 mM) sulfur particle concentration the trend in surface tension change is different, it attains a minimum surface tension value of 53.4 mN/m in the presence of small sized particles (158 nm) and then gradually increases with the increasing particle size. Similar behavior was also observed in case of TiO2 nanofluids; for 136 nm particles the surface tension decreases sharply to 47.1 mN/m; then gradually increases to 57.3 mN/m in the presence of 400 nm particles at a constant TiO2 concentration of 0.8 mg/l (0.01 mM). These facts can be qualitatively explained as follows, the small sized nanoparticles have a more Brownian motion and negligible gravitational force in suspension; as a result they are having mobility towards both air–water interface as well as bulk phase. However, for larger particle gravitational force gradually increases and mobility towards the interface decreases. The presence of particles at the air–water interface affects the cohesive force among the water molecules at the interface, and subsequently reduce the surface tension of nanofluids; the effect is similar in the presence of both hydrophobic and hydrophilic particles at the interface. Since the number density of the bigger sized particles to be present at the interface are less they behave comparatively hydrophilic (higher surface tension) than smaller sized particles of the same material [36] and gravitation

force also favors the larger particles to stay inside the bulk media; therefore, in the presence of TiO2 (0.8 mg/l) and sulfur particles with low concentration (0.32 mg/l) surface tension initially decreases sharply then increases gradually with the increasing particle size. In the presence of higher particle concentration (16 mg/l for sulfur) the increasing trend of surface tension was not observed with increasing particles size. This is attributed as the wide distribution of the particles at higher average particle size; while distribution is wide and concentration is also high, sufficient number of small sized particle would present at the interface to reduce surface tension. When the particles come to the interface, depending on material property and density they may be classified into two categories: (i) immersion (partially immersed into the liquid layer), (ii) flotation (floating at the interface). For both cases particles at the interface will experience an attractive capillary force [37]. The Fflotation (capillary force for flotation) is negligible when the particle size is smaller than a critical size (Particle radius (rp) 90° the particles can move easily to the edge of the three phase contact line, however, for 90° the particles may be easily reached to the three phase contact line. When the particle size is large the number of deposited particle layers will be less than that in the presence of small size particles, as the height of the liquid film layer of the vertex may not be sufficient to accommodate a multi layer. At the same time for smaller particles, because of the low gravitational force the probability of deposition of the particles on the surface is also less than the larger particles. Finally, it has been observed that the contact angle decreases gradually with the increasing particle size on the PTFE surface. In some reported studies, it has been reported that disjoining pressure becomes significant when the particle concentration is 20 vol% or more [26,43]. However, in this study it is essential to mention that the overall particle concentration was lower than 20 vol% but at the triple junction point it would be sufficiently high to show significant disjoining pressure because of the accumulation of particles, as evaporative flux of the sessile drop drives the particles towards drop edge. Additionally, salt presents in the reaction mixture, creates a concentration gradient within the drop during evaporation, which in turn helps to move and deposit the particles near to the triple line along with the salt and contribute in disjoining pressure. It is also noteworthy to mention that in our study particles are present inside the sessile drop, whereas, in the reported studies by Wasan and co-works [10,14,29] oil droplet was present inside the nanofluids. In that case, the external angle was considered and the angle was also small. As a result, the present study is quite different. The polydispersity of particles is also an important parameter for wetting of nanofluids. In general, ploydispersity reduce the wetting behavior than that of monodisperse particles [44]. Actually, with increasing polydispersity the particles cannot form an ordered structure, as a result the structural disjoining pressure decreases. In our case, since the smaller sized particles are also present, we are expecting the smaller sized particles can reach near to triple line and then may form multiple layer then the larger sized particles will organize far away from the triple line, where they touch the liquid film depending on size. The reduction of contact angle can also be supported by the reduction of surface tension in the presence of nanoparticles using Young’s equation. Additionally, when particles are deposited on the flat surface, the surface can be considered as nano-level roughness with higher roughness factor. The increase in roughness factor eventually helps to decrease in contact angle [24,25,45]. The particle size and concentration is also important in this case. In contrast to the PTFE surface, the contact angle increases on the glass surface for similar studies, although nanofluids show lower surface tension. Although excess pressure is developed within the fluids because of the presence of particles at the triple line which should favor the flow of the liquids by reducing the contact angle as discussed before. However, in this case as the contact angle increases, some opposing factor could be predominant over the driving force of disjoining pressure. The opposing force is mainly because of increasing hydrophobicity of the glass surface after deposition of the hydrophobic sulfur particles. The behavior is attributed to the similar phenomenon of increasing of contact angle on the glass surface in the presence of surfactant solutions [46–48]. After the initial sharp increase, the contact angle slowly increases with the increase in particle size and attains a maximum then there is a little decrease in the presence of 860 nm particles because of the excess structural disjoining pressure.

R. Ghosh Chaudhuri, S. Paria / Journal of Colloid and Interface Science 434 (2014) 141–151

times more [8]. So, the higher adsorption density of sulfur particles at glass–water interface than that of PTFE–water interface is the main reason for higher extent of contact angle change on the glass surface (30°) than the PTFE surface (7°) by the same nanofluid. Fig. 9(a) and (b) shows the effect of particles size and concentration on the work of adhesion [WA = c(1 + cos h)] on the PTFE and glass surfaces in aqueous sulfur suspension. Work of adhesion is the work applied to an interface to separate the phases. The work of adhesion is more if there is more attractive interaction between the phases. Fig. 9(a) shows the effect of particle size on the WA in the presence of a constant particle concentration (16 mg/l). The Figure clearly indicates for the glass surface WA decreases sharply in the presence of low sized particles then continued slowly and attains to a plateau value of 59.1 mJ/m2 in the presence of 860 nm sulfur particles; the results are consistent with the contact angle. In the case of the PTFE surface, WA also decreases with the increasing particle size, which is contradictory with the contact angle results; as contact angle decreases gradually, WA should increase. This is mainly because of the ranges of contact angle values, as all the contact angle values are above 90° (where, cos h = negative) and surface tension is decreasing, so ultimately WA also decreases. Fig. 9(b) shows the effect of particle concentration in the presence of fixed sized particles (860 nm) on WA for glass and PTFE surfaces. For the glass surface, at a low particle concentration the WA sharply decreases, then almost constant, whereas for the PTFE surface at low particle concentration (16.0–80.2 mg/l) the WA is almost constant, but at a high particle concentration (160.3 mg/l) it decreases because of increase in contact angle.

3.5. Wetting of PTFE and glass surfaces by TiO2 nanofluids Similar to sulfur nanofluids, concentration and size effects on the wetting of PTFE and glass surfaces using TiO2 nanofluids were 45

PTFE Glass

100 90 80

35

70 60 0

200

400

600

800

50

Particle Size (nm) 42

y (Glass) = -76.56 + 1.72x R2= 0.967

50

(b)

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30 20

120 110

2

2

-30

A

PTFE Glass

LG

-20

γ cosθ (Glass)

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W , PTFE (mJ/m )

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-15

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y (PTFE) = 19.25 - 0.70x R2= 0.976

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WA, Glass (mJ/m2)

The effect of increasing particle concentration in the presence of a fixed particle size (860 nm) shows the contact angle decreases for the PTFE and increases for the glass surfaces, similar to that of the size effect. This effect is attributed to similar to that of surfactant concentration, in the presence of low particle concentration the chances of monolayer formation with less dense structure is more; however, with the increasing particle concentration particles adsorption density increases even at higher particle concentration chances of multilayer formation is also more. The results show with the increasing particle concentration the contact angle decreases because of increasing particle adsorption density on the solid surface. When the particles are close enough to form a monolayer, they are more stable because of the increase in particle–particle van der Waals force of attraction, as the particle–particle distance of separation decreases, which in turn leads to increase in disjoining pressure. Additionally, in the contact angle reduction process, a roughness factor because of the deposition of particles also plays a positive role as mentioned before. The contact angle again increases above 80.2 mg/l concentration, which could be because of the formation of multilayer, which is again related to disjoining pressure. For the glass surface, maximum contact shows at low particle concentration, and then there is a decreasing trend, but the lowering is more prominent at higher particle concentration. As adsorption density depends on particle concentration, we believe 16 mg/l particle concentration is sufficient to form the monolayer and shows highest contact angle, above that concentration the contact angle decreases gradually. Unlike the PTFE surface contact angle is less than 90° for the glass surface, so at higher particle concentration particle–particle interaction in the bulk phase is more, in this situation particles cannot reach a very close distance to the triple point at the vertex; that may be the probable reason of decreasing contact angle. Now comparing both surface tension and contact angle results it is observed that small sized particles are more effective to reduce the surface tension whereas large sized particles are responsible to change the contact angle. The wetting behavior can also be explained in terms of adhesion tension (cLG cos h); it depends on both surface tension and contact angle. Similar to the surfactant solutions, in case of nanofluids a linear relation between adhesion tension and surface tension is observed for both surfaces. Fig. 8 shows the adhesion tension vs. the surface tension plot for PTFE and glass surfaces using sulfur nanofluid. For both surfaces, linear relationships are observed,   cos h and the numerical value of the slope dcLG is higher for the dcLG glass surface than that of PTFE surface. It has been found that the slope for the PTFE surface is 1 (1.72). These results clearly indicate surface excess of particles at PTFE–water interface is lower (0.70 times) than that of air–water interface; however at glass–water interface it is 1.72

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38

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30 50

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Fig. 8. Plot of adhesion tension (cLG cos h) vs. surface tension (cLG) on glass and PTFE surfaces for sulfur nanofluids (irrespective of particle size and concentration).

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Particle Concentration (mg/l) Fig. 9. Effect of sulfur particle size (a) and particle concentration (b) on work of adhesion (WA) at glass and PTFE surfaces.

R. Ghosh Chaudhuri, S. Paria / Journal of Colloid and Interface Science 434 (2014) 141–151

studied as shown in Fig. 10(a) and (b). Fig. 10(a) shows the effect of particle size on the contact angle at a fixed particle concentration (0.8 mg/l) on PTFE and glass surfaces. In case of PTFE surface, the contact angle continuously decreases to 104.5° in the presence of 381.5 nm particles and after that it is almost constant. For TiO2 nanofluids the final decrease in contact angle is 4°more compared to that of sulfur; although interaction between PTFE surface and TiO2 particle is expected to be less because of the hydrophobic nature of PTFE surface, still lowering of contact is mostly because of the increase in hydrophilicity of the TiO2 deposited PTFE surface. Whereas, on the glass surface the change in contact angle does not follow a particular trend, the contact angle initially increases sharply to 65.3° in the presence of 135 nm TiO2 particles then it increases slowly and attains a maximum (69.2°) in the presence of 240 nm particles; but after that it again decreases to 52.4° in the presence of 400 nm sized particles. When TiO2 particles are deposited on the glass surface near to the triple line, there will be an excess disjoining pressure, but at the same time, similar to the PTFE surface the surface hydrophobicity will also change because of particle deposition on the surface. In this case as TiO2 is less hydrophilic than glass, the surface hydrophilicity dominates over the disjoining pressure; as a result the contact angle initially increases with increasing particle size. In the presence of 240 nm particles the contact angle reaches to 69.2°, which is close to the contact angle of pure water on a pure solid TiO2 surface (72–74°) [49,50]. The contact angle value also depends on the phase and as well as the crystal plane of TiO2 [49]. So, the result indicates that in the presence of 240 nm particles TiO2 may form an uniform layer on the glass surface so that contact angle increases and wetting property decreases. When the particle size further increases the larger particles may not be able to reach a close distance of the triple line of the vertex, as a result, the disjoining pressure again dominates as a controlling factor to reduce the contact angle.

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Particle Concentration (mg/l) Fig. 10. The variation of contact angle on PTFE and glass surfaces by TiO2 nanofluids: (a) effect of particle size and (b) effect of particle concentration.

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Similar to the PTFE surface, the maximum change in contact angle was observed in the presence of sulfur nanofluids (77.4°) compare to TiO2 nanofluids (69.2°) because of the more hydrophobic nature of sulfur materials. The optical images of sessile drops of TiO2 nanofluid containing 380 nm sized particles at 0.8 mg/l concentration on PTFE and glass surfaces are shown in (c) and (d) parts of Fig. 6 to show the spreading behavior. The Fig. 10(b) shows the particle concentration dependent contact angle on both PTFE and glass surfaces in the presence of 240 nm sized TiO2 particles. Similar to sulfur nanofluids the contact angle decreases slowly with the increasing particles concentration and attains a value of 97.8° at 20 mg/l concentration on the PTFE surface continuing the decreasing trend without saturation. This continuous decreasing trend of contact angle with the increasing particle concentration is attributed as follows. As TiO2 particle size is smaller and surface charge (f = 30.4 mV) is also higher than sulfur, particle–particle van der Waals attractive force reduces because of the electrostatic repulsive force, as a result, particle density at the surface decreases. In this condition, probably the required particle concentration is more to form a complete monolayer on the surface. Additionally, as the contact angle is always greater than 90° particles may also form multilayer at higher concentration without any confinement of the particles at the edge of the vertex. Whereas in case of the glass surface similar to size affect the contact angle do not follow a particular trend. The contact angle initially increases sharply to 64.3° in the presence of 0.8 mg/l particle concentration, then it starts decrease and reaches a minimum value of 46.8° at 8 mg/l concentration and beyond that again it increases to 66.1° at 20 mg/l concentration. The initial increase in contact angle may be because of the increasing hydrophobicity after the scattered deposition of particles on the glass surface. In the next step of decreasing trend, when the particle concentration increases deposited particles may form an uniform monolayer near to the triple line where disjoining pressure may predominant. In the third step of increasing contact angle, at further higher particle concentration chances of multilayer formation is more; possibly hydrophobicity dominates over the disjoining pressure there to get the final contact angle. Considering the effect of both S and TiO2 nanoparticles on both PTFE and glass surfaces, it is clear that for the hydrophobic surface the disjoining pressure gradient is more important to control the wetting, whereas for the hydrophilic glass surface the particle-solid van der Waals interaction is more important for final contact angle. In the case of the PTFE surface, the maximum decrease in contact angle occurs by TiO2 nanofluids because of higher hydrophilicity of the TiO2 particles than the S, whereas for hydrophilic glass surface maximum increase occurs in the presence of sulfur nanofluids because of its higher hydrophobicity than TiO2. Similar to sulfur nanofluids, in case of TiO2 nanofluid a linear relationship is also observed (as shown in Fig. 11) between adhesion tension and surface tension, but with different slope and intercept values than that of sulfur nanofluids. In case of TiO2 nanofluids also the slope for glass surface (1.37) is higher than that of PTFE surface (0.86), which in turn leads to more change in contact angle on the glass surface (22°) than the PTFE surface (18°) because of similar reasons. Interestingly, while comparing both nanofluids it has been observed that the adsorption density of sulfur particles is more on glass surface, and it just reverse for TiO2. The behavior can be explained in terms of surface charge, as the glass surface and the TiO2 nanoparticles both are negatively charged, adsorption density on the glass surface is lower because of the electrostatic repulsive force between the particles and surfaces. Fig. 12(a) and (b) shows the effects of particle size and concentration on the work of adhesion (WA) at PTFE and glass surfaces with the TiO2 nanofluids. From the Fig. 12 it can be seen that for both surfaces the change of WA follows a similar trend, but the

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ranges are different. In the case of the size effect, WA decreases first and in the second regime increases continuously. The Fig. 12(a) shows at a fixed particle concentration (0.8 mg/l) with the increasing size WA reaches a minimum value of 32.4 and 66.8 mJ/m2 respectively, for PTFE and glass surfaces at 135 nm particle size, then in the second regime WA increases gradually with increasing particle size. In the case of the PTFE surface, the initial decrease in WA is not consistent with the contact angle results; as the contact angle value decreases throughout the particle size range. This fact is simply because of the low surface tension value obtained at that particular particle size. For the glass surface, a change in WA values is mostly consistent with the contact angle results. The Fig. 12(b) shows with the increasing particle concentration using 240 nm sized particles the WA changes into three regimes; decreasing, increasing, and again decreasing respectively on both PTFE and glass surfaces. In the case of the PTFE surface, at a low particle concentration the WA initially slightly decreases, but at the higher particle concentration the WA increases to a maximum value 140 PTFE Glass

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50.5 mJ/m2 at 8 mg/l particle concentration then it again decreases. These changes are mainly because of the combined effect of the change in surface tension and contact angle. Similarly, for the glass surface, at the low particle concentration the WA is sharply decreases to 75.4 mJ/m2 at 0.8 mg/l particle concentration because of the increase in contact angle as well decrease in surface tension, then further increase in particles concentration the WA increases to 105.1 mJ/m2 at 12 mg/l particle concentration as both factors (decreasing contact angle and increasing surface tension) are favorable to increase the work of adhesion. In the third regime, at very high particle concentration (20 mg/l) the WA again decreases to a value 69.2 mJ/m2 because of similar reasons.

The wettability of hydrophilic and hydrophobic surfaces by different nanofluids has been studied by the sessile drop technique. The observed results show the nanofluids can strongly influence the wettability of both solid surfaces. Nanofluids also show strong surface tension reduction ability depending on particle size, concentration, and material property similar to that of surfactant solutions. Herein, we studied surface tension of nanofluids containing hydrophilic and hydrophobic particles and their wetting properties on hydrophilic and hydrophobic surfaces to explain different behaviors. The results as obtain in this study can be summarized as, Surface tension gradually decreases with the decreasing particle size and TiO2 nanofluids show lower surface tension than that of sulfur. In the presence of minimum particle size (157 nm for sulfur and 135 nm for TiO2) surface tension of pure water reduces to 53.4 and 47.1 mN/m for sulfur and TiO2 nanofluids respectively. Nanofluids are able to change the contact angle on a solid surface because of the reduction in surface tension, as well as the disjoining pressure of thin liquid films. The contact angle on the PTFE surface is more influenced by TiO2 nanofluids than S nanofluids, the contact angle decreases to a minimum value (104.9°) in the presence of larger sized TiO2 particles (400 nm). Similarly, with the increasing particle concentration, a minimum contact angle of 97.8° was achieved in the presence of 20 mg/l particle concentration. Whereas, in case of the glass surface, S nanofluid is more effective to increase the contact angle to a maximum extent (77.4°) in the presence 600 nm particles. From the wettability studies of nanofluids it has been found that, in case of the PTFE surface, the structural disjoining pressure is important; whereas, for the glass surface the change in surface hydrophilicity because of the deposition of particles is most important.

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The financial supports from the Department of Science and Technology (DST) under Nanomission, New Delhi, India, Grant No. SR/S5/NM-04/2007, and CSIR, New Delhi, India, Grant No. 22(0527)/10/EMR-II, for this project are gratefully acknowledged.

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Particle Concentration (mg/l) Fig. 12. The variation of the WA on PTFE and glass surfaces by TiO2 nanofluids: (a) effect of particle size and (b) effect of particle concentration.

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