Nanofluids mediating surface forces

Advances in Colloid and Interface Science 179-182 (2012) 68–84

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Historical perspective

Nanofluids mediating surface forces Georgia A. Pilkington, Wuge H. Briscoe ⁎ School of Chemistry, University of Bristol, BS8 1TS, United Kingdom

a r t i c l e

i n f o

Available online 30 June 2012 Keywords: Surface forces Nanofluids Nanoparticles Complex fluids Colloidal interactions

a b s t r a c t Fluids containing nanostructures, known as nanofluids, are increasingly found in a wide array of applications due to their unique physical properties as compared with their base fluids and larger colloidal suspensions. With several tuneable parameters such as the size, shape and surface chemistry of nanostructures, as well as numerous base fluids available, nanofluids also offer a new paradigm for mediating surface forces. Other properties such as local surface plasmon resonance and size dependent magnetism of nanostructures also present novel mechanisms for imparting tuneable surface interactions. However, our fundamental understanding, experimentally and theoretically, of how these parameters might affect surface forces remains incomplete. Here we review recent results on equilibrium and dynamic surface forces between macroscopic surfaces in nanofluids, highlighting the overriding trends in the correlation between the physical parameters that characterise nanofluids and the surface forces they mediate. We also discuss the challenges that confront existing surface force knowledge as a result of this new paradigm. © 2012 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanofluids: many applications . . . . . . . . . . . . . . . . . . . Surface forces: a brief overview . . . . . . . . . . . . . . . . . . . Previous studies . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. The effects of nanostructure size . . . . . . . . . . . . . . . 4.2. The effects of nanostructure shape . . . . . . . . . . . . . . 4.3. The effects of nanostructure polydispersity . . . . . . . . . . 4.4. The effects of nanostructure concentration . . . . . . . . . . 4.5. The effects of the solution condition of base fluids . . . . . . . 4.6. The effects of coexistence with surfactant and polymers . . . . 4.7. Soft vs hard nanostructures . . . . . . . . . . . . . . . . . 4.8. The effects of nanostructure surface chemistry . . . . . . . . 4.9. Interactions between nanofluids and soft matter mesophases . 5. Outstanding questions and challenges . . . . . . . . . . . . . . . . 5.1. Three confinement regimes and many outstanding questions . 5.2. Complications and modifications to the DLVO theory . . . . . 5.3. Invasion of surface soft matter structures by nanofluids . . . . 5.4. Tuneable surface forces through controlling nanofluid properties 6. Concluding remarks and outlook . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction ⁎ Corresponding author. Tel.: +44 117 331 8256; fax: +44 117 925 1295. E-mail address: [email protected] (W.H. Briscoe). 0001-8686/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2012.06.007

The term nanofluid, as known in the engineering community, was first coined by Choi in 1995 [1] to describe liquid suspensions

G.A. Pilkington, W.H. Briscoe / Advances in Colloid and Interface Science 179-182 (2012) 68–84

containing nanometer-sized structures with at least one principle dimension less than 100 nm (cf. Fig. 1). For colloid scientists, these are nano-colloids that have been studied for some time. The nanostructures can be made from a wide variety of materials including metals such as gold [2,3], silver [3] and copper [4], metal oxides (e.g. titanium [5], zinc [6] and iron oxides [7]), as well as polymeric compounds (e.g. polystyrene [8] and poly(methyl methacrylrate)) [9]. They can vary in size aspect ratio and take various shapes [10] such as spheres [2], plates [11], ellipsoids [12], rods [13] and cubes [2]. The definition of nanofluids may also be expanded to broadly include fluids containing soft nanostructures, such as micelles, dendrimers and liposomes (cf. Fig. 2(e)). Depending on their surface chemistry, these nanostructures can be dispersed in a large number of base fluids [14], including water, ethylene glycol and motor oils. Their surface chemistry can also be readily tailored with covalently anchored or physically adsorbed soft condensed matter, including polymers and surfactants, to furnish their stability and achieve desired surface properties. Furthermore, their interactions in suspensions can be mediated through tuning the conditions of suspending base fluids, for example through addition of salt or modifying the solution pH in the case of an aqueous medium. Nanofluids are increasingly found in engineering, technological and biomedical applications, which exploit their unique physicochemical properties. These properties depend intricately on the inter-nanostructure forces in nanofluids, as well as the interactions between the nanostructures and the surrounding medium. In the next section we will briefly review some of recent applications, and it will transpire that the efficacy and functionality of nanofluids in these applications are also hinged on their interactions with macroscopic surfaces they confront and on how they may mediate the interactions between the macroscopic surfaces that confine nanofluids. We thus suggest that nanofluids represent a new paradigm for mediating desired interactions between macroscopic surfaces. Such interactions (called surface forces F) [15] are critical in achieving desirable material properties and facilitating designed technological processes (e.g. dispersed pigment particles in household paint), and underpin many natural and biological phenomena (e.g. moving cartilages in a mammalian joint), where surfaces come to close proximity, in the range of 0 to some hundreds of nm. Given such importance, much effort has been dedicated to improving our understanding of surface forces, both theoretically and experimentally [15,16]. Thus, we now have many tricks up our sleeves to achieve desired surface forces, for example by changing the pH or salt concentration of the intervening medium or by anchoring polymers and surfactants on the surface. Attempts have also been made recently to study the inter-surface forces across nanofluids. Such nanofluid-mediated surface forces are likely to be determined by both the inter-nanostructure forces and

d h

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Fig. 1. Nanostructures of well-defined sizes (i.e. d, l, t etc.), shapes and surface chemistry are readily available, and may be suspended in various base fluids, giving rise to a large repertoire of nanofluids with tuneable physicochemical properties.

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nanostructure-surface forces, and in turn depend on a plethora of parameters, such as the nanostructure size, shape, concentration, solution pH and the surface chemistry of confining surfaces. Understanding the complex relationships between these parameters and the unique properties of nanofluids and their effects on surface forces is critical to our ability to tailor nanofluids for specific engineering applications. It is also relevant to predict the fate, behaviour and toxicity when they come in intimate contact with biological systems. After a brief overview of surface forces, we will review recent experimental efforts on surface force measurements involving nanofluids and highlight the overriding trends in the correlation between the above parameters that characterise nanofluids and the surface forces they mediate. We will also pursue a discussion on the challenges that nanofluids would impose on the current understanding of surface forces, in particular how they might behave when they are highly confined by macroscopic surfaces and how they might invade surface structures of polymer and surfactants, thereby modifying inter-surface interactions. 2. Nanofluids: many applications Nanofluids have attracted considerable interest in the last decade due to their unique and enhanced chemical and physical properties, e.g. enhanced viscosity, thermal conductivity and compressibility, compared to their base fluids and analogous suspensions of larger colloidal particles. They are already found in many engineering applications, as summarised in several recent reviews [17,18]. In the last decade or so it has been well established that nanofluids exhibit enhanced thermal physical properties such as thermal conductivity, heat transfer coefficients and thermal diffusivity compared to those of their base fluids [19–22]. These improved properties of nanofluids are believed to be related to the large surface areato-volume of nanostructures, which leads to a significantly larger number of their atoms participating in heat transfer than for larger colloidal particles. Furthermore, due to their small mass and thus a reduction in gravitational effects, nanofluids have also shown increased flowability. These properties have led to the use of nanofluids in a multitude of heat transfer applications such as in the cooling of electronic devices [23,24] and automotive engines [25,26] (cf. Fig. 2(a)). Due to their higher boiling points, nanofluids have also been found to improve the efficiency of petrofluids by increasing their specific heat and enhance the reliability of brake fluids by reducing their tendency to vapour-lock [27,28]. Moreover, due to the sensitivity of their thermal conductivity to various parameters including nanostructure size, shape and concentration, nanofluids have also been recognised as possible “smart fluids” to control the flow of heat from energetic sources [29]. However, due to a lack of consistency and fundamental study of the underlying mechanisms of nanofluid heat transfer, a clear understanding of how these factors affect the heat transfer properties of nanofluids has not yet been ascertained. Another application of nanofluids is in tribological systems due to their enhanced load carrying capacity, anti-wear and friction reduction properties [28,30–33] (cf. Fig. 2(b)). In general the improved tribological properties of nanofluids have been attributed to three main mechanisms. i) The nanostructures form a surface film over the rubbing surfaces, in which they fill surface furrows and troughs, and thus effectively reduce the roughness of the surfaces, facilitating smoother sliding between them. ii) The nanostructures can act as spacers which separate the surfaces, reducing the plastic deformation of surface asperities. iii) Spherical or cylindrical nanostructures may also roll between the rubbing contacts in sliding friction, thus decreasing the friction coefficient markedly and facilitating third body material transfer which may otherwise lead to an increase in surface roughness. Such properties have so far been exploited in the application of nanofluids in automotive lubrication and hydraulic systems. Nanofluids have also been found to exhibit enhanced spreading and wetting behaviours on solid surfaces compared to simple fluids,

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which cannot be described by classical concepts [34–36]. In recent dynamic spreading studies these unique properties of nanofluids have been related to the concentrating of nanostructures at the three phase wedge-like region resulting in an increased disjoining pressure at the oil-nanofluid-surface interface [37] (cf. Fig. 2(c)). This additional pressure would induce spreading of the nanofluid at the oil-nanofluid-surface interface, therefore reducing the oil droplets contact area with the surface. This offers a new mechanism for oil removal which could be used in numerous applications, ranging from the removal of stains and grease from clothes to the cleaning of rocks and sediments in the remediation of oil spills [38]. There are also a growing number of possible biomedical roles for nanofluids. For example, some nanofluids containing noble metal nanostructures have been found to exhibit antibacterial activity, whereby the small size of nanostructures allows them to cross cell membranes without any hindrance and their large surface area maximises their interactions with a target cell [39–43]. In particular nanofluids containing silver nanostructures of diameter d ∼1–10 nm have been found to directly interact with the cell membranes of bacteria, such as

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Escherichia coli and Vibrio cholerae, and drastically disturb their functions like permeability and respiration [41] (cf. Fig. 2(d)). In addition nanostructures have also been found to invade the bacteria causing further damage to bacterial cells by interacting with sulphur and phosphorus containing compounds such as DNA, and releasing silver ions which have additional bactericidal effects [44]. In another application, nanofluids containing pharmaceutical nanocarriers, including liposomes, micelles, and nanospheres have been widely used for experimental (and in some cases clinical) delivery of therapeutic and diagnostic drugs [45] (cf. Fig. 2(e)). The tuneable surface and material properties of nanostructures have been exploited to provide effective and selective drug release. In addition, due to their small mass and consequential reduction of inertial forces on their flow, nanofluids have been found to supply a more uniform drug concentration to target areas [46]. Through modifying the surfaces of nanostructures with stabilising molecules (such as polymers) the stability and longevity of nanodrug carriers in the body have also been shown to be improved. Furthermore, surface modifications of nanostructures with specific ligands such antibodies and peptides also enable

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Fig. 2. (a) Nanofluids containing copper (Cu) nanospheres (d ~50–200 nm) suspended in ethylene glycol have been found to exhibit enhanced heat conductivity compared to conventional coolants. The particles' small mass alleviates the gravitational effects on particle flow and reduces damage to heat exchange surfaces, whereas their large surface-to-volume ratio improves the fluid's ability to absorb and transfer heat [22]. (b) Nanofluids, containing nanostructures such as inorganic fullerene-like particles (d ~50–100 nm), have shown enhanced tribological properties compared to those of their base fluids including increased carrying ability, lower friction and wear reduction. These improved properties have been attributed to three mechanisms: i) nanostructure rolling, ii) the nanostructures acting as spacers between the rubbing surface asperities, and iii) third body material transfer. (c) Silica–water nanofluids (d ~19 nm) have been found to exhibit enhanced spreading properties at oil–liquid–solid interfaces. This behaviour had been related to the structuring of nanoparticles near the three-phase oil–nanofluid–solid contact line (at the wetting wedge), leading to an excess structural disjoining pressure gradient towards the wedge from the bulk solution [37]. (d) Silver (Ag) nanofluids (d ∼1–10 nm) have been found to directly interact with cell membranes of bacteria, such as E. coli and V. cholerae, and drastically disturb cell functions like permeability and respiration. The invasion of bacteria cell by Ag nanoparticles has also been found to cause further damage due to nanoparticle interaction with sulphur and phosphorus containing compounds such as DNA and release silver ions which have an additional contribution to the bactericidal effect [41]. (e) Nanofluids containing pharmaceutical nanocarriers, including liposomes, micelles, and nanospheres, have been widely used for experimental (and in some cases clinical) delivery of therapeutic and diagnostic drugs. Surface modification of these carriers is often used to improve their properties, for example stability, and control their functionality, for instance stimuli responsiveness or interaction with specific target molecules [46]. (f) Gold (Au) nanofluids as photo-absorbing agents in laser photothermal therapy. When exposed to laser light, plasmonic nanostructures such as Au nanospheres (d ~30 nm) can induce local heating due to their strong and tuneable linear absorption in the near infrared region where tissues are optically transparent. This can lead to selective cell or tissue alterations through hyperthermia, coagulation or vaporization processes [49]. (g) Various nanofluids containing inorganic nanoparticles, such as iron oxide (FeO) nanoparticles (db 50 nm), have been used as magnetic resonance imaging (MRI) contrast agents due to their ability to shorten the spin–spin relaxation time (T2) which permits negative contrast enhancement and, thus, darker images of the regions of interest in molecular and cellular imaging [52].

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nanostructures to interact with complex cellular functions in new ways and facilitate novel mechanisms for specific site targeting [45,47]. The coating or complexing of nanostructures with soft matter structures, such as polymers or lipids, has similarly been also shown to enable controlled and specific drug delivery via local or external triggers, such as pH and heat [48]. For example, plasmonic nanostructures including gold nanospheres have been shown to be novel classes of photo-absorbing agents in laser photothermal therapy due to their strong and tuneable linear absorption in the near infrared region where tissues are optically transparent [49] (cf. Fig. 2(f)). In other therapeutic applications, magnetic nanostructures, such as iron based nanofluids, have similarly shown great potential in drug delivery, particularly in cancer therapeutics [50]. Magnets can be used to guide the magnetic nanofluids in the body and deliver high doses of drugs or radiation, minimising effects on nearby healthy cells. Furthermore, magnetic nanostructures have been found to have an increased adhesion to tumour cells compared to non-malignant cells which is not observed for larger (micron) sized structures. Thus, damage to nearby healthy cells is further reduced. Further to therapeutics, various nanofluids have also been used in biomedical imaging applications [50,51]. In particular, inorganic nanostructures have been used as magnetic resonance imaging (MRI) contrast agents due to their unique properties and efficient contrasting effects. Upon functionalisation, nanostructures with different surface chemistries have also been used to target imaging sites of specific interest. In particular magnetic iron oxide nanospheres of db 50 nm have been used extensively as MRI contrast agents due to their ability to shorten the spin–spin relaxation time (T2) which permits negative contrast enhancement giving darker images of the regions of interest in molecular and cellular imaging [52,53](cf. Fig. 2(f)). Gold nanostructures [54] and nanographene oxide sheets [55] have also attracted a lot of attention due to their good biocompatibility and near-infraredfluorescence (NIRF) quenching properties. From the above examples, it transpires that the applications of nanofluids exploit the unique and novel properties facilitated by their nano-dimensions, not encompassed in their base fluids or colloidal suspensions with larger particles. These properties in essence are determined by the inter-nanostructure interactions. In addition, it should be recognised that the efficacy of these applications also depends strongly on how nanofluids interact with the macroscopic surfaces they confront in these applications, and also how nanofluids modify their surface properties. However, despite the numerous applications of nanofluids, our fundamental understanding of how they mediate surface forces through inter-nanostructure forces and inter-surface forces, which corroborate together to give rise these macroscopic properties, is not well understood. In addition, how other characteristics of nanofluids, such as their size dependent magnetism and local plasmon surface resonance (LPSR), might mediate desired and tuneable surface forces remain to be fully explored. 3. Surface forces: a brief overview Surface forces can be obtained from a summation of the inter-atomic forces between all the constituent atoms of a system [15,56]. When this is carried out, two features are paramount: 1) surface forces F are largely determined by the outermost atoms or molecules on the surfaces (i.e. by the surface property) and those of the intervening medium between them; and 2) the range of F is much more extended than the Ångström-ranged inter-atomic or inter-molecular forces, i.e. they can be felt up to some hundred nanometers (nm) away from the surface. Accordingly, various strategies have been designed to mediate the surface forces of desired characteristics: the magnitude of F; how F varies with surface separation; and its sign (attractive or repulsive). The universal van der Waals (Fvdw) force between surfaces arises from their correlating atomic and molecular dipoles, where Fvdw decays as D −n (n = 2 or 3, depending on the geometry of the interacting

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surfaces) [15]. By coating surfaces with different molecules and varying the refractive index of the intervening medium, the magnitude and the sign of Fvdw may be adjusted. When surfaces are in intimate contact (D ~ 0), Fvdw dominates the interactions and the surface forces are referred to as adhesion forces. Solids often acquire a surface charge (of density σ) in a polar medium such as water via ion adsorption or desorption according to some chemical equilibrium with the medium (cf. Fig. 3(a)). A diffuse layer of oppositely charged ions (counterions) consequently develops at the vicinity of the surface with a characteristic thickness κ−1, called the Debye length. The surface charge and the diffuse counterion layer constitute an electric double layer (EDL). When two such charged surfaces come close to each other, so that their diffuse counterion layers overlap, the entropic unwillingness of the counterions to be crowded together results in an EDL repulsion Fedl. The magnitude of Fedl is determined by σ, and it decays exponentially with surface separation, i.e. Fedl ∝e−κD. The chemical equilibrium responsible for the surface charging, and in turn σ, may be adjusted by changing the solution conditions, such as the pH. The Debye length κ−1 may be compressed by increasing the ionic strength or adding multivalent ions in the medium; for instance, κ−1 is ~100 nm in a 0.01 mM 1:1 electrolyte solution and ~1.8 nm in a 10 mM 2:1 electrolytes solution. Therefore, both the magnitude and range of Fedl can be varied as a result. The well-known Derjaguin–Landau–Verwey–Overbeek (DLVO) theory [57,58] in colloid science states that colloidal stability in polar media can be achieved by establishing a repulsive Fedl between colloidal particles, preventing them from falling into adhesive contact due to Fvdw. Conversely, the EDL forces in nonpolar media are less well studied, with

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Fig. 3. (a) Classically, surface forces are mediated by surface charges and soft structures such as surfactant aggregates, polymer brushes and liposomes. The range and magnitude of the forces are determined by solution conditions (e.g. pH and electrolyte concentration), and the molecular architecture and size of the surfactants and polymers. (b) Intriguing surface forces have been observed when surface separation D becomes comparable to the dimensions of the molecules or molecular aggregates. These include the depletion force due to the eviction of the non-adsorption polymers or micelles, and the structural force due to packing of confined molecules. The adhesion force and friction force between surfaces in contact are also dominated by the properties of confined molecules or their assembled structures, which could differ significantly from their bulk properties. In addition, the capillary force can arise from the formation of a liquid bridge, as characterised by the meniscus of Kelvin radius rk, due to a finite contact angle θ between the liquid and two surfaces in close proximity.

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related experimental reports few and far between [59–61]. It remains unclear if the surface charging and stabilisation mechanisms of aqueous media would be operational in a nonpolar liquid due to its low dielectric constant. There is evidence that the EDL force would decay with the surface separation as a power law, i.e. Fedl ∝1/D [59,62], but further research is required to further substantiate this observation. Soft condensed matter such as surfactants and polymers can also be effectively employed to mediate desired surface forces (cf. Fig. 3(a)). Surfactant molecules of different molecular architectures (e.g. singlechained, Gemini and semi-fluorinated) readily adsorb at the solid– liquid interface to form surface aggregates (e.g. monolayers, bilayers and hemi-cylinders) and consequently surface forces are largely determined by the interactions between these aggregates [63–65]. The surface coverage and detailed structure of the surface aggregates are dependent on both the surfactant concentration in the solution and also the molecular structure of the surfactant. Thus, surface forces may in turn be tuned by varying these conditions. For instance, a hydrophilic surface in water can be made hydrophobic by surfactant adsorption from a dilute solution, with the hydrocarbon tails pointing outwards. A hydrophobic attraction with a range of up to some tens, and even hundreds, of nanometres has been observed between two such surfaces [66–68]. Similarly polymers may also adsorb on surfaces with multiple anchoring points or alternatively they may be strongly end-anchored to a surface to form a polymer brush [69–71]. In either case, their presence at the surface dominates surface forces, whose range and magnitude are determined respectively by the thickness L and the density (s −2) of the polymers on the surface [70]. These parameters are accessible by using polymers of different molecular weights Mw, adjusting the solvency (how the polymers would interact with the medium), or varying the strength of the anchorage of the polymer on the surface [72,73]. Intriguing surface force characteristics have been observed when surface separation D becomes comparable to the dimensions of the molecules that mediate the interactions (cf. Fig. 3(b)). For instance, when a simple hydrocarbon liquid is confined to a few times its molecular dimension, the surface forces exhibit oscillations between attraction and repulsion with an oscillation period the size of the hydrocarbon molecule. This oscillatory structural force has been ascribed to the packing of the molecules into an ordered structure under confinement [74,75]. In the case that surfaces are immersed in a medium with non-adsorbing polymers, when surface separation D becomes comparable to the polymer Flory radius of gyration RF, the eviction of polymers from the gap between the surfaces leads to an attraction called depletion attraction, due to the imbalance between the osmotic pressure in the bulk and that between the surfaces. The range of this depletion force is determined by RF, and in turn may be adjusted by varying the Mw 0.6 of the polymer (as RF ~ Mw ) and the solvency of the medium; whereas its magnitude may be controlled by varying the concentration of the polymer in the medium. Such depletion forces are also likely to occur in the presence other soft matter structures, such as liposomes and micelles. It should be pointed out that the above surface forces are equilibrium forces. When two surfaces are in relative motion (at speed v), additional hydrodynamic forces become relevant and important. For surface separation D > ~1 μm, the hydrodynamic surface forces resisting the relative motion between the surfaces are determined by the bulk viscosity η of the intervening medium [76], as described in the Reynolds equation Fh ∝vη /D. The viscosity η reflects the inter-molecular interactions in the medium, i.e. η ~ Gτ. The interaction modulus G (or interaction energy per unit volume) is invariably of the order of a few kT per molecule for all liquids; however, the relaxation time τ can be modified by many orders of magnitude. For example, by adding small amounts of polymers to water, the relaxation time τ could be enhanced from ~10 −12 s to a few ms [77]; this in turn dramatically modifies the hydrodynamic surface forces.

The dynamic tangential force resisting surfaces in relative motion at intimate contact (i.e. D ~0) is commonly known as friction. Although the relation between friction and the equilibrium surface forces, particularly adhesion and capillary force (due to a liquid meniscus of Kelvin radius rk around the contact (cf. Fig. 3(b))); is yet to be fully understood; it is well accepted that the presence of polymers and surfactants on the surface tends to dominate the lateral as well as the normal surface forces, and some examples of their effectiveness in friction reduction have been reported [78–83]. With these many tricks up our sleeves to arrange and modify surface forces, it is fair to say that our understanding of surface forces is well advanced. These tricks most notably include adjusting solution conditions and adding soft condensed matter to the surface and the intervening medium. Current research has focused on measuring interactions in complex biological systems, tackling difficult phenomena such as surface deformation [84] and going beyond some of the early theoretical assumptions. There are also other categories of surface forces such as chemical bonding and hydration forces that may become important in some circumstances. However, the surface forces reviewed above are the most important and relevant to nanofluids. Although well studied, how such forces would be affected by the presence of nanostructures of different size, shape and surface chemistry, still remains unclear. These effects will be examined in the next section which reviews previous surface force and engineering studies which have been conducted on nanofluids and highlights the key trends seen in varying such parameters.

4. Previous studies A number of studies on surface forces mediated by nanofluids have been carried out. Primarily these have been conducted using the surface force apparatus (SFA) [85–92] and colloidal probe atomic force microscopy (CP-AFM) [93–101], with results from X-ray motorised confinement cavities [102,103] and microfluidic channels [104] also reported. The majority of previous studies have focused on the equilibrium behaviour of nanofluids and several studies have also investigated their dynamic properties, such as their friction [89,90,102,105] and rheological behaviours [97,106]. In general the presence of nanofluids has been found to lead to deviations in the interactions between surfaces from the behaviours predicted by classical theories. In some cases, these interactions have been found to be dominated by additional, non-classical effects superimposing on or convoluting with classical interactions [93,94,98,100,103]. For example, in 1992 Parker et al. observed an additional attractive depletion well or structural forces which superimposed on double layer forces across aqueous ionic micellar solutions of the cationic surfactant CTAB, at different surfactant concentrations using SFA [85]. Similar behaviours were also observed in reversed dioctyl sodium sulfosuccinate (AOT)–water–heptane micellar solutions by Richetti and Kekicheff in 1992 [86] and in aqueous cerium oxide (CeO2) nanosphere nanofluids by Spalla and Kekicheff in 1997 [87]. By and large, nanofluids have been found to lead to an increase in the repulsive interaction between surfaces at close proximity as compared to their base fluids, when the nanoparticles are not depleted from the approaching surfaces. This is most likely due to steric forces related to their finite size. For instance, in a study by Sennerfors et al. in 2000 the addition of silica (SiO2) nanospheres (d ~12 nm) at various concentrations ranging from 25 to 250 ppm in polyelectrolyte solutions was found to significantly increase the repulsive force measured between two borosilicate glass surfaces using a version of the SFA [107]. In general the magnitude and range of the interactions mediated by nanofluids depend on a number of parameters related to properties of the nanostructures. In the following subsections a current overview of the effects of these parameters on the equilibrium and dynamic forces in nanofluids will be presented.

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4.1. The effects of nanostructure size The size (i.e. d, l, t etc. in Fig. 1) of the nanostructures dispersed in a nanofluid has a strong influence on the range, magnitude and nature of the interactions between surfaces which are intervened by nanofluids. In the case of structural forces, an additional dependence of the periodicity of the oscillatory interaction on the nanostructure size has been commonly observed. For example, in 2002 Piech and Walz reported that an increase in the volume fraction of larger nanospheres in aqueous polystyrene latex (d ~ 21 and 31 nm) binary nanofluids led to an amplification of the structural forces between two silica surfaces [93] (cf. Fig. 4(a)). Upon increasing the proportion of larger spheres the oscillation periodicity was also observed to increase. This behaviour reflects the reduced ability of larger nanoparticles to pack effectively between the confining surfaces. However, in a related CP-AFM [98] study across SiO2-water nanofluids by Klapp et al. in 2010, the dependence of structural forces on nanostructure size was attributed to the increase in the total charge of the particles with particle diameter. The dependence of the interactions mediated by nanofluids on nanostructure size was also reported in a theoretical study by Bohinc et al. in 2009 who found the electrostatic potential, volume charge density and interaction free energy between two plates in a Lennard–Jones liquid would decrease in magnitude with decreasing nanosphere size (d ~0.5–5 nm) [108].

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A dependence on nanostructure size has similarly been seen in the dynamic behaviours of nanofluids. For example, in 2009 Vafaei et al. observed that the wettability of aqueous bismuth telluride (Bi2Te3) nanofluids on silicon wafer and glass substrates was significantly enhanced by reducing the size of the nanospheres dispersed in the fluids from 10.4 to 2.5 nm [109]. Likewise, Chevalier et al. in 2007 observed an enhancement in the viscosities of SiO2–ethanol nanofluids containing nanospheres of various sizes (d ~ 35, 94, 190 nm) upon reducing the nanosphere size (cf. Fig. 4(b)) [106]. Taken as a whole, these studies have shown the magnitude and range of the equilibrium forces scale with the size of the nanostructures suspended in nanofluids. This is likely to be related to the steric effects of the nanostructures on the overall force and also to their packing efficiency under confinement. Similarly, the dynamic properties of nanofluids have also been found to be sensitive to nanostructure size. Although the mechanisms behind such effects are currently unclear, it is postulated the increased mobility of the nanostructures with decreasing size may play an important role. Such dynamic properties will be important to the non-equilibrium and hydrodynamic forces that nanofluids mediate. 4.2. The effects of nanostructure shape Analogous to size, the shape or aspect ratio of the nanostructures dispersed in nanofluids has been found to have a significant effect on the nature, magnitude and range of the surface forces measured across the nanofluids. In a recent comprehensive study by Akbulut et al. in 2007, the dependence of the surface forces on nanostructure shape was demonstrated in SFA measurements across dodecane nanofluids containing surfactant-coated zinc sulphide (ZnS) nanostructures of different sizes and shapes (nanospheres d ~4 nm; nanorods ϕ ~4 nm and l ~8 nm; nanowires ϕ ~4 nm and l ~200 nm) [88]. Overall, both the magnitude and range of the equilibrium forces were observed to increase upon increasing the aspect ratio of the nanostructures, as shown in Fig. 5(a). These observations were attributed to the increased steric contribution of the nanostructures to the overall surface interaction associated with their increasing size and their reduced ability to pack efficiently due to their decreasing symmetry. The effect of nanostructure shape on surface forces was also shown in another SFA study by Min et al. in 2007 whereby introducing curvature to surfactant-coated ZnS nanowires (ϕ ~4 nm, l ~200 nm) dispersed in dodecane was found to have a strong influence on the equilibrium and shear forces [90]. Upon confinement the curved nanowires gave rise to a smoother, longer ranged interaction with a hard-wall distance four times larger than measured across straight nanowire suspensions. On the other hand, under shear, the curved nanowires were found to provide a comparable friction coefficient to straight nanowires at low to intermediate loads. However, at larger loads the curved nanowires showed significantly higher friction forces. This contrast in the frictional behaviour of these nanofluids was attributed to the increased entanglement and subsequent inability of the curved nanowires to pack as efficiently as straight nanowires under confinement and shear. Such shape effects have also been probed in a number of theoretical studies. For example, in 2003 Qin and Fichthorn studied the interaction between particle pairs of different size, shape and surface chemistry in a fluid using molecular dynamics simulations [110]. In their measurements the interaction energy between two solvophilic nanocubes (h ~6 nm) was found to be significantly larger than that between similarly sized nanospheres. It was suggested that the nanospheres would bear a surface roughness, associated with their shape, which disrupts solvent layering; whereas the cubes' smooth, flat surfaces would promote the ordering of the solvent into more well defined layers. In a recent paper by Timofeeva et al. the shape or size ratio of nanostructures has also been found to have a significant effect on the viscosity and rheological behaviours of water–ethylene glycol

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Overall, varying the aspect ratio of the nanostructures dispersed in nanofluids has been found to have a profound effect on the equilibrium forces mediated by nanofluids. In dynamic measurements across nanofluids their physical properties have also been found to strongly depend on the aspect ratio of nanostructures. This is convoluted with the size of the nanostructures and their concentrations in nanofluids.

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In general, the nanostructures used in most commercial nanofluids will be subject to some size distribution or polydispersity. Therefore, understanding the influence of nanostructure polydispersity on the interactions mediated by nanofluids is important. In a number of recent studies, polydispersity has been found to have a significant effect on both the equilibrium and dynamic forces mediated by nanofluids. For example, in 2006 Drelich et al. found that polydispersity (d ~5–50 nm) in the size of aluminium oxide (Al2O3) nanospheres dispersed in water led to a reduced periodicity and magnitude of the structural forces measured across the nanofluids as compared to those across monodispersed SiO2 (d ~10 nm) nanosphere suspensions using CP-AFM [101]. This suggests that polydispersity relaxes the geometrical constrains on nanostructure packing under confinement. In another study by Nikolov and Wasan in 1992, polydispersity was also noted to have a significant effect on the structural behaviour of

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based nanofluids containing various shapes of alumina nanostructures (nanoplatelets (d ~ 9 nm), nanodisks (d ~ 60 nm, t ~ 10 nm), nanorods (ϕ ~ 10 nm, l ~ 80 nm), and nanocubes (h ~ 40 nm)) [111]. The viscosity of the nanofluids was not found to scale with aspect ratio but with the volume fraction of the nanostructures, reflecting the larger volume of the nanofluid occupied by the nanostructures at a given concentration (cf. Fig. 5(b)). In shear measurements the shape of the nanostructures was found to affect the rheological behaviour of the nanofluids in a different way to viscosity. The viscosities of the nanoplatelets and nanodisks varied by a factor of 2, but at the same concentration they both showed Newtonian behaviour, i.e. their viscosity is independent of shear rate. On the other hand, the fluids containing nanocubes and nanorods were found to show shear thinning, i.e. their shear viscosity decreased with increasing shear rate, at higher nanostructure concentrations. This behaviour indicates that some nanostructure agglomeration, might be present at higher shear rates the agglomerates broke up into individual nanostructures or smaller nanoclusters, thus reducing the structural restriction on the fluid flow. However, the dependence of this behaviour on shape is unclear but could reflect the geometry of the agglomerates formed by different nanostructures. As agglomerates are usually elongated [112], they behave in similar fashion to high aspect ratio structures, such as nanorods, by restricting their rotational and translational Brownian motions.

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G.A. Pilkington, W.H. Briscoe / Advances in Colloid and Interface Science 179-182 (2012) 68–84

4.4. The effects of nanostructure concentration Upon increasing the concentration or volume fraction of nanostructures dispersed in a fluid it is intuitive that a concurrent increase in confinement will lead to increased inter-particle interactions and force the nanostructures to pack more closely. This could induce structuring of nanofluids, an effect that has been observed in a number of systems, as exemplified by the stratification of aqueous suspensions of silica nanospheres, non-ionic (d ~ 10) and anionic (d ~ 5) micelles reported by Nikolov and Wasan in 1992 [113]. Such enhanced structuring of nanostructures in the system has also been shown to affect the structural forces in nanofluids in a number of CP-AFM experiments. For example, in 2004 Piech and Walz reported the periodicity of the oscillatory structural forces measured across SiO2–water nanofluids to be dependent on the concentration of nanostructures (d~ 26 nm) and scale as n −1/3, where n is the nanosphere number density [94] (cf. Fig. 7(a)). This scaling has been observed in several other studies, including across SiO2–water nanosphere (d ~ 22 nm) and anionic (SDS) micellar solutions by Tulpar et al. in 2006 [95], as well as in a number of theoretical studies on structural forces in aqueous electrolyte solutions containing charged nanospheres [98,116]. Similarly, nanostructure concentration has also been reported to modify the longer ranged forces experienced across nanofluids. For example, in 2007 Akbulut et al. found that both the magnitude and range of the normal forces measured between mica surfaces across surfactant coated ZnS nanostructures suspensions in dodecane of different sizes and shapes (nanospheres d ~4 nm, nanorods ϕ ~4 nm and l ~8 nm, and nanowires ϕ ~4 nm and l ~200 nm) increased with nanostructure concentration [88]. These observations were attributed to an increased steric repulsion for the larger volume fraction of particles at a given confinement. In more concentrated dispersions, features such kinks or discontinuities were also more evident in the measured force–distance curves, alluding to the tendency of nanostructures to aggregate with increasing concentration. The dynamic properties of nanofluids have also been shown to be affected by nanostructure concentration. In the SFA study by Akbulut et al. in 2006, friction across dodecane nanofluids containing surfactant coated ZnS nanorods (ϕ ~1 nm, l ~5 nm) was found to be either lower or higher than that in pure dodecane, depending on the nanorod concentration and load, as shown in Fig. 7(b) [89]. Overall, an intermediate particle concentration of 2 mg ml−1 mediated lower friction than pure dodecane at all loads. It was suggested that for such a friction reduction

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silica nanospheres in aqueous suspensions [113]. Polydispersity (d ~7 and 28 nm) was found to disrupt the ordering of the nanospheres and increase the number of vacancies in the nanostructure layers compared to in monodispersed systems (d ~19 nm, 230 nm). Similar effects on the structuring of nanofluids have also been reported in a theoretical study by Chu et al., where polydispersity of the nanostructures was found to reduce the magnitude and increase the range of the structural forces across nanofluids containing hard nanospheres (d ~9 nm) [114], as shown in Fig. 6(a). In contrast, the influence of polydispersity on the dynamic properties of nanofluids has not been well studied. In a recent study Pastoriza-Gallego et al. observed a significant effect caused by polydispersity on the rheological properties of monodisperse (d ~11 nm) and polydisperse (d ~23–37 nm) CuO–water nanofluids [115]. Upon increasing the volume fraction of the CuO nanospheres it was found that the viscosity of the nanofluids containing polydispersed nanostructures increased less rapidly than those of similarly sized monodispersed nanostructures, as shown in Fig. 6(b). This suggests that different sized nanospheres might pack more efficiently and the resultant clusters would impede the fluid flow. Conversely, it means that nanofluids with polydispersed nanostructures are less effective in viscosity enhancement.

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at all loads the nanorod concentration must be low enough to not lead to significant nanorod aggregation and subsequent irregular shearing, but also high enough to prevent all the nanorods from being expelled under high load and shear rates. In a CP-AFM study in 2006, the hydrodynamic interaction between two surfaces across aqueous nanofluids was also found to be concentration dependent [97]. At high SiO2 nanosphere (d ~22 nm) volume fractions (9.9 and 11.1%) a hydrodynamic force was observed, which was not present at lower nanosphere concentrations. This behaviour was attributed to the increased viscosity of the nanofluid which would reduce the mobility of the AFM colloidal probe in the fluid. There have also been several studies which have reported the relationship between the viscosity of nanofluids and nanostructure volume fraction. In general, a linear relationship with volume fraction has been observed, for example by Lee et al. in aqueous dispersions of Al2O3 nanospheres [117] and by Chen et al. in ethylene glycol based nanofluids containing TiO2 nanospheres [118]. However, other studies, such as by Tseng and Lin in 2003 [119] who studied the rheology of aqueous nanofluids containing TiO2 nanospheres (d ~ 7–20 nm), have shown that the viscosity of the nanofluids increases exponentially with nanostructure volume fraction. Furthermore, the concentration of nanostructures has also been found to affect the wetting or spreading properties of nanofluids. For instance, in 2006 Vafaei et al. reported that the contact angle of aqueous nanofluids containing Be2Te3 nanospheres functionalized with thioglycolic acid (d ~2.5–10.4 nm) on glass and silicon surfaces increased with increasing nanosphere concentration up to a maximum

G.A. Pilkington, W.H. Briscoe / Advances in Colloid and Interface Science 179-182 (2012) 68–84

4.5. The effects of the solution condition of base fluids The base fluid in which colloids are suspended can also have a significant influence on the interactions nanofluids mediate. Via the intrinsic properties of the base fluid, such as polarity and viscosity, the nature of the chemical and physical properties of suspended colloids can be modified. Thus, changes in the base fluid conditions, such as addition of salt or change in pH in aqueous solutions, are likely to have a strong effect on the interactions mediated by the nanostructures. Such effects have already been observed in a number of nanofluid systems. For example, in 1997 Spalla and Kékicheff reported that the magnitude and nature of the interaction between two mica surfaces coated with cerium oxide (CeO2) nanospheres (d ~ 7 nm) across aqueous solutions were strongly influenced by solution pH [87], as shown in Fig. 8(a). This behaviour reflects the modification of the nanosphere electrochemical surface potential by pH changes, leading to the dispersion or aggregation of the nanostructures depending on the nanostructure surface chemistry. A strong dependence on pH of both the range and magnitude of the structural forces was also observed in aqueous suspensions of SiO2 nanospheres (d ~ 10 nm) by Drelich et al. in 2006 [101]. Similarly, this was ascribed to an increased repulsion between the spheres with the increased surface charge density. However, this strong pH dependence of the interactions mediated by nanofluids was not reported in a number of other related studies. For instance, in a CP-AFM study by Piech and Walz in 2004, the pH of aqueous SiO2 nanosphere (d ~ 26 nm) suspensions was found to have a less significant effect on the structural forces than the addition of salt [94]. Upon increasing the solution pH, the periodicity of the oscillatory interaction was found largely unaltered and the amplitude of the oscillations only weakly increased, indicating that the electrostatic interactions in the system remained approximately the same at different pH values. However, upon the addition of salt, the magnitude of the structural oscillations was found to significantly reduce, pointing to electrostatic interactions contributing to structural forces. Mixtures of organic fluids, such as ethylene glycol, with water are commonly used in many heat-transfer applications of nanofluids. A number of studies have shown that the introduction of water to organic nanofluids has a significant effect on the normal and lateral forces they mediate. For example, Min et al. reported in 2008 that the introduction of water to dodecane nanofluids containing ZnS nanorods (ϕ ~1.5 nm, l ~8 nm) or nanowires (ϕ ~1.5 nm, l ~200 nm) had a strong influence on their equilibrium and dynamic behaviours [92]. Upon increasing exposure to water the normal interaction measured across the nanorod solutions both increased in magnitude and in range. This behaviour was attributed to nanostructure aggregation due to an increase in capillary

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and then decreased when the concentration was increased further [120]. A similar concentration dependence of the wetting behaviour of Al nanosphere–ethanol nanofluids has also been reported by Sefiane et al. [34]. In general this dependence of the wetting or spreading behaviour of nanofluids on nanostructure concentration has been attributed to changes in the structuring of the nanostructures in the fluid wedge of the three-phase contact line, modifying the pressure gradient within the fluid. To summarise, increasing the nanostructure concentration in nanofluids has been found to generally increase the magnitude and range of the structural forces in nanofluids. However, for nanofluids containing nanospheres the range of the structural forces has been found to be reduced by increasing the nanostructure concentration as it forces the spheres to pack more efficiently. The dynamic properties of nanofluids have also shown a dependence on nanostructure concentration. However, the enhancement of the dynamic properties of nanofluids with nanostructure concentration, e.g. friction reduction and wetting, are limited by increased aggregation of nanostructures at high concentrations.

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adhesion between them. In the same system, the addition of water was also found to increase the friction between the confining mica surfaces and to lead to a non-linear increase in friction with load, characteristic of adhesion-controlled frictional behaviour (cf. Fig. 8(b)). This behaviour was similarly suggested to arise from increased adhesion between the nanostructures due to capillary bridges between them. Overall, modifying the base fluid conditions, e.g. pH or addition of water to organic base fluids, has been observed to have a profound effect on the equilibrium and dynamic properties of nanofluids. Such modifications can mediate different inter-nanostructure and solventnanostructure interactions by altering van der Waals and double layer forces present or imparting capillary forces, thereby altering the stability, structuring or mobility of the nanostructures. 4.6. The effects of coexistence with surfactant and polymers Different types of soft matter, particularly surfactants or polymers, are commonly used to modify surface forces and tune the properties of fluids. Surfactants and polymers can readily decorate nanostructure surfaces, thereby altering their surface chemistry, and form various structures in a fluid themselves. Thus one can anticipate them to lead to complex behaviours once they are added to nanofluids. This complexity offers an opportunity to design sophisticated mechanisms to tailor nanofluid properties using the convoluted interactions of polymers, surfactants and nanostructures.

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In the SFA study by Min et al. in 2008 the addition of anionic surfactant SDS to aqueous solutions of multiwalled, close-ended carbon nanotubes (CNTs) (ϕ ~ 10–50 nm, l ~ 2 μm) coated with SDS was found to increase the range of the repulsive interactions, as well as the hard wall repulsion distance [92] (cf. Fig. 9(a)), although the magnitude of the interaction remained approximately constant. However, in another study by Zeng et al. the addition of differently charged surfactants, i.e. anionic (SDS), cationic (CTAB) and non-ionic (β-dodecylmaltoside), was not found to affect the nature, range or periodicity of the structural forces measured between a micron-size SiO2 sphere and an air bubble across aqueous suspensions of SiO2 nanospheres (d ~ 25 nm) [100]. However, the magnitudes of the interactions were found to be lower in the presence of each surfactant than those measured in the pure nanofluid. This was attributed to the lowering of the surface tension and the effective charge of the nanostructure surfaces. In contrast to surfactants, the effects of polymers on the equilibrium and dynamic properties to nanofluids have not been well studied. However, in a recent pioneering study, upon increasing the concentration of polyvinylpyrrolidone (PVP), a significant decrease in the viscosity of silver nitrate (AgNO3)–ethanol nanofluids was observed [121] (cf. Fig. 10(b)). This was attributed to the enhanced stability of nanostructures against agglomeration by entropic repulsion between adsorbed polymer layers at the nanostructure surface.

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Overall, the addition of surfactants and polymers has been shown to be a powerful tool for controlling the interactions between nanostructures in nanofluids and subsequently their equilibrium and dynamic properties. In particular, the concentration of these soft molecules added has significant effects on the electrostatic and steric forces they mediate. However, especially in the case of polymers, the effects of other parameters, such as molecular weight and molecular architecture, remain to be fully explored. 4.7. Soft vs hard nanostructures Nanostructures in nanofluids can be either hard or soft. The former can be metallic (e.g. gold), inorganic (e.g. silica), or polymeric (e.g. latex), whereas the latter are often self-assembled aggregates of surfactants and polymers such as liposomes, micelles and polymer nanogels, or hard nanostructures coated with soft surface layers, such as polymer brushes [117,122]. In general the interactions mediated by soft nanostructures have been found to show similar features to those of hard nanostructures. For example, in 1992 Richetti and

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Kékicheff reported that the magnitude of the structural forces measured in reversed (AOT/water/heptane) micellar solutions at high concentrations increased with micelle concentration and the periodicity of the oscillatory interaction was approximately comparable to the diameter of the micelles [86], as observed for many nanofluids containing hard nanostructures [93–95,98]. This behaviour was also observed across aqueous solutions of sodium dodecyl sulphate (SDS) micelles by Tulpar et al. in 2006 [95] (cf. Fig. 10(a)) and Tabor et al. in 2010 [123]. However, some differences in the range and magnitude of hard and soft nanostructure equilibrium forces have been reported. For instance, Tulpar and Walz in 2006 found that the periodicity of the structural forces observed across anionic SDS micellar solutions scaled as 0.86n −1/3, where n is the nanoparticle number density [95]; whereas for the wavelength of structural forces measured across SiO2 aqueous nanofluids the periodicity scaled as n −1/3. The “softness” of nanostructures has also been found to have an effect on the dynamic properties of nanofluids. For example, in a recent paper by Zheng et al. in 2009 the frictional properties of crosslinked poly(2-cinnamoyloxyethyl acrylate) (PCEA) diblock copolymer nanospheres dispersed in base oil were found to correlate with the degree of crosslinking in the nanosphere cores when confined between rubbing stainless steel surfaces, as shown in Fig. 10(b) [124]. Compared to softer self-assembled PCEA micelles and pure base oil, the polymeric-base oil nanofluids showed a significant reduction of friction over a large range of rubbing velocities. This behaviour was related to the ability of the nanospheres to retain their elasticity and structural integrity under high pressure, thus preventing contact of the rubbing surface asperities and facilitating the support of load. In general the interactions mediated by nanofluids with soft nanostructures have not been well studied. However, it is understood so far that, compared to hard nanostructures, soft nanostructures have a reduced structural contribution to the total interaction between surfaces but can retain sufficient structural integrity to facilitate similar dynamic behaviours, e.g. friction–reduction properties. Furthermore, nanostructure “softness” has also been shown to be a novel parameter which can be used to tune and control surface forces.

forces), was observed. On the other hand, between two solvophobic nanostructures, which experience a stronger interaction with each other than with the solvent molecules, the force profile was found to be monotonic and weakly attractive, suggesting depletion forces to be present instead due to the repulsive interaction between the nanostructures and the solvent molecules. Likewise, the surface chemistry of the surfaces which confine nanofluids has also been found to have a significant effect on the interactions they mediate. For instance, in 2006 Drelich et al. showed that changing one of two confining surfaces from silica to mica led to a significant change in the nature of the interaction measured across an aqueous dispersion of SiO2 nanospheres [101]. Similarly, in a theoretical study in 2009 Bohinc et al. illustrated that upon increasing the surface charge density of two confining surfaces the interaction mediated by charged spheres of various size (d ~ 0.5, 2 and 6 nm) might be changed from repulsive to attractive if the nanostructures were large enough (d ~ 2, 6 nm) [108]. Across “softer” nanofluids composed of PCEA diblock copolymer nanospheres or micelles dispersed in base oil the surface chemistry of nanostructures has also been reported to affect the frictional properties of nanofluids measured between stainless steel surfaces by Zheng et al. [124], as shown in Fig. 11(b). Overall, they found that by changing the surface groups of several nanospheres and micelles the magnitude of the friction measured across the nanofluids was modified, whilst the trend between rubbing velocity and friction remained similar for each type of nanostructure core. The surface groups which had a higher affinity for the confining stainless steel

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The surface chemistry of both the nanostructures dispersed in nanofluids and the surfaces which confine them can have a significant effect on the inter-nanostructure, nanostructure-surface and nanostructure-solvent interactions. These effects have been reported in a number of studies and present a promising method of tuning the surface forces nanofluids mediate. For example, in 2007 Hakim et al. showed, by applying ultra-thin Al2O3 surface coatings to SiO2 (d ~40 nm) and TiO2 (d ~10 nm) nanospheres in aqueous dispersions, both the short and long range interactions between the nanospheres and a silicon nitride (Si3N4) AFM tip were strongly modified (cf. Fig. 11(a)) [125]. In addition, the Al2O3 coatings on the nanospheres were also found to affect their flowability and dispersability. The changes to the interactions measured across the nanosphere dispersions were attributed to changes in the Hamaker constant of the nanospheres, which leads to an increased inter-nanostructure van der Waals attraction and subsequent nanostructure aggregation. The importance of nanostructure surface chemistry has also been investigated by molecular dynamics simulation. For example, in a study performed by Qin and Fichthorn in 2003 the interactions between pairs of nanostructures of different size, shape and surface chemistry (spheres d ~ 2 nm and 6 nm, cubes h ~ 6 nm) were found to strongly depend on their surface chemistry [110]. Between the solvophilic nanostructures, which have a stronger interaction with solvent molecules than with each other, an oscillatory interaction, due to the solvent ordering and layering transitions (solvation

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4.9. Interactions between nanofluids and soft matter mesophases It is well known that amphiphilic polymers and surfactants can self-assemble into various mesophases and their morphologies are governed by the intermolecular forces between such molecules. It has already been observed in a number of systems that upon exposure to nanofluids the morphologies of such mesophases would be perturbed or modified. Thus, nanofluids present a new method for controlling the morphologies of soft matter systems and constructing new nanocomposite materials. In a neutron reflectometry study by Lauter-Pasyuk et al. in 1998, it was reported that the morphology of nanosphere-diblock copolymer thin films, made from of polystyrene (PS), polybutylmethacrylate (PBMA) and iron oxide (γ-Fe2O3) nanospheres, depended on the nanosphere size (d ~ 4, 6 nm) [126]. In each case the lamellar structure of the diblock copolymer films was maintained; however the distribution of the nanospheres was found to vary with nanosphere size. In the case of the smaller nanospheres, they were found to concentrate at the copolymer interface, whilst the larger nanospheres localised in the centre of the higher surface energy PS domains. Similarly, in a theoretical study by Thompson et al. variations in the size and volume fraction of nanospheres were found to lead to different morphologies in diblock copolymer–nanosphere composites, as shown in Fig. 12(a) [127]. At a high nanosphere volume fraction (ϕ = 0.15) the larger spheres (r = 0.3R0), where r is the sphere radius and R0 is the natural radius of the polymer, were found to localise within the cavity between the polymer chains, forming a well ordered core shell structure with the nanospheres forming nanosheets. However, when the nanostructure volume was lowered (ϕ=0.03) the composite no longer formed a highly ordered phase as too few nanospheres were present. Alternatively, for smaller nanospheres (r=0.2R0) an edge-assembled morphology was observed at the larger volume fraction (ϕ=0.15). This size dependent behaviour was attributed to the smaller nanospheres imparting less stretching energy on the polymer chains. Hence, suggesting their structure is dominated by the translational entropy of the particles. In a later theoretical study model by Lee et al. in 2002, the effect of nanosphere size on the phase behaviour of diblock copolymer– nanosphere mixtures was also studied [128]. Similarly, upon changing the size of the nanostructures, given as a fraction of R0, a change in the morphology of the composite films was also observed. Similar to Thompson et al.'s observations, the larger nanospheres (r ~0.18R0) were found to concentrate in one domain, where as the smaller nanospheres (r ~0.06R0) were found to mainly concentrate at the interface between the two polymer segments and distribute more uniformly throughout the two domains. In the case of the larger nanospheres, it was suggested that the enthalpic gain obtained from reducing the interfacial tension between the polymer block was greater than the loss of the nanospheres translational entropy. In addition to the distribution of the nanospheres, the exact mesophase of the diblock copolymer system depended on the size of the nanospheres at high particle concentrations [128] (cf. Fig. 12(b)). For example, in the presence of the larger spheres the morphology of the diblock copolymer–nanosphere

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surfaces were also found to perform better at friction reduction. This behaviour was related to the higher affinity of the exposed surface groups on the nanostructures for the rubbing surfaces, leading to an increased number of nanospheres or micelles retained between the surfaces under shear. It is not surprising the surface chemistry of the nanostructures dispersed in nanofluids as well as the surfaces which confine them strongly affect the equilibrium and dynamic forces they mediate, since it is well established that surface forces are largely determined by the outermost atoms or molecules on the surfaces. However, more work needs to be done in order for us to understand how the surface forces in nanofluids may be tailored by controlling the surface chemistry of nanostructures and confining surfaces.

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composites was found to change from a lamellar to a cylindrical mesophase. A similar dependence on nanostructure size has also been reported in a theoretical study by Balazs et al. in 2003, where in a binary system of small (r ~0.1R0) and large nanospheres (r ~0.3R0), the larger nanospheres were found to concentrate in the centre of the domain, whereas the smaller nanospheres were observed to preferentially concentrate at the edge of the interface between the two block copolymers [129]. Such structural effects found upon varying the size of nanostructures have also been reported in polyelectrolyte layers. For example, in 2000 Sennerfors et al. investigated the effect of silica nanosphere size and charge on the properties of polyacrylamide multilayers using ellipsometry and AFM [107]. Both the thickness and adsorption rate of the nanospheres were found to scale with their size (cf. Fig. 13(a)). The latter observation was attributed to the diffusivity of Brownian particles decreasing with increasing size, as predicted by the Einstein formula. In addition, the density and surface coverage of the films was also found to increase with the charge density of the nanospheres. This behaviour was ascribed to a decrease in the tendency for aggregation and higher affinity for the polyelectrolyte with increasing charge density. Similarly, the absorption behaviour of citrate stabilised gold nanospheres (d ~ 3 and 16 nm) dispersed in charged (poly(Nisopropylacrylamide), PNIPAAM; poly((diethy- lamino)ethyl methacrylate, PDEAMA)) and uncharged (polyacrylamide, PAAM) polymer brush layers was also shown to depend on nanosphere size by Bhat and Genzer in 2006 [130]. In the case of the larger nanospheres, the nanostructures were found to reside primarily at the top of the brush layer and the amount of particles absorbed was found to increase with increasing polymer brush grafting density and polymer molecular weight. Alternatively, in the case of the smaller nanospheres, they were believed to penetrate the brush layers and their concentration initially increased with increasing polymer brush grafting density,

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a

confining surfaces on the properties of nanofluids will help to fulfil such a potential. There are other unique size-induced and shapedependent properties associated with noble metal nanostructures that, although widely used in biosensing [143], which remain relatively unexplored in the context of surface forces. As discussed below, these properties could cause nanostructures to interact and correlate with each other, and once the nanostructures are under confinement of two surfaces, such inter-nanostructure correlation could modify the forces between the confining surfaces in novel and sophisticated ways.

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reaching a maximum before decreasing with further increase of the grafting density, as shown in Fig. 13(b). This behaviour was related to the increase in osmotic pressure due to the absorption of the nanospheres in the brush layers. Such modifications to the properties of polymer brushes and polyelectrolyte surface layers, dependent on and thus tuneable by the size and concentration of nanostructures in nanofluids, represent novel mechanisms for mediating surface force and remain to be fully explored. 5. Outstanding questions and challenges In light of the above overview of recent studies, it transpires that the range and magnitude of surface forces can be readily influenced by nanofluids, and that nanostructures of narrow size distribution, various shapes (such as spheres [131–133], rods [134,135], plates [136–138], cubes [2,139] and polyhedrons [140], etc.) and different materials (such as metal oxides [141], noble metals [131–133], polymers [142], and DNA [140]) now pose as a new paradigm for mediating surface forces. Further systematic work, both experimental and theoretical, on the effects of the nanostructure size, shape, surface chemistry, as well as the surrounding base fluid conditions and nature of the

Despite many previous studies on the behaviour of nanofluids and their numerous applications, our understanding of nanofluids under confinement, and how they mediate interactions between confining surfaces, still remains incomplete, with many outstanding questions remaining. When fluids containing nanostructures are subjected to confinement from a separation D ~ μms to intimate contact (i.e. D ~ 0) the interactions between the confining surfaces are largely dominated by the degree of confinement the nanostructures are subjected to, as determined by the characteristic length scales or finite sizes of the nanostructures compared to the surface separation D. Referring to Fig. 1, one can consider the degree of confinement as a ratio ζ between D and the characteristic length scales of nanostructures, i.e. ζ = D / (d, ϕ, l, t or b). As the surfaces are brought closer to each other and ζ decreases it can then be postulated a nanofluid will undergo several distinct regimes of confinement. In the mild confinement regime or at large D, e.g. ζ >~10 [2], the volume and charge distribution of the nanostructures are likely to come into play and they may act as giant molecules or ions whose electric and dielectric properties are non-negligible when surface forces are considered. For instance in aqueous media, if charged (e.g. by carrying a stabilising polyelectrolyte), the nanostructures may not be simplistically thought of as multiply-charged counterions of volumeless point charges but their own surface charge distribution, shape and size must be considered, as discussed further in the next section. The multibody interaction between the nanostructures, the confining surfaces and the surrounding solution are also likely to become more apparent and modify the total surface forces. In a stronger confinement regime, e.g. ζ b ~10, the surfaces are expected to experience structural forces or solvation forces due to the packing of nanostructures, thus exhibiting oscillations between attraction and repulsion with an oscillation period comparable to the characteristic length scales of the nanostructures [86,93–95,98]. In this regime some of the features of the liquid crystal phase transitions observed in the bulk dispersion have been observed to be recovered. For example, in nanofluids containing nano-plates, such bulk phase transitions from isotropic to nematic, to columnar phases have been found to occur as the volume fraction increases [137,138]. During this packing process, the orientational entropy loss due to the alignment of the nanostructures must be compensated by a translational entropy gain. In contrast, confinement of nanostructures could induce a solidification of the nanofluid or a glass transition which may lead to a drastically augmented viscosity and thus, an increase in the shear forces experienced by the confining surfaces. In binary mixtures of two different sizes or shapes of nanostructures (e.g. spheres with two very different diameters or sphere/rod mixtures), the two characteristic oscillatory structural force curves of each component have also been observed to superimpose on each other in this regime [93]. However, how the dynamic behaviours of nanofluids would depend on the size, shape and surface charge of the nanostructures in binary mixtures still remains unclear. In the very high confinement regime or close to contact, e.g. ζ ~ 2–4, the surface forces have been shown to be dominated by the

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interactions between the surface and the nanostructures. For example, if the nanostructures do not adsorb on the surface, a depletion attraction between the surfaces is expected at ζ ~ 1 due to the expulsion of the nanostructures between the surfaces, similar to the depletion attraction mediated by non-adsorbing polymers. Specifically for nanostructures with two different length scales (nanoplates and nanorods), it is likely that two different corresponding onset separations corresponding to the two length scales for such depletion attractions would exist. Conversely if the nanostructures adsorb to the surface, their size and geometry may be translated to surface roughness modifying the frictional properties of the surfaces which have been found to depend on the size [144] and surface coverage [145] of the nanostructures. 5.2. Complications and modifications to the DLVO theory The DLVO theory of colloid stability has been experimentally verified by numerous surface force measurements across a wide range of systems and separation distances. However, the limitations of the DLVO theory, especially at small separations, are well recognised [146]. Previously, there have been several attempts to modify or extend the DLVO theory, for example to include ion specific effects such as finite ion size or polarisability [147,148]. However, the applicability of the DLVO theory to surface forces mediated by nanofluids is yet to be verified. In the DLVO theory both the surfaces and the intervening medium in a system are treated theoretically as structureless continuums. However, in the case of nanofluids the intervening medium is no-longer structureless but instead contains a background fluid and nanostructures which have their own shape, size and surface charges associated with them. Thus, the van der Waals forces can no longer be solely determined by the interaction between the medium and the macroscopic surfaces. Instead, the many body interactions between the nanostructures, surfaces and surrounding base fluid molecules may be cumulative and added to the DLVO forces in a fashion similar to Hofmeister effects [147]. However, how this would contribute to the total surface force measured across nanofluids is unclear. In addition, if the nanostructures are charged and adsorb onto oppositely charged surfaces, the van der Waals forces associated with the surfaces will not be uniform. In aqueous media and if charged, the nanostructures may also not be simplistically thought of as multiply-charged counterions and the assumption that ions act like point-like charges in the DLVO theory is likely to no longer hold. Alternatively, the nanostructures size, shape and surface charge distribution are likely to modify the organisation of charged nanostructures away from a charged interface, leading to a density distribution which can no longer be simply described by the Poisson–Boltzmann equation. In addition, if the charged nanostructures adsorb onto oppositely charged surfaces the resultant surface charge distribution of the surfaces will be far from uniform and may also be mobile, facilitated by the mobility of nanostructures on the surface. Already the effect of discrete surface charges on the electrical double layer has been found to have a pronounced effect by number of theoretical studies, particularly in the presence of multiply charged ions and large average surface charges [149]. Such ion correlation effects have been difficult to probe experimentally. Carefully designed experiments involving nanofluids, by controlling the distribution, density and mobility of charged nanostructures at the surface, would represent model systems in which theoretical models on ion-correlation effects could be tested. 5.3. Invasion of surface soft matter structures by nanofluids Further to the considerations associated with the finite size of nanostructures, the interactions mediated by nanofluids may be further complicated in the presence of soft matter surface structures

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(e.g. polymer brushes and surfactant bilayers). The influence of nanostructures on the interactions between absorbed soft matter structures has only been investigated in a small number of studies. From such studies it has already been shown the nanostructures can have a pronounced effect on the surface forces and dynamic surface properties of these structures. How nanofluids might perturb and invade such surface structures depends on the nanostructure shape, size, surface chemistry and concentration, combined with the parameters that characterise soft surface structures, such as the surface coverage and density, layer thickness, polymer brush grafting density and surface charge. How these parameters corroborate to mediate desired surface forces remains to be further investigated. Such interactions between nanofluids and soft matter structures also bear fundamental relevance to how nanostructures interact with complex biological tissue surfaces such as biopolymers and lipids in the topical and recently developed subject termed nanotoxicology [150]. For instance, in a pioneering clinical application of nanoparticles in human brains, iron oxide nanoparticles coated with dextran (a sugar) were found to slip through the cell lining of the walls of blood vessels in the brain and adhere favourably to brain tumour and lesion cells, subsequently enabling magnetic resonance (MR) visualization of the loci of these cancer cells. Bare gold (Au) nanoparticles (d ~ 2 nm) have also been found to have similar capabilities of targeting and binding to tumour cells [151]. It is also worth noting that nanoparticles are most commonly intravenously introduced and thus are subjected to surface forces from biological cells and tissue surfaces before reaching their designated targets. It is thus the current emerging view that the effects of nanostructures on the interactions between biological surfaces would ultimately determine their effectiveness in biomedical applications and their (nano)toxicity [152–154], although our understanding of such interactions on a fundamental level is lacking. 5.4. Tuneable surface forces through controlling nanofluid properties In addition to the pertinent sizes of the nanostructures, there are other unique size-induced properties associated with noble metal nanostructures that are yet to be fully explored. For example, in a phenomena known as localised surface plasmon resonance (LSPR), noble metal nanoparticles (e.g. Au nanospheres or rods) exhibit a UV–visible absorption band absent in the spectrum of the bulk metal when the incident photon frequency is resonant with the collective excitation of their conduction band electrons [155,156]. The excitation of these surface plasmon modes can lead to a number of unique phenomena including additional dielectric dipoles and, if the input photon energy is sufficiently large, local heating of the medium and even alloying of nanoparticles. Furthermore, the resonant frequency (or the optical absorption wavelength) of LSPR has been found to depend on the shape and size of the nanostructures [141,156,157]. As a result, noble metal nanostructures may be exploited as an externally tuneable parameter in mediating surface forces. For example, the self-alignment of nanostructures could be promoted via enhancing their dipole–dipole interactions and surfaces or their solvency tuned by local heating, if they are coated with temperature responsive soft matter e.g. poly(NIPAAm) polymer brushes [158]. As another consequence of local heating, nanobubbles may also be generated which in turn could contribute to surface forces [141]. Due to the dependence of LSPR on the shape and size of nanostructures, shape factors may also couple with the LSPR, to give rise to unusual optical properties and in turn enable a range of novel applications such as surface-enhanced spectroscopy, optical filters, and sensors [157]. Further unusual properties related to the low-dimensionality of nanostructures include enhanced magnetization in metal nanostructures, such as cobalt (Co) [133], palladium (Pd) [159] and gold (Au) nanospheres [160]. By applying a magnetic field the alignment and

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interactions between the nanostructures may be adjusted and thus employed to control the surface forces that magnetic nanostructures mediate. In addition, self-assembled secondary structures may be formed by these magnetic nanoparticles. For example, whilst d ~ 20 nm Co nanoparticles (coated with a surfactant) remain dispersed in toluene, Co nanoparticles of similar size d ~ 25–30 nm have been found to self-assemble into bracelets (or rings) with a diameter of d ~ 100 nm [133]. These size-dependent and tuneable secondary structures are expected to give rise to interesting characteristics in the surface forces they mediate. 6. Concluding remarks and outlook Interactions between surfaces in intimate contact or at close proximity are critically important in many technological applications and biological processes. Classically, the range and magnitude of these surface forces are readily furnished by various simple and complex molecules, particularly soft condensed matter such as surfactants and polymers, both at the surfaces and between them. However, with the rapid development of nanotechnology, nanofluids containing nanostructures of various shapes, sizes and surface chemistry now pose as a new paradigm for mediating surface forces. There have already been a number studies which have investigated the effects of such properties of nanofluids on the equilibrium and dynamic forces they mediate. In general the magnitude and range of the equilibrium forces measured across nanofluids have been found to be strongly affected by nanostructure size, aspect ratio and concentration due to their enhanced steric contribution, packing efficiency considerations and tendency for aggregation and secondary structure formation. Moreover, they have also been found to be sensitive to the surrounding solution conditions, such as salt concentration, pH or the presence of soft matter, as well as the surface chemistry of the nanostructures, which can modify their interaction with the surrounding solution and confining surfaces. Similarly, the dynamic properties of nanofluids, such as their viscosity, have been shown to be influenced by the properties of nanostructures and their surrounding base fluid conditions. In general the wettability and viscosity of nanofluids have been found to increase with decreasing nanostructure size and upon increasing their concentration. In addition, the friction–reduction ability of nanofluids has also been found to be improved by increasing the aspect ratios of nanostructures. Similarly, the base fluid conditions and the presence of polymers and surfactants have been shown to impart a strong influence on the dynamic properties of nanofluids, such as their wetting behaviour. Although, many surface force and dynamic studies have been conducted on nanofluids and various trends between their characteristic features and properties identified, many challenges and opportunities remain. In particular, “soft” nanofluids containing self-assembled aggregates or hard nanostructures coated with soft structures remain relatively under-explored compared to “hard” nanofluid systems. Furthermore, how nanostructures may invade or perturb surface anchored polymer and surfactant layers and how this would affect their equilibrium and dynamic behaviours also remain unclear. In particular, the understanding of such mechanisms in lipid or biopolymer membranes would provide important insight into the interactions of nanofluids with biological systems such as cell membranes, a topic that is relevant to the toxicity of nanofluids. Furthermore, new tuneable and unique properties, such as the electronic excitation of noble metal nanofluids via LSPR and enhanced magnetism in metal nanofluids, also present exciting and novel mechanisms for controlling surface forces which are yet to be fully explored. In order to address the outstanding questions and challenges in the field, it will require the careful design of experimental procedures and the judicious choice of well-defined nanofluids, so that the complex and correlating effects can be distinguished and examined.

Whilst the SFA has been shown to be a definitive technique for the direct measurement of the surface forces, colloidal probe AFM is a versatile technique that can be applied to a wider range of systems. For instance, the effect of LSPR on surface forces of confined nanostructures may be explored by applying resonant wavelengths into the micro- or nano-cavity formed by confining surfaces in the SFA and AFM. Similarly, by applying an external magnetic field the effect of the alignment of magnetic nanoparticles along the direction of the applied field on the surface forces mediated by magnetic nanostructures may be investigated. In addition to force measurements, the characteristics of nanostructures and their inter-nanostructure interactions (i.e. many body interactions) in nanofluids need to be characterised in order for surface force measurements across nanofluids to be fully analysed and understood. Small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) are well established methods for obtaining such information for suitable nanofluid systems, and both the size, shape of the nanostructures and their inter-nanostructure interactions can be elucidated from the structure and form factors attainable from SAXS and SANS. In addition to experimental techniques and methods, there are also opportunities and challenges for new theoretical and simulation studies in this field. That is, how the existing surface force theories (DLVO, Alexander-de Gennes, etc.) should be modified to account for the complex effects of size, shape, volume fraction and surface chemistry of nanostructures in combination with the physicochemical properties of the base fluid of an intervening nanofluid on the surface forces they mediate. Acknowledgements Funding from the Engineering and Physical Science Research Council (EPSRC; EP/H034862/1), the Royal Society (UK) and the European Research Council (ERC) is gratefully acknowledged. GAP is the recipient of an EPSRC DTA PhD studentship at School of Chemistry, the University of Bristol. We would also like to thank Tim Snow for his careful reading of and helpful comments on our manuscripts. References [1] Choi US. Enhancing thermal conductivity of fluids with nanoparticles. New York: The American Society of Mechanical Engineers; 1995. [2] Grzelczak M, Perez-Juste J, Mulvaney P, Liz-Marzan LM. Chem Soc Rev 2008;37: 1783–91. [3] Brust M, Kiely CJ. Colloids Surf A Physicochem Eng Asp 2002;202:175–86. [4] Mott D, Galkowski J, Wang L, Luo J, Zhong C-J. Langmuir 2007;23:5740–5. [5] Chen X, Mao SS. Chem Rev 2007;107:2891–959. [6] Fan Z, Lu JG. J Nanosci Nanotechnol 2005;5:1561–73. [7] Gupta AK, Gupta M. Biomaterials 2005;26:3995–4021. [8] Lu M, Zhou J, Wang L, Zhao W, Lu Y, Zhang L, et al. J Nanomater 2010:2010. [9] Shim SE, Lee H, Choe S. Macromolecules 2004;37:5565–71. [10] Sau TK, Rogach AL. Adv Mater 2010;22:1781–804. [11] Chen S, Carroll DL. Nano Lett 2002;2:1003–7. [12] Yang Z, Huck WTS, Clarke SM, Tajbakhsh AR, Terentjev EM. Nat Mater 2005;4: 486–90. [13] Gou L, Murphy CJ. Chem Mater 2005;17:3668–72. [14] Wang X-Q, Mujumdar AS. Int J Therm Sci 2007;46:1–19. [15] Israelachvili JN. Intermolecular and surface forces. London: Academic Press; 1991. [16] Horn RG. J Am Ceram Soc 1990;73:1117–35. [17] Wong KV, De Leon O. Adv Mech Eng 2010:2010. [18] Yu WH, France DM, Routbort JL, Choi SUS. Heat Transfer Eng 2008;29:432–60. [19] Masuda H, Ebata A, Teramae K, Hishinum N. Netsu Bussei (Japan) 1993;4: 227–33. [20] Choi SUS, Zhang ZG, Lockwood FE, Grulke EA. Appl Phys Lett 2001;79:2252. [21] Eastman JA, Choi SUS, Li S, Yu W, Thompson LJ. Appl Phys Lett 2001;78:718–20. [22] Liu MS, Lin MCC, Huang IT, Wang CC. Int Commun Heat Mass Transfer 2005;32: 1202–10. [23] Jang SP, Choi SUS. Appl Therm Eng 2006;26:2457–63. [24] Nguyen CT, Roy G, Gauthier C, Galanis N. Appl Therm Eng 2007;27:1501–6. [25] Kole M, Dey TK. Exp Therm Fluid Sci 2010;34:677–83. [26] Leong KY, Saidur R, Kazi SN, Mamun AH. Appl Therm Eng 2010;30:2685–92. [27] Kao M-J, Chang H, Wu Y-Y, Tsung T-T, Lin H-M. J Chin Soc Mech Eng 2007;28: 195–200. [28] Kao M-J, Ting C-C, Lin B-F, Tsung T-T. J Test Eval 2008;36:186.

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