Nanoparticle Self-Structuring in a Nanofluid Film Spreading on a Solid

May 3, 2010 - Revised Manuscript Received April 22, 2010. Liquids containing ... thin liquid film, separating its two surfaces confining the nano- flu...
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Nanoparticle Self-Structuring in a Nanofluid Film Spreading on a Solid Surface Alex Nikolov, Kirti Kondiparty, and Darsh Wasan* Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 Received March 5, 2010. Revised Manuscript Received April 22, 2010 Liquids containing nanoparticles (nanofluids) exhibit different spreading or thinning behaviors on solids than liquids without nanoparticles. Previous experiments and theoretical investigations have demonstrated that the spreading of nanofluids on solid surfaces is enhanced compared to the spreading of base fluids without nanoparticles. However, the mechanisms for the observed enhancement in the spreading of nanofluids on solid substrates are not well understood. The complex nature of the interactions between the particles in the nanofluid and with the solid substrate alters the spreading dynamics [Wasan, D. T.; Nikolov, A. D. Nature 2003, 423, 156]. Here, we report, for the first time, the results of an experimental observation of nanoparticles self-structuring in a nanofluid film formed between an oil drop and a solid surface. Using a silica-nanoparticle aqueous suspension (with a nominal diameter of 19 nm and 10 vol %) and reflected light interferometry, we show the nanoparticle layering (i.e., stratification) phenomenon during film thinning on a smooth hydrophilic glass surface. Our experiments revealed that the film thickness stability on a solid substrate depends on the film size (i.e., the drop size). A film formed from a small drop (with a high capillary pressure) is thicker and contains more particle layers than a film formed from a large drop (with a lower capillary pressure). The data for the film-meniscus contact angle verses film thickness (corresponding to the different number of particle layers) were obtained and used to calculate the film structural energy isotherm. These results may provide a better understanding of the complex phenomena involved in the enhanced spreading of nanofluids on solid surfaces.

Introduction Uniformly sized spherical nanoparticles, or surfactant micelles, globular proteins, and macromolecules are known to form ordered microstructures (layering) between the two solid surfaces or in thin liquid films between bubbles or drops, such as those associated with foam and emulsion systems.1-14 These ordered microstructures exert the structural disjoining pressure (i.e., the excess pressure in a film relative to that in the bulk solution) in a thin liquid film, separating its two surfaces confining the nano*To whom correspondence should be addressed. E-mail: [email protected]. (1) Nikolov, A. D.; Wasan, D. T. J. Colloid Interface Sci. 1989, 133, 1. (2) Nikolov, A. D.; Wasan, D. T. Langmuir 1992, 8, 2985. (3) Lobo, L.; Wasan, D. T. Langmuir 1993, 9, 1668. (4) Sethumadhavan, G. N.; Nikolov, A.; Wasan, D. Langmuir 2001, 17, 2059. (5) Wasan, D. T.; Nikolov, A. D.; Chu, X. L. In Micelles, Microemulsions and Monolayers, Science and Technology; Shah, D. O., Ed.; Marcel Dekker, Inc.: New York, 1998; pp 124-144. (6) Parker, J. L.; Richetti, P.; Kekicheff, P.; Sarman, S. Phys. Rev. Lett. 1992, 68, 1955. (7) Bergeron, V. J. Phys.: Condens. Matter 1999, 11, R215. (8) Kleinschmidt, F.; Stubenrauch, C.; Delacotte, J.; von Klitzing, R.; Langevin, D. J. Phys. Chem. B 2009, 113, 3972. (9) Wasan, D.; Nikolov, A. Curr. Opin. Colloid Interface Sci. 2008, 13, 128. (10) Wasan, D.; Nikolov, A.; Henderson, D.; Trokhymchuk, A. In Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed.; Marcel Dekker, Inc.: New York, 2002, p 1181-1192. (11) Klitzing, R. Adv. Colloid Interface Sci. 2005, 114-115, 253. (12) Basheva, E. S.; Nikolov, A. D.; Kralchevsky, P. A.; Ivanov, I. B.; Wasan, D. T. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York, 1991; Vol. 11, p 467. (13) Basheva, E. S.; Danov, K. D.; Kralchevsky, P. A. Langmuir 1997, 13, 4342. (14) Wasan, D.; Nikolov, A.; Henderson, D. AIChE J. 2003, 49, 550. (15) Chu, X. L.; Nikolov, A. D.; Wasan, D. T. J. Chem. Phys. 1995, 103, 6653. (16) Trokhymchuk, A.; Henderson, D.; Nikolov, A.; Wasan, D. T. Langmuir 2001, 17, 4940. (17) Wasan, D. T.; Nikolov, A.; Trokhymchuk, A.; Henderson, D. Condens. Matter Phys. 2001, 4, 361. (18) Stubenrauch, C.; von Klitzing, R. J. Phys.: Condens. Matter 2003, 15, R1197. (19) Basheva, E. S.; Kralchevsky, P. A.; Danov, K. D.; Ananthapadmanabhan, P.; Lips, A. Phys. Chem. Chem. Phys. 2007, 9, 5183.

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fluid, thereby imparting stability to these systems.15-22 The structural disjoining pressure has an oscillatory exponential decay with the increasing film thickness (gap), with both the period of oscillation and decay factor equal to the average effective diameter of the nanoparticles.15-17,19 The structural disjoining pressure dominates on scales larger than the effective diameter of a nanoparticle, below which other disjoining pressure components (such as van der Waals, electrostatic and solvation forces) are prevalent.23,24 Recent experiments have revealed that ordered structures also form near the three-phase contact line (wetting wedge) of a drop or bubble on a solid surface, which promotes the spreading of liquids containing nanoparticles (nanofluids).25-30 Understanding the complex nature of the interactions between the particles in the nanofluid and with the solid substrate is critical to our comprehension of the enhanced spreading behavior of nanofluids on solids under the action of the structural disjoining pressure. Using the combined differential and common reflected light interferometric methods, we investigated the complex mechanism (20) Christov, N. C.; Danov, K. D.; Zeng, Y.; Kralchevsky, P. A.; von Klitzing, R. Langmuir 2010, 26, 915. (21) Vesaratchanon, J.; Nikolov, A.; Wasan, D. T. Ind. Eng. Chem. Res. 2009, 48, 80. (22) Gouin, H. Int. J. Eng. Sci. 2009, 47, 691. (23) Kralchevsky, P. A.; Danov, K. D.; Denkov, N. In Handbook of Surface and Colloid Chemistry; Birdi, K. S., Ed.; CRC Press: 2002; pp 4-156. (24) Ruths, M.; Berman, A. D.; Israelachvili, J. N. In Nanotribology and Nanomechanics: An Introduction; Bhushan, B., Ed.; Springer: 2005; p 389. (25) Wasan, D. T.; Nikolov, A. D. Nature 2003, 423, 156. (26) Chengara, A.; Nikolov, A. D.; Wasan, D. T.; Trokhymchuk, A.; Henderson, D. J. Colloid Interface Sci. 2004, 280, 192. (27) Chengara, A. V.; Nikolov, A. D.; Wasan, D. T. Adv. Polym. Sci. 2008, 218, 117. (28) Matar, O. K.; Craster, R. V.; Sefiane, K. Phys. Rev. E 2007, 76, 056315. (29) Sefiane, K.; Skilling, J.; MacGillivray, J. Adv. Colloid Interface Sci. 2008, 138, 101. (30) Craster, R. V.; Matar, O. K.; Sefiane, K. Langmuir 2009, 25, 3601.

Published on Web 05/03/2010

DOI: 10.1021/la100928t

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Figure 1. (A) Schematic of the optical arrangement for studying the solid-nanofluid-oil interactions. (B) Photomicrograph depicting nanofluid film formation.

Figure 2. Sequence of micrographs depicting various stages of silica suspension aqueous film thinning of an oil droplet approaching a solid and the development of particle layering phenomena. (a) Film with dimple in the shape of a horseshoe and (b) film with four particle layers. The bright green color corresponds to a one particle layer. The green color corresponds to two particle layers, the dark green area at the central part corresponds to three particle layers, and black area corresponds to four particle layers. (c) With time, the black area with four particle layers diminishes due to stratification phenomena (layer-by-layer thinning) resulting in a film with three layers of particles.

involved in the solid-nanofluid-oil interactions by directly observing, for the first time, the phenomenon of nanoparticle self-layering (i.e., stratification) due to confinement of nanoparticles in a thin film. The effect of film size on stability of nanofluid films on a solid substrate was also studied.

Experiments and Results We studied the solid-oil interactions in the presence of a silica nanoparticle aqueous suspension (nanofluid), using a combined differential and common reflected light interferometry by observing the three-phase contact region.31-33 A sketch of the optical arrangement is shown in Figure 1. This technique was used in conjunction with a specially designed glass cell. The cell was designed to eliminate light reflection from the solid substrate (glass) and to observe nanoparticle self-structuring in the nanofluid film confined between an oil drop and the solid surface. In order to obtain high quality digital images, the objective of the microscope was submerged in an immersion oil with a refractive index matching that of the glass surface. The cell was placed on the stage of the Max Zhender differential interference microscope, (31) Nikolov, A. D.; Dimitrov, A. S.; Kralchevsky, P. A. Opt. Acta 1986, 33, 1359. (32) Dimitrov, A. S.; Kralchevsky, P. A.; Nikolov, A. D.; Wasan, D. T. Colloids Surf. 1990, 47, 299. (33) Nikolov, A. D.; Wasan, D. T. In Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC Press: New York, 1995; pp 209-214.

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which is mounted on a vibration-free table to keep any external disturbances from affecting the film thinning process. The lower part of the cell was filled with a 10 vol % aqueous silica suspension with a nominal diameter of 19 nm (a product of Johnson Matthey Electronics). A drop of canola oil expelled into the nanofluid in the cell approaches an optically smooth glass surface, forming a nanofluid film (96 μm) corresponding to a drop with an equatorial diameter of ∼300 μm. In the reflected light mode of the microscope, monochromatic light (wavelength 546 nm) through the top of the glass cell is incident on the film surface. As the nanofluid film between the oil drop and the glass surface thins, the film thickness changes, producing interference patterns. The video camera, in conjunction with the monitor and digital video recorder, records the process of film thinning. Film Stratification. Initially, the oil droplet separated by the aqueous nanofluid film rolls under the glass surface. It forms a dimple (seen as a dark fringe in the photomicrograph in Figure 2) in the shape of a horseshoe; this is produced by the reflected light using a differential microscope. The video clip in the Supporting Information shows the various stages of the thinning of the aqueous nanofluid film between the oil droplet and the glass surface. Figure 2 shows a sequence of photomicrographs depicting the development of the particle layering phenomenon during the film thinning process. Figure 3 shows a photomicrograph depicting four different particle structural transitions inside the 868 μm nanofluid film, corresponding to a drop size of 1.5 cm in equatorial diameter. The Langmuir 2010, 26(11), 7665–7670

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Figure 3. Photomicrograph depicting particle layering of 10 vol % aqueous silica suspension (19 nm particle size) on a solid surface (film size = 868 μm).

Figure 4. (A) Photomicrograph depicting the nanofluid film and the adjoining meniscus. (B) Schematic of differential interferometry in reflected light. (C) Interferogram depicting the film-meniscus profile.

dark green color corresponding to a film thickness of 152 nm contains three particle layers: a bright green color corresponding to a film thickness of 109 nm containing two particle layers, a green color for a film thickness of 66 nm with one particle layer, and a dark color for a film thickness of 23 nm without any particles inside it. Our experiments revealed that the nanofluid film thickness stability (i.e., the film’s ability to spontaneously decrease its thickness corresponding to fewer particle layers) on a solid substrate depends on the film size (i.e., drop size). For example, a small oil drop with an equatorial diameter of 456 μm with a film size of 152 μm and with an equilibrium thickness of 66 nm contains one layer of particles (Figure 4). When the droplet size increases, the film size also increases, and the nanofluid film remains at the equilibrium thickness with no nanoparticles inside it. For example, the photomicrograph in Figure 3 shows that the film with a diameter of 868 μm, which corresponds to a drop size of 1.5 cm in equatorial diameter, remains stable with no particles in it. Our experimental observations clearly show that the equilibrium film thickness of small films formed by small droplets (with a higher capillary pressure) are thicker, stable and contain more particle layers than the larger films formed by large drops (with a lower capillary pressure). Langmuir 2010, 26(11), 7665–7670

Film-Meniscus Profile. We employed combined differential and common-reflected light interferometry for the simultaneous monitoring of the film-meniscus profile, the three-phase contact angle dynamics, and the wetting film thickness when an oil drop dispersed in an aqueous nanofluid approaches a smooth, horizontal hydrophilic glass surface. The differential light interferometric technique is particularly suited for measuring the film thickness profile in turbid and nontransparent liquids, and in highly curved film-meniscus surfaces at both smooth and rough solid surfaces. In interferometry using reflected light, each fringe is created by the interference of two beams, reflected by the two surfaces of the film-meniscus region between the glass-nanofluid and the oil-nanofluid (Figure 4B). The photomicrograph in Figure 4A shows the nanofluid film formed between the oil drop and the glass surface surrounded by the meniscus, indicated by the consecutive dark and bright Newton interference rings around the periphery of the film. The interferogram depicting the film-meniscus profile is shown in Figure 4C. The successive maxima and minima in the intensity of reflected light represent a change in the meniscus region thickness. The film has a uniform thickness (∼66 nm); moving away from the film area, the thickness gradually increases DOI: 10.1021/la100928t

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Figure 5. Profile of the nanofluid film-meniscus region as probed by differential interferometric method.

in the meniscus region, as indicated by the intensity of reflected light changing from bright to dark. This corresponds to the local constructive and destructive interference. The film-meniscus thickness, h, is calculated from the following relation: h ¼

kλ 4nref

ð1Þ

where λ is the wavelength of light, nref is the refractive index, and k is the order of the interference. k is even for the dark fringes and odd for the bright ones. The first bright interference pattern (Figure 4A) appears at a meniscus thickness of 102 nm, and the first dark one at a meniscus thickness of 204 nm (Figure 4C). The variation of the light intensity across the digitized photomicrograph (Figure 4A) of the interference patterns is scanned using ImagePro Software. The spacing between the interference patterns provides the information about the X coordinate (i.e., distance along the meniscus) and the order of interference provides the information about the Y coordinates (i.e., meniscus thickness). For calculating the meniscus thickness, the refractive index of the silica nanofluid measured by the refractometer at a wavelength of 546 nm was found to be 1.338 at 25 C. Figure 5 shows the film-meniscus region profile where the experimental data are marked with arrows corresponding to the maxima and minima in the interferogram. We fitted the data with a second order polynomial (with a regression coefficient of 0.998). The intercept of the fitted curve with the Y axis gives the film thickness (66 nm). The difference between a film thickness corresponding to one- and two-particlelayer thick films was found to be about 43 nm. Thus, the effective diameter of the nanoparticles was deduced to be about 43 nm. The meniscus capillary pressure, P, is related to the local curvature of the meniscus profile and the oil-nanofluid interfacial tension, σ, by the equation: Pmin ¼

2σ Rmin

ð2Þ

where Rmin is the local radius of curvature and can be calculated from the local slope of the profile by the following expression: 1 Rmin

d2 h dx2 ¼"  2 #3=2 dh 1þ dx

ð3Þ

where h is the local meniscus thickness. The slope, dh/dx (the change in the meniscus thickness), can be measured with an accuracy better than 10% ((5 nm). 7668 DOI: 10.1021/la100928t

The interfacial tension for canola oil pre-equilibrated with the nanofluid was determined (0.5 mN/m) by using the drop shape analysis method and the apex radius of the curvature of the drop by fitting the drop profile with the Laplace equation. Table 1 lists the capillary pressure together with the thickness of the film-meniscus region corresponding to one particle layer and thin film with no particles. Film-Meniscus Microscopic Contact Angle. The contact angle, θeq, is the angle subtended between the film and the meniscus and can be obtained from the local slope of the meniscus profiles, which is experimentally determined using light interferometry at the point of intersection with the nanofluid film (with an accuracy of 10%, e.g., 0.4 when the film contact angle is 5). Figure 6 shows the experimental values of the film-meniscus microscopic contact angle versus film thickness corresponding to the number of particle layers. The photomicrographs of the interference patterns corresponding to the nanofluid films containing three, one, and no particle layers are also shown in this figure. The distance between the interference patterns for a film without any particles is small, while the interference patterns are relatively widely spaced for a thicker film corresponding to three particle layers. This indicates that the meniscus profile is relatively steep with a large contact angle for a thin film compared to a thicker film, which has a smaller contact angle. Figure 6 shows a decrease in the film-meniscus microscopic contact angle with the increasing film thickness. Structural Interaction Energy between Film Surfaces. The film-meniscus microscopic contact angle is related to the disjoining pressure, given by the Frumkin-Derjaguin equation:34-44 Z ¥ σo=l ðcos θeq - 1Þ ¼ Π0 h0 þ ΠðhÞ dh ð4Þ h0

where σ is the interfacial tension between the oil (o) and the nanofluid (l), h0 is the equilibrium film thickness, Π0 is represented by the sum of the capillary pressure (Pc) and hydrostatic pressure, and Π is the disjoining pressure (represented by three major terms: Π = Πvw þ Πd þ Πst); Πvw represents the short-range van der Waals force, Πd accounts for forces which are electrostatic or stearic in nature, and Πst represents the long-range structural forces arising from the ordering of particles in nanofluids in the film-meniscus region (Figure 3). The second term on the right side of eq 4, which is the integral of the disjoining pressure, is the interaction energy between the film surfaces, Wst(h): Z Wst ðhÞ ¼

¥ h0

Πst ðhÞ dh

ð5Þ

Therefore, the film energy can be calculated from the measured values of the contact angle versus film thickness using the Frumkin-Derjaguin eq 4. It can be deduced from eq 4 that the thinner the film, the larger the contact angle, and this is what we observed experimentally (Figure 6) using our combined differential and common reflected light interferometric method. (34) Derjaguin, B. V. Theory of Stability of Colloids and Thin Films; Johnson, R. K., Ed.; Consultants Bureau: New York, 1989. (35) Churaev, N. V. Liquid and Vapor Flows in Porous Bodies: Surface Phenomena; Galwey, A., Ed.; Overseas Publishers Association: Amsterdam, 2000. (36) Churaev, N. V.; Zorin, Z. M. Adv. Colloid Interface Sci. 1992, 40, 109. (37) Churaev, N. V. Colloids Surf., A 1993, 79, 25. (38) Churaev, N. V. Adv. Colloid Interface Sci. 2003, 103, 197. (39) Hirasaki, G. J. SPE Form. Eval. 1991, No.June, 217. (40) Churaev, N. V. Adv. Colloid Interface Sci. 2003, 104, xv–xx. (41) Starov, V. M. Adv. Colloid Interface Sci. 1992, 39, 147. (42) Bangham, D. H.; Razouk, R. I. Trans. Fraday Soc. 1937, 33, 1459. (43) Starov, V. M.; Velarde, M. G. J. Phys.: Condens. Matter 2009, 21, 464121. (44) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827.

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Table 1. Structural Film Energy and Corresponding Number of Particle Layers in a Nanofluid Film Stratifying on a Solid Surface no. of particle layers

wedge film thickness h0 (nm)

film-meniscus contact angle θeq (deg)

The term Πh0 in eq 4 (J/m2)

filmRenergy [(D2/kT) ¥ h0 Π(h) dh]

0 1

23.6 ( 5 66.0 ( 8

3.20 ( 0.40 1.40 ( 0.23

6.0  10-7 4.0  10-7

-0.61 -0.25

Figure 6. Film-meniscus microscopic contact angle versus film thickness and corresponding number of particle layers in a stratifying nanofluid film on a solid surface. (film size = 868 μm).

It is important to emphasize that the oscillatory structural energy, Wst (due to the nanoparticle layering within the film-meniscus region), is the main component in the total film interaction energy between the film surfaces, and it dominates the van der Waals and electrostatic contributions to the film energy given by the conventional DLVO theory. Our experimental data on the stratification of nanofluid films revealed that particles tend to self-organize at a film thickness corresponding to an integral number of the effective particle diameters. This is due to the collective particle-particle effect, and the particles self-assemble into multilayers in the confined nanofluid film (Figure 3). Therefore, the structural film energy or disjoining pressure is oscillatory; for hard spheres, it follows the relation:45     2πh h Wst ðhÞ ¼ A cos exp D D

ð6Þ

where D is the period of oscillation and decay factor, while A is the amplitude of oscillation. The period of oscillation and decay factor, D, is the effective diameter of the particle (including the double layer) of the nanoparticle and is related to the layer step height (for example, of 43 nm in a 10 vol % silica nanoparticle suspension) measured in our particle microlayering (i.e., stratifying) experiments using the reflected light microinterferometric method (Figure 1). Combining eqs 5 and 6 and substituting in eq 4, we get a relation between the amplitude of oscillation, A, and the experimentally measured parameters, namely, contact angle, θ, and film thickness, h, as shown in eq 7:     2πh h exp ¼ σo=l ðcos θeq - 1Þ - Π0 h0 A cos D D

ð7Þ

We chose the film thickness corresponding to one particle layer to evaluate the amplitude of the oscillations. This is because the interference patterns corresponding to one particle layer are very sharp compared to that for the film with a greater number of particles. For film thicknesses below one layer of particles, the structural forces due to the nanoparticle ordering effect are not prevalent. The values of each term in eq 7 corresponding to one (45) Henderson, D.; Lozada-Cassou, M. J. Colloid Interface Sci. 1986, 114–180.

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Figure 7. Calculated oscillatory film structural energy isotherm as a function of film thickness scaled by the effective particle diameter. (System: 10 vol % silica nanoparticle aqueous suspension.)

particle layer film thickness, estimated from experimental data, are presented in Table 1. The average effective diameter of particle, D, estimated from the difference in thicknesses of stratifying nanofluid film on a solid surface is 43 nm. This value agrees well with the value reported by us for the same silica hydrosol system (2). The amplitude of the oscillations, A, is calculated by substituting these values in eq 7 and solving for A. Figure 7 displays the calculated oscillatory structural energy isotherm for a 10 vol % (effective volume of 40%) aqueous dispersion of silica nanoparticles with a nominal diameter of 19 nm (the effective particle diameter is 43 nm). The structural energy was nondimensionalized using D, effective diameter of the nanoparticle, and ‘kT’, the kinetic energy of the particles. The solid substrate (glass) used in our experiments has a double layer of about 23 nm in the nanofluid without any added electrolyte at a pH of 9.0. The film thickness corresponding to one particle layer, including the effective particle diameter and the film double layers, is 66 nm (Figure 7); this is 1.5 times the effective diameter of the particle. Figure 7 shows that this film thickness corresponds to the first local minimum of the oscillatory structural energy isotherm. The oscillatory structural disjoining pressure as a function of the film thickness can be calculated by differentiating the film structural energy isotherm (Wst) with respect to the film thickness. Our main objective for calculating the oscillatory structural film energy (and thereby, the structural disjoining pressure) from the microscopic contact angle data is to obtain quantitatively the film tension (or the structural disjoining pressure) gradient resulting from the nanoparticle self-multilayering within the wedge film that promotes the spreading dynamics of nanofluids. This will be presented in our forthcoming paper (in progress) which deals with modeling nanofluid spreading dynamics.

Concluding Remarks We have reported here, for the first time, the results of an experimental observation of the nanoparticle self-ordering (layering) and stepwise thinning of the nanofluid film formed between an oil drop and a solid surface. We also present the measured values of the film-meniscus microscopic contact angle versus film thickness corresponding to the number of particle layers on a solid surface using differential interferometry. These measurements were used to calculate the film energy due to the nanoparticle layering within the nanofluid film. The data DOI: 10.1021/la100928t

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presented here are for a silica nanoparticle aqueous suspension of 10 vol %. The nanoparticle concentration, particle charge, particle size, and polydispersity are among the technological factors that greatly influence particle ordering (self-multilayering), the microscopic contact angle in the film-meniscus region, and the spreading dynamics of nanofluids. These factors warrant further study. Also, the collective particle-particle interactions in different solvents affect particle self-layering in the wedge film and thereby the microscopic contact angle, disjoining pressure and film energy. These interactions need to be assessed. Also, experiments need to be performed to examine microstructure formation, microscopic contact angle dynamics, and film thickness

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stability using various drop (film) sizes, since the nanofluid film stability is governed by the film (drop) size. Also, experiments need to be conducted using a solid substrate other than hydrophilic glass to confine the nanoparticles and enhance the structural disjoining pressure in the wedge film. Acknowledgment. This work was supported by the National Science Foundation under grant CTS-0553738. Supporting Information Available: Movie clip depicting stratification of nanofluid film on solid surface seen in reflected light using differential microscope. This material is available free of charge via the Internet at http://pubs.acs.org.

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