Article pubs.acs.org/Langmuir
Dynamic Spreading of Nanofluids on Solids. Part I: Experimental Kirtiprakash Kondiparty, Alex D. Nikolov, Darsh Wasan,* and Kuan-Liang Liu* Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, United States ABSTRACT: Nanofluids have enhanced thermophysical properties compared to fluids without nanoparticles. Recent experiments have clearly shown that the presence of nanoparticles enhances the spreading of nanofluids. We report here the results of our experiments on the spreading of nanofluids comprising 5, 10, and 20 vol % silica suspensions of 19 nm particles displacing a sessile drop placed on a glass surface. The contact line position is observed from both the top and side views simultaneously using an advanced optical technique. It is found that the nanofluid spreads, forming a thin nanofluid film between the oil drop and the solid surface, which is seen as a bright inner contact line distinct from the conventional three-phase outer contact line. For the first time, the rate of the nanofluidic film spreading is experimentally observed as a function of the nanoparticle concentration and the oil drop volume. The speed of the inner contact line is seen to increase with an increase in the nanoparticle concentration and decrease with a decrease in the drop volume, that is, with an increase in the capillary pressure. Interestingly, the formation of the inner contact line is not seen in fluids without nanoparticles.
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INTRODUCTION Nanofluids are used in various industrial and biological processes, such as cleaning and cooling microchips, drug delivery, heat transfer, enhanced oil recovery, hydraulic fracturing of gas and oil to enhance well productivity, and detergency, because of their enhanced thermophysical properties.1,2 The spreading of nanofluids (such as suspensions of nanosized particles and polymer latexes, globular proteins, and surfactant micelles or microemulsions) on solid surfaces is important. The spreading of nanofluids composed of liquid suspensions of nanoparticles is different from the spreading of liquids without nanoparticles. Recent studies have clearly demonstrated that the well-established concepts concerning the spreading of simple liquids do not apply to nanofluids because of the complex interactions between the nanoparticles and the solid substrate.3−10 Experiments have revealed that nanoparticles form ordered structures in the confinement of the three-phase contact region formed by a bubble or drop on a solid surface.8−10 Using the combined differential and common light interferometric method, Nikolov et al.9 investigated the complex solid−nanofluid−oil interactions and directly observed, for the first time, the self-structuring of nanoparticles (i.e., stratification) due to their confinement in a thin film. The ordering of these microstructures in the wedge-film region results in excess pressure (i.e., the structural disjoining pressure) in a film relative to that in the bulk solution, separating the two surfaces confining the nanofluid (Figure 1a). The structural disjoining pressure has an oscillatory exponential decay with increasing film thickness (Figure 1b), with both the period of oscillation and the decay factor equal to the effective diameter of the nanoparticles.11 The structural disjoining © 2012 American Chemical Society
pressure dominates at scales longer than the effective diameter of a nanoparticle, below which other disjoining pressure components (such as van der Waals, electrostatic, and solvation forces) are prevalent.9,11,12,16,18−21 Wasan and Nikolov8 estimated the spreading coefficient given by de Genne’s theory3 by considering only the structural component of the disjoining pressure. They showed that the spreading coefficient increases exponentially with a decrease in the film thickness, that is, with a decrease in the number of particle layers inside the film. Their results showed that the inlayer particle structure within the wedge film can enhance the spreading of the nanofluid on solids. As the film thickness decreases toward the wedge vertex, the structural disjoining pressure increases. The structural disjoining pressure corresponding to a film thickness with one layer of particles is observed to be higher than that at a two-particle-layer-thick film. Wasan and Nikolov8 applied this concept of the structural disjoining pressure and showed that the driving force for the spreading of the nanofluid is the structural disjoining pressure gradient or film tension gradient (Δγ) directed toward the wedge from the bulk solution; the film tension is high near the vertex because of the nanoparticle structuring in the wedge confinement. As the film tension increases toward the vertex of the wedge, it drives the nanofluid to spread at the wedge tip (the three-phase contact line moves), thereby enhancing the dynamic spreading behavior of the nanofluid (Figure 1a). They demonstrated the practical applicability of this phenomenon in the removal of an oil drop from the solid surface using an Received: July 13, 2012 Revised: September 7, 2012 Published: September 11, 2012 14618
dx.doi.org/10.1021/la3027013 | Langmuir 2012, 28, 14618−14623
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Article
Figure 1. (a) Nanoparticle structuring in the wedge film resulting in a structural disjoining pressure gradient at the wedge. (b) Presure on the walls of the wedge for a 0.5° contact angle at the vertex as a function of the radial distance.10
Figure 2. Photomicrograph taken using reflected-light interferometry depicting the inner and outer contact lines and the nanofluid film region.
receding contact line velocity was found to increase with concentrations up to 1 wt %. However, for concentrations higher than 1 wt %, the contact line velocity decreased marginally or was constant. The authors hypothesized that the observed result could be explained by two possible mechanisms. They believed that the structural disjoining pressure would drive the spreading of the nanofluid, as proposed by Wasan et al.8 However, at concentrations higher than 1 wt %, they believed that the viscous forces dominated the structural disjoining pressure. The authors also indicated that the nanoparticles might adsorb on the solid surface. The liquid would then slip on the particles like it does on a superhydrophobic structured surface as a result of the reduction in friction. Matar et al.,15 however, studied the spreading dynamics of a nanofluid drop on a hydrophilic surface by solving the mass and momentum conservation (Navier−Stokes) equation with the lubrication approximation. The structural component of the disjoining pressure was considered while solving these equations for a film thickness greater than the diameter of the nanoparticles. The nanoparticle dynamics were assumed to be driven by the convective diffusion. The results of their study showed the deformation of the fluid−liquid meniscus and
aqueous surfactant micellar solution. They observed the spreading dynamics of the surfactant micellar film between the oil drop and glass surface using reflected-light digital video microscopy. The first experimental evidence of the enhanced spreading of nanofluids (micellar solutions) on solid surfaces was reported by Kao et al.13 Using interference microscopy in reflected-light mode, they studied the profile of a crude oil−micellar solution interface when a sessile drop of oil was removed from a hydrophilic glass surface. They noticed two distinct contact lines (Figure 2): an outer one (among the oil droplet, solid, and water film) and an inner one (among the oil droplet, solid, and a mixed oil−water film) when seen from the top. The spreading of a mixed oil−water film was later understood to be driven by the structural disjoining pressure gradient arising from the ordering of the micelles in the wedge-film region based on the mechanism proposed by Wasan and Nikolov.8 Sefiane et al.14 studied the forced spreading dynamics of nanofluid drops on hydrophobic surfaces. They used a nanoparticle suspension of polydisperse alumina particles in ethanol. A drop of this nanofluid was forced to spread on a Teflon-coated solid surface by expelling or withdrawing the nanofluid at a fixed volumetric flow rate. The advancing/ 14619
dx.doi.org/10.1021/la3027013 | Langmuir 2012, 28, 14618−14623
Langmuir
Article
Figure 3. Apparatus and experimental setup for monitoring nanofluid spreading dynamics and micrographs depicting the top and side views of the drop.10
terracing, or “foot” formation. The displacement in the position of the foot was found to increase with the concentration and time. The foot corresponds to the inner contact line observed experimentally by Kao et al.13 In this article, we experimentally investigate the spreading dynamics of nanofluids using the advanced optical technique. We monitored the position of the contact line with time and measured the rate of nanofluid spreading driven by the structural disjoining pressure by varying the size of the oil drop (i.e., the capillary pressure) and the concentration of the nanofluid.
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nm in diameter containing 3 mM SDS (sodium dodecyl sulfate, much less than the critical micelle concentration, cmc, in water). SDS was used below the cmc to reduce the contact angle for the effective confinement and structuring of the nanoparticles in the wedge-film region. Experimental Setup and Procedure. We used a recently developed advanced microscope technique,10 utilizing an objective with a high magnification (90×) and a long focal length (∼4 cm) to observe the spontaneous spreading of the nanofluid directly on a solid surface. The spreading dynamics of the nanofluid were monitored using the digital optical apparatus shown in Figure 3. An oil drop of a specific volume was deposited from a syringe under the lower surface of a hydrophilic, optically smooth, thoroughly cleaned glass surface. Then, the glass slide with the oil drop was immersed in the nanofluid in a transparent experimental cell (length = 8.5 cm, breadth = 8.5 cm, height = 3 cm) in a controlled environment to form a sessile drop. Two square glass supports were used to lift the glass slide in order to provide for good visual observation. A sessile oil drop was formed when the air was displaced by the nanofluid around the oil drop on the glass surface. The buoyancy in this configuration presses the drop toward the supporting glass surface. The experimental cell, along with the high-magnification lens, was placed on an optical bench so that the experimental cell was right below the objective of the microscope (90× magnification, 4 cm focal length); the magnification lens (7× magnification, focal length ∼7 cm) was on the side of the experimental cell focused on the oil drop profile. This optical setup is unique because it enables the simultaneous monitoring of the oil drop from both the top and side views. The top view gives the formation and spreading of the nanofluid film. The position of the spreading edge of the nanofluid film was observed with time. The side view gives the drop-shape profile. This profile is used in evaluating the interfacial tension (and the capillary and hydrostatic pressures) by fitting the Laplace equation to it. Twenty-one data points from the profile were used for the drop-shape analysis. The value of the interfacial tension thus measured agreed favorably with that obtained from analyzing the drop profile from the goniometric technique (Kernco Instruments Co.). The error in the interfacial tension measurement using our experimental setup was found to be about 10%. The top view was observed in the reflected light mode of the microscope, and the objective was connected to a charge-coupleddevice (CCD) camera. The image was recorded using a video
EXPERIMENTS
Materials. Canola oil (density = 0.905 g/cm3 at 25 °C, Przybylski, 2005), produced by J. M. Smucker (Crisco), was used in experiments as a sessile drop of oil on a solid surface surrounded by a nanofluid. A small amount (
