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Four nonhalogenated ionic liquids (ILs) based on the same phosphonium cation are investigated in terms of the anion suitability for enhancing the lubricity of a biodegradable oil. For all test cond ... more...

It is shown that the air–liquid interface can be made to display the same rich curvature phenomena as common lyotropic liquid crystal systems. Through mixing an insoluble, naturally occurring, branched fatty acid, with an unbranched fatty acid of the same length, systematic variation in the packing constraints at the air–water interface could be obtained. The combination of atomic force microscopy and neutron reflectometry is used to demonstrate that the water surface exhibits significant tuneable topography. By systematic variation of the two fatty acid proportions, ordered arrays of monodisperse spherical caps, cylindrical sections, and a mesh phase are all observed, as well as the expected lamellar structure. The tuneable deformability of the air–water interface permits this hitherto unexplored topological diversity, which is analogous to the phase elaboration displayed by amphiphiles in solution. It offers a wealth of novel possibilities for the tailoring of nanostructure. more...

The effect of water on the electroactive structuring of a tribologically relevant ionic liquid (IL) when dispersed in a polar solvent has been investigated at a gold electrode interface using neutron reflectivity (NR). For all solutions studied, the addition of small amounts of water led to clear changes in electroactive structuring of the IL at the electrode interface, which was largely determined by the bulk IL concentration. At a dilute IL concentration, the presence of water gave rise to a swollen interfacial structuring, which exhibited a greater degree of electroresponsivity with applied potential compared to an equivalent dry solution. Conversely, for a concentrated IL solution, the presence of water led to an overall thinning of the interfacial region and a crowding-like structuring, within which the composition of the inner layer IL layers varied systematically with applied potential. Complementary nanotribotronic atomic force microscopy (AFM) measurements performed for the same IL concentration, in dry and ambient conditions, show that the presence of water reduces the lubricity of the IL boundary layers. However, consistent with the observed changes in the IL layers observed by NR, reversible and systematic control of the friction coefficient with applied potential was still achievable. Combined, these measurements provide valuable insight into the implications of water on the interfacial properties of ILs at electrified interfaces, which inevitably will determine their applicability in tribotronic and electrochemical contexts. more...

Neutron reflectivity (NR) measurements have been employed to study the interfacial structuring and composition of electroresponsive boundary layers formed by an ionic liquid (IL) lubricant at an electrified gold interface when dispersed in a polar solvent. The results reveal that both the composition and extent of the IL boundary layers intricately depend on the bulk IL concentration and the applied surface potential. At the lowest concentration (5% w/w), a preferential adsorption of the IL cation at the gold electrode is observed, which hinders the ability to electro-induce changes in the boundary layers. In contrast, at higher IL bulk concentrations (10 and 20% w/w), the NR results reveal a significantly larger concentration of the IL ions at the gold interface that exhibit significantly greater electroresponsivity, with clear changes in the layer composition and layer thickness observed for different potentials. In complementary atomic force microscopy (AFM) measurements on an electrified gold surface, such IL boundary layers are demonstrated to provide excellent friction reduction and electroactive friction (known as tribotronics). In agreement with the NR results obtained, clear concentration effects are also observed. Together such results provide valuable molecular insight into the electroactive structuring of ILs in solvent mixtures, as well as provide mechanistic understanding of their tribotronic behaviours. more...

Control of the interfacial structures of ionic liquids (ILs) at charged interfaces is important to many of their applications, including in energy storage solutions, sensors and advanced lubrication technologies utilising electric fields. In the case of the latter, there is an increasing demand for the study of non-halogenated ILs, as many fluorinated anions have been found to produce corrosive and toxic halides under tribological conditions. Here, the interfacial structuring of a series of four imidazolium ILs ([C(n)C(1)Im]) of varying alkyl chain lengths (n = 5, 6, 7, 10), with a non-halogenated borate-based anion ([BOB]), have been studied at charged interfaces using sum frequency generation (SFG) spectroscopy and neutron reflectivity (NR). For all alkyl chain lengths, the SFG spectra show that the cation imidazolium ring responds to the surface charge by modifying its orientation with respect to the surface normal. In addition, the combination of SFG spectra with electrochemical NR measurements reveals that the longest alkyl chain length (n = 10) forms a bilayer structure at all charged interfaces, independent of the ring orientation. These results demonstrate the tunability of IL interfacial layers through the use of surface charge, as well as effect of the cation alkyl chain length, and provide valuable insight into the charge compensation mechanisms of ILs. more...

A simple, insoluble monolayer of fatty acid is shown to induce 3D nanotexturing of the water–air interface. This advance has been achieved through the study of monolayers of a methyl-branched long chain fatty acid, analogous to those found on the surface of hair and wool, directly at the water–air interface. Specular neutron reflectometry combined with AFM probing of deposited monolayers shows pronounced 3D surface domains, which are absent for unbranched analogues and which are attributed to hydrocarbon packing constraints. The resulting surface topographies of the water far exceed the height perturbation that can be explained by the presence of capillary waves of a free liquid surface. These have hitherto been considered the only source of perturbation of the flatness of a planar water interface under gravity in the absence of topographical features from the presence of extended, globular or particulate matter. This amounts to a paradigm shift in the study of interfacial films and opens the possibility of 3D texturing of the water–air interface. more...

We investigate the interfacial properties of the non-halogenated ionic liquid (IL), trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P6,6,6,14][BMB], in proximity to solid surfaces, by means of surface force measurement. The system consists of sharp atomic force microscopy (AFM) tips interacting with solid surfaces of mica, silica, and gold. We find that the force response has a monotonic form, from which a characteristic steric decay length can be extracted. The decay length is comparable with the size of the ions, suggesting that a layer is formed on the surface, but that it is diffuse. The long alkyl chains of the cation, the large size of the anion, as well as crowding of the cations at the surface of negatively charged mica, are all factors which are likely to oppose the interfacial stratification which has, hitherto, been considered a characteristic of ionic liquids. The variation in the decay length also reveals differences in the layer composition at different surfaces, which can be related to their surface charge. This, in turn, allows the conclusion that silica has a low surface charge in this aprotic ionic liquid. Furthermore, the effect of temperature has been investigated. Elevating the temperature to 40 &deg, C causes negligible changes in the interaction. At 80 &deg, C and 120 &deg, C, we observe a layering artefact which precludes further analysis, and we present the underlying instrumental origin of this rather universal artefact. more...

Understanding and harnessing the coupling between lubrication pressure and elasticity provides materials design strategies for applications such as adhesives, coatings, microsensors, and biomaterials. Elastic deformation of compliant solids caused by viscous forces can also occur during dynamic force measurements in instruments such as the surface forces apparatus (SFA) or the atomic force microscope (AFM). We briefly review hydrodynamic interactions in the presence of soft, deformable interfaces in the lubrication limit. More specifically, we consider the scenario of two surfaces approaching each other in a viscous fluid where one or both surfaces is deformable, which is also relevant to many force measurement systems. In this article the basic theoretical background of the elastohydrodynamic problem is detailed, followed by a discussion of experimental validation and considerations, especially for the role of elastic deformation on surface forces measurements. Finally, current challenges to our understanding of soft hydrodynamic interactions, such as the consideration of substrate layering, poroelasticity, viscoelasticity, surface heterogeneity, as well as their implications are discussed. more...

Using neutron reflectivity, the electro-responsive structuring of the non-halogenated ionic liquid (IL) trihexyl(tetradecyl)phosphonium-bis(mandelato)borate, [P6,6,6,14][BMB], has been studied at a ... more...

We present the multifunctional force microscope (MFM), a normal and lateral force-measuring instrument with in situ imaging. In the MFM, forces are calculated from the normal and lateral deflection of a cantilever as measured via fiber optic sensors. The motion of the cantilever is controlled normally by a linear micro-translation stage and a piezoelectric actuator, while the lateral motion of the sample is controlled by another linear micro-translation stage. The micro-translation stages allow for travel distances that span 25 mm with a minimum step size of 50 nm, while the piezo has a minimum step size of 0.2 nm, but a 100 μm maximum range. Custom-designed cantilevers allow for the forces to be measured over 4 orders of magnitude (from 50 μN to 1 N). We perform probe tack, friction, and hydrodynamic drainage experiments to demonstrate the sensitivity, versatility, and measurable force range of the instrument. more...

Despite its importance in many applications and processes, a complete and unified view on how nano- and microscale asperities influence hydrodynamic interactions has yet to be reached. In particular, the effects of surface structure can be expected to become more dominant when the length scale of the asperities or textures becomes comparable to that of the fluid flow. Here we analyze the hydrodynamic drainage of a viscous silicone oil squeezed between a smooth plane and a surface decorated with hexagonal arrays of lyophilic microsized cylindrical posts. For all micropost arrays studied, the periodicity of the structures was much larger than the separation range of our measurements. In this thin channel geometry, we find the hydrodynamic drainage and separation forces for the micropost arrays cannot be fully described by existing boundary condition models for interfacial slip or a no-slip shifted plane. Instead, our results show that the influence of the microposts on the hydrodynamic drag exhibits three distinct regimes as a function of separation. For large separations, a no slip boundary condition (Reynolds theory) is observed for all surfaces until a critical (intermediate) separation, below which the position of the no-slip plane scales with surface separation until reaching a maximum, just before contact. Below this separation, a sharp decrease in the no-slip plane position then suggests that a boundary condition of a smooth surface is recovered at contact. more...

Understanding dendrimer structures and their interactions in concentrated solutions is important to a wide range of applications, such as drug delivery and lubrication. However, controversy has persisted concerning whether, when confined to proximity, dendrimers would entangle as observed for polymer systems, or act as deformable spheres. Furthermore, how such behavior may be related to their size-dependent molecular architecture remains unclear. Using small-angle X-ray scattering (SAXS), the intermolecular interactions and structures in aqueous nanofluids containing three generations of carboxyl-terminated poly(amidoamine) (PAMAM) dendrimers (G0.5, Rg = 9.3 Å; G3.5, Rg = 22.6 Å; G5.5, Rg = 39.9 Å, where Rg is the radius of gyration) over a mass fraction range 0.005 ≤ x ≤ 0.316 have been studied. In the highly concentrated regime (x ≥ 0.157), we observe that the solution properties depend on the dendrimer generation. Our results suggest that the smaller G0.5 dendrimers form a highly entangled polymer melt, while the larger dendrimers, G3.5 and G5.5, form densely packed and ordered structures, in which the individual dendrimers exhibit some degree of mutual overlap or deformation. Our results demonstrate the tunability of interdendrimer interactions via their molecular architecture, which in turn may be harnessed to control and tailor the physical properties of dendrimer nanofluids. more...

Understanding the frictional properties of nanostructured surfaces is important because of their increasing application in modern miniaturized devices. In this work, lateral force microscopy was used to study the frictional properties between an AFM nanotip and surfaces bearing well-defined nanodomes comprising densely packed prolate spheroids, of diameters ranging from tens to hundreds of nanometers. Our results show that the average lateral force varied linearly with applied load, as described by Amontons’ first law of friction, although no direct correlation between the sample topographic properties and their measured friction coefficients was identified. Furthermore, all the nanodomed textures exhibited pronounced oscillations in the shear traces, similar to the classic stick–slip behavior, under all the shear velocities and load regimes studied. That is, the nanotextured topography led to sustained frictional instabilities, effectively with no contact frictional sliding. The amplitude of the stick–slip oscillations, σf, was found to correlate with the topographic properties of the surfaces and scale linearly with the applied load. In line with the friction coefficient, we define the slope of this linear plot as the stick–slip amplitude coefficient (SSAC). We suggest that such stick–slip behaviors are characteristics of surfaces with nanotextures and that such local frictional instabilities have important implications to surface damage and wear. We thus propose that the shear characteristics of the nanodomed surfaces cannot be fully described by the framework of Amontons’ laws of friction and that additional parameters (e.g., σf and SSAC) are required, when their friction, lubrication, and wear properties are important considerations in related nanodevices. more...

Despite extensive studies with many experimental techniques, the morphology and structure of the self-assembled aggregates of quaternary alkyl ammonium bromides (CnTABs; where n denotes the number of hydrocarbons in the surfactant tail) at the solid-liquid interface remains controversial, with results from atomic force microscopy (AFM) imaging pointing to a variety of surface aggregates such as cylinders and surface micelles, whilst surface force measurements and neutron reflectivity (NR) measurements reporting bilayer structures. Using a home-built liquid cell that employs the "bending mica" method, we have performed unprecedented synchrotron X-ray reflectometry (XRR) measurements to study the adsorption behaviour of a C nTAB series (n = 10, 12, 14, 16 and 18) at the mica-water interface at different surfactant concentrations. We find that our XRR data cannot be described by surface aggregates such as cylindrical and spherical structures reported by AFM studies. In addition we have observed that the bilayer thickness, surface coverage and the tilt angle all depend on the surfactant concentration and surfactant hydrocarbon chain length n, and that the bilayer thickness reaches a maximum value at approximately the critical micellisation concentration (∼1 cmc) for all the CnTABs investigated. We propose that CnTABs form disordered bilayer structures on mica at concentrations below cmc, whilst at ∼1 cmc they form more densely packed bilayers with the tails possibly tilted at an angle θt ranging from ∼40 to 60° with respect to the surface normal in order to satisfy the packing constraints due to the mica lattice charge, i.e. so that the cross-section area of the tilted chain would match that of the area of the lattice charge (As ≅ 46.8 Å2). As the surfactant concentration further increases, we find that the bilayer thickness decreases, and we ascribe this to the desorption of surfactant molecules, which recovers certain disorder and fluidity in the chain and thus leads to interdigitated bilayers again. In light of our XRR results, previously unattainable at the mica-water interface, we suggest that the surface aggregates observed by AFM could be induced by the interaction between the scanning probe and the surfactant layer, thus representing transient surface aggregation morphologies; whereas the CnTAB bilayers we observe with XRR are intrinsic structures under quiescent conditions. The suggestion of such quiescent bilayers will have fundamental implications to processes such as lubrication, self-assembly under confinement, detergency and wetting, where the morphology and structure of surfactant layers at the solid-liquid interface is an important consideration. © 2013 The Royal Society of Chemistry. more...

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. more...

With nanotextured surfaces and interfaces increasingly being encountered in technological and biomedical applications, there is a need for a better understanding of frictional properties involving such surfaces. Here we report friction measurements of several nanostructured surfaces using an Atomic Force Microscope (AFM). These nanostructured surfaces provide well defined model systems on which we have tested the applicability of Amontons' laws of friction. Our results show that Amontonian behaviour is observed with each of the surfaces studied. However, no correlation has been found between measured friction and various surface roughness parameters such as average surface roughness (R(a)) and root mean squared (rms) roughness. Instead, we propose that the friction coefficient may be decomposed into two contributions, i.e., μ = μ(0) + μ(g), with the intrinsic friction coefficient μ(0) accounting for the chemical nature of the surfaces and the geometric friction coefficient μ(g) for the presence of nanotextures. We have found a possible correlation between μ(g) and the average local slope of the surface nanotextures. more...

The self-assembly behaviour, structure, and consequently the electronic properties of electroactive organic molecules can differ significantly from those of the bulk material when confined to thin films. Here we have examined the self-organised in-plane and out-of-plane structures of aniline oligomers in thin films using surface-sensitive grazing-incidence X-ray scattering (GIXS). Thin films of the aniline tetramer (TANI) and octamer (OANI) were prepared both in their native emeraldine base (EB) oxidation state and in the doped emeraldine salt (ES) state (combined with the acid surfactant bis(ethyl hexyl)phosphate (BEHP)), using a simple drop-casting and solvent annealing process. It was found that the presence of the acid surfactant induced self-organisation into highly ordered structures. The details of these structures, such as the morphology, orientation relative to the underlying substrate and the degree of orientation were found to depend on the molecular architecture of the oligomer. The BEHP-doped TANI system formed a highly oriented hexagonal unit cell (lattice parameters: a = b = 2.53 nm, c = 2.91 nm, γ = 120°), whereas the BEHP-doped OANI complex adopted a randomly oriented lamellar structure (d-spacing = 2.25 nm). Such detailed structural information reveals that the self-assembly behaviour and the packing of oligomer–BEHP complexes, when confined to thin films, are indeed different to that of the bulk phase materials. Furthermore, the molecular architecture of the oligomers directly influenced the structural changes of the doped films in response to in situ thermal treatment. These results demonstrate that through a simple processing route the morphology of electroactive oligomer films can be tailored by molecular design. These findings are important to future applications where thin film structure is a crucial consideration for device function and performance. more...