Modified Surface Forces Apparatus Research Articles

Direct Measurement of Lateral Correlations under Controlled Nanoconfinement.

Lateral correlations along hydrophobic surfaces whose separation can be varied continuously are measured by x-ray scattering using a modified surface force apparatus coupled with synchrotron radiation, named SFAX. A weak isotropic diffuse scattering along the equatorial plane is revealed for mica surfaces rendered hydrophobic and charge neutral by immersion in cationic surfactant solutions at low concentrations. The peak corresponds to a lateral surface correlation length ξ≈12 nm, without long-range order. These findings are compatible with the atomic force microscopy imaging of a single surface, where adsorbed surfactant stripes appear surrounded by bare mica zones. Remarkably, the scattering patterns remain stable for gap widths D larger than the lateral period but change in intensity and shape (to a lesser extent) as soon as D<ξ. This evolution codes for a redistribution of counterions (counterion release from antagonistic patches) and the associated new x-ray labeling of the patterns. The redistribution of counterions is also the key mechanism to the long-range electrostatic attraction between similar, overall charge-neutral walls, reported earlier.

Advanced Characterization of a Stabilized Solid Electrolyte Interphase on Silicon Anodes for Lithium-Ion Batteries By in-Situ Electrochemical Surfaces Forces Apparatus

The unsatisfactory behaviour of the solid electrolyte interphase (SEI) on silicon-based anodes still prevents the commercial introduction of silicon in lithium-ion batteries – a milestone which would be a huge leap forward towards a sustainable energy use.[1] Our group has recently developed an electrolyte mixture improving the cycling performance of a silicon-based lithium-ion battery. For understanding the stabilizing effect of this novel electrolyte on the SEI, advanced characterization techniques had to be employed, such as a modified Surface Forces Apparatus (SFA) and synchrotron-based Hard X-ray Photoelectron Spectroscopy (HAXPES). So far, the SFA was used to research fundamental biologic and solution based interfacial processes.[2] Here we present a detailed investigation of the SEI on a silicon anode by designing a battery SFA cell for in-situ electrochemical measurements. This enabled us to monitor the SEI thickness during the different stages of the SEI formation on both silicon and graphite surfaces. Additionally, we report the mechanical properties of the SEI for different electrolytes which can also be addressed by the SFA setup. HAXPES data collected on a modified SEI on silicon based anodes reveal a complex nanostructure and provide an additional insight into the stabilization mechanism of the electrolyte mixture. Coin-cell testing confirm the stabilizing effect of the novel electrolyte mixture on the battery performance. The electrochemical battery SFA will be an important tool for evaluating novel electrolyte compositions and their effect on the mechanical properties and growth mechanism of the interfacial SEI. The introduction of this in-situ battery test cell for SFA opens the way for both fundamental interfacial insights as well as applied material studies in battery research. REFERENCES 1. E.C. Evarts, To the limits of lithium, Nature, 2015, 526, 593-595.2. J. Israelachvili et al., Recent advances in the surface forces apparatus (SFA) technique, Rep. Prog. Phys., 2010, 73, 1-16. Figure 1

Variation of Local Surface Properties of an Air Bubble in Water Caused by Its Interaction with Another Surface.

Surface and hydrodynamic forces acting between an air bubble and a flat mica surface in surfactant-free water and in 1 mM KCl solution have been investigated by observing film drainage using a modified surface force apparatus (SFA). The bubble shapes observed with the SFA are compared to theoretical profiles computed from a model that considers hydrodynamic interactions, surface curvature, and disjoining pressure arising from electrical double layer and van der Waals interactions. It is shown that the bubble experiences double-layer forces, and a final equilibrium wetting film between the bubble and mica surfaces is formed by van der Waals repulsion. However, comparison with the theoretical model reveals that the double-layer forces are not simply a function of surface separation. Rather, they appear to be changed by one of more of the following: the bubble's dynamic deformation, its proximity to another surface, and/or hydrodynamic flow in the aqueous film that separate them. The same comments apply to the hydrodynamic mobility or immobility of the air-water interface. Together the results show that the bubble's surface is "soft" in two senses: in addition to its well-known deformability, its local properties are affected by weak external forces, in this case the electrical double-layer interactions with a nearby surface and hydrodynamic flow in the neighboring aqueous phase.

Stick-slip friction of gecko-mimetic flaps on smooth and rough surfaces.

The discovery and understanding of gecko 'frictional-adhesion' adhering and climbing mechanism has allowed researchers to mimic and create gecko-inspired adhesives. A few experimental and theoretical approaches have been taken to understand the effect of surface roughness on synthetic adhesive performance, and the implications of stick-slip friction during shearing. This work extends previous studies by using a modified surface forces apparatus to quantitatively measure and model frictional forces between arrays of polydimethylsiloxane gecko footpad-mimetic tilted microflaps against smooth and rough glass surfaces. Constant attachments and detachments occur between the surfaces during shearing, as described by an avalanche model. These detachments ultimately result in failure of the adhesion interface and have been characterized in this study. Stick-slip friction disappears with increasing velocity when the flaps are sheared against a smooth silica surface; however, stick-slip was always present at all velocities and loads tested when shearing the flaps against rough glass surfaces. These results demonstrate the significance of pre-load, shearing velocity, shearing distances, commensurability and shearing direction of gecko-mimetic adhesives and provide us a simple model for analysing and/or designing such systems.

JKR Theory for the Stick–Slip Peeling and Adhesion Hysteresis of Gecko Mimetic Patterned Surfaces with a Smooth Glass Surface

Geckos are highly efficient climbers and can run over any kind of surface with impeccable dexterity due to the typical design of their hierarchical foot structure. We have fabricated tilted, i.e., asymmetric, poly(dimethylsiloxane) (PDMS) microflaps of two different densities that mimic the function of the micrometer sized setae on the gecko foot pad. The adhesive properties of these microflaps were investigated in a modified surface forces apparatus; both for normal pure loading and unloading (detachment), as well as unloading after the surfaces were sheared, both along and against the tilt direction. The tilted microflaps showed directional, i.e., anisotropic adhesive behavior when sheared against an optically smooth (RMS roughness ≈ 10 ± 8 nm) SiO2 surface. Enhanced adhesion was measured after shearing the flaps along the tilted (gripping) direction and low adhesion when sheared against the tilted (releasing) direction. A Johnson-Kendall-Roberts (JKR) theory using an effective surface energy and modulus of rigidity (stiffness) quantitatively described the contact mechanics of the tilted microflaps against the SiO2 surface. We also find an increasing adhesion and stick-slip of the surfaces during detachment which we explain qualitatively in terms of the density of flaps, considering it to increase from 0% (no flaps, smooth surface) to 100% (close-packed flaps, effectively smooth surface). Large energy dissipation at the PDMS-silica interface caused by the viscoelastic behavior of the polymer results in stick-slip peeling and hence an enhanced adhesion energy is observed during the separation of the microflaps surface from the smooth SiO2 surface after shearing of the surfaces. For structured multiple contact surfaces, hysteresis as manifested by different loading and unloading paths can be due entirely to the elastic JKR micro-contacts. These results have important implications in the design of biomimetic adhesives.

Friction and Adhesion of Gecko-Inspired PDMS Flaps on Rough Surfaces

Geckos have developed a unique hierarchical structure to maintain climbing ability on surfaces with different roughness, one of the extremely important parameters that affect the friction and adhesion forces between two surfaces. Although much attention has been paid on fabricating various structures that mimic the hierarchical structure of a gecko foot, yet no systematic effort, in experiment or theory, has been made to quantify the effect of surface roughness on the performance of the fabricated structures that mimic the hierarchical structure of geckos. Using a modified surface forces apparatus (SFA), we measured the adhesion and friction forces between microfabricated tilted PDMS flaps and optically smooth SiO(2) and rough SiO(2) surfaces created by plasma etching. Anisotropic adhesion and friction forces were measured when sliding the top glass surface along (+y) and against (-y) the tilted direction of the flaps. Increasing the surface roughness first increased the adhesion and friction forces measured between the flaps and the rough surface due to topological matching of the two surfaces but then led to a rapid decrease in both of these forces. Our results demonstrate that the surface roughness significantly affects the performance of gecko mimetic adhesives and that different surface textures can either increase or decrease the adhesion and friction forces of the fabricated adhesives.

Bubble−Solid Interactions in Water and Electrolyte Solutions

Surface forces between an air bubble and a flat mica surface immersed in aqueous electrolyte solutions have been investigated using a modified surface force apparatus. An analysis of the deformation of the air bubble with respect to the mutual position of the bubble and the mica surface, the capillary pressure, and the disjoining pressure allows the air-liquid surface electrical potential to be determined. The experiments show that a long-range, double-layer repulsion acts between the mica (which is negatively charged) and an air bubble in water and in various electrolyte solutions at low concentration, thereby indicating that the air bubble surface is negatively charged. However, there is clear evidence that charge regulation occurs at the air-water interface to maintain a constant surface potential, and as a result of this, the charge at this interface changes from negative to positive as the bubble approaches the mica surface. Because of the attraction that arises as a result of the charge reversal, a finite force is required to separate the bubble from the mica, though the mica remains wetted by the aqueous phase. At the low concentrations investigated, the potential on the gas-liquid interface is independent of the electrolyte type within experimental uncertainty.

Hydrodynamic forces involving deformable interfaces at nanometer separations

A model is developed to describe the dynamic forces acting between two deformable drops, or between one drop and a solid surface, when they are in relative axisymmetric motion at separations of ≲100nm in a Newtonian liquid. Forces arise from hydrodynamic pressure in the draining liquid film that separates the interfaces and from disjoining pressure due to repulsive or attractive surface forces. Predictions of the model are successfully compared with recent experimental measurements of the force between two micrometer-scale surfactant stabilized decane drops in water in an atomic force microscope [S. L. Carnie, D. Y. C. Chan, C. Lewis, R. Manica, and R. R. Dagastine, Langmuir 21, 2912 (2005); R. R. Dagastine, R. Manica, S. L. Carnie, D. Y. C. Chan, G. W. Stevens, and F. Grieser, Science 313, 210 (2006)] and with subnanometer resolution measurements of time-dependent deformations of a millimeter-scale mercury drop approaching a flat mica surface in a modified surface force apparatus [J. N. Connor and R. G. Horn, Faraday Discuss. 123, 193 (2003); R. G. Horn, M. Asadullah, and J. N. Connor, Langmuir 22, 2610 (2006)]. Special limits of the model applicable to small and moderate deformation regimes are also studied to elucidate the key physical ingredients that contribute to the characteristic behavior of dynamic collisions involving fluid interfaces.

Friction and adsorption of aqueous polyoxyethylene (Tween) surfactants at hydrophobic surfaces

The nanotribological responses of a series of nonionic polyoxyethylene surfactants (Tween 20, Tween 40, Tween 60, and Tween 80) were investigated after they were adsorbed from aqueous solution onto atomically smooth hydrophobic substrates. The hydrophobic surfaces were composed of a condensed monolayer of octadecyltriethoxysilane (OTE; contact angle θ > 110 ° ). The nanorheological measurements were performed using a modified surface forces apparatus after coating atomically smooth mica with these OTE monolayers, while adsorption measurements were performed using phase-modulated ellipsometry on silicon wafers coated with these same monolayers. The minimum surface–surface separation observed under high load in friction studies agreed quantitatively with the thickness obtained from ellipsometry. For Tweens 20, 40, and 60, the thickness of the adsorbed film increases with increasing alkyl chain length. Systematic investigations of the nanorheological response showed that there is a “solid-like” elastic response from confined surfactant layers, which is the case for the smallest separations to separations up to slightly larger than twice the adsorbed film thickness. In kinetic friction, these confined layers are characterized by a shear stress of approximately 3 MPa with minimal dependence on shear rate. The magnitude of the sliding shear stress is the same as the apparent yield stress at ≈3 MPa; it is independent of alkyl chain length within the Tween family of surfactants and corresponds to a nominal friction coefficient of μ ∼ 1 . A similar friction coefficient is observed for boundary lubrication on the macroscopic scale in a tribometer utilizing hydrophobic surfaces and μ ≈ 1.1 for Tweens 20, 40, and 60. These results suggest that while Tween molecules adsorb onto hydrophobic surfaces to form a robust separating layer, the lubricating properties of these layers are dominated by a highly dissipative slip plane, the same for all alkyl chain lengths.

Self-Assembled Monolayers on Mercury Probed in a Modified Surface Force Apparatus

Self-assembled monolayers (SAMs) of three thiol compounds formed on mercury are investigated by a combination of cyclic voltammetry, electrocapillary curves, and a novel method of measuring electrical double-layer properties. The last method involves a modified surface force apparatus in which a flat mica surface is pressed down toward a fixed mercury drop held beneath it, while both are immersed in aqueous electrolyte solution. Optical interference measurements are made of the mica-mercury separation as a function of electrical potential applied to the mercury, which yields information on the double-layer interaction between the two surfaces. Mercury is decorated by SAMs of 11-mercapto-1-undecanoic acid, which is shown to bring negative charge to the mercury/aqueous interface due to dissociation of the carboxylic acid groups; 11-mercapto-1-undecanol, which although it is uncharged changes the dipole potential of the interface; and 1-undecanethiol, which likewise changes the dipole potential, but by a different amount. The difference between the changes in dipole potential (90 mV) can be related to the different terminal groups of these two SAMs, -CH3 compared to -OH, that are in contact with the aqueous phase.

Triboelectrification between Smooth Metal Surfaces Coated with Self-Assembled Monolayers (SAMs)

Using a modified surface forces apparatus, we have simultaneously measured the friction and triboelectrification between both similar and dissimilar molecularly smooth hexadecanethiol-coated metal surfaces on mica substrates. On shearing dissimilar surfaces, the tribocurrent increases dramatically as the load or pressure is increased, with large fluctuations about the mean. Neither charge transfer nor fluctuations are observed when the symmetric surfaces are sheared against each other. We also find that the type of friction, i.e., stick-slip or smooth sliding, the load and friction force, the sliding distance, and recent previous history have additional fine influences on the triboelectrification. Our results suggest that frictional dissipation induces electron-hole formation and charge transfer between two shearing surfaces due to molecular-level roughness and defects and local dielectric constant changes, giving rise to the observed tribocurrents.

Effect of temperature on the frictional forces between polystyrene brushes

AbstractNormal and shear forces between opposing polystyrene (PS) brushes made from preferentially assembled PS–polyvinylpyridine diblock copolymers were measured in toluene and in near‐theta cyclohexane at 32, 40, and 50 °C, using a modified surface forces apparatus. In cyclohexane, over the temperature window probed, the normal forces of interaction are repulsive and the range of those force profiles changes only slightly; however, for both of the PS brushes studied, the onset of shear forces in near‐theta cyclohexane is strongly influenced by changes in temperature: As the temperature is increased, the onset of the frictional interactions between the brushes in cyclohexane shifts to smaller distances, approaching the distances where frictional forces are observed for brushes in the good solvent toluene. The pattern of behavior seen in the frictional response between the limits of good and theta condition is attributed to composition fluctuations, which increase near the theta condition because of the decrease in excluded volume interactions. These fluctuations may give rise to increased drag during shearing motion because of interfacial roughness or interchain coupling across the brush–brush interface. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 649–655, 2006

Nanoscratch and friction: An innovative approach to understand the tribological behaviour of poly(amide) fibres

The analysis of the wear resistance of polymeric fibres requires a better understanding of both their abrasive scratch behaviour and their frictional response. These aspects have been investigated at the nanometre scale using the resources of a modified surface force apparatus. In an attempt to simulate the abrasive wear losses, nanomachining experiments have been carried out which consists in the repeated scratching of a portion of the fibre surface by the rigid indenter. However, an analysis of the resulting surface topography indicated a significant plastic grooving of the fibre surface with no evidence of wear losses as it was observed at the macroscopic scale. Single pass nanoscratch experiments realised at various sliding speeds also allow discussing the relative contributions of both the material viscoplasticity and the tip/material local interactions on the frictional response. When the sliding speed was incrementally changed during a scratch experiment, it was observed that the associated friction variation was accommodated on a 50 nm distance, independently of the sliding speed.

Surface forces measured between an air bubble and a solid surface in water

Surface forces between an air bubble and a flat mica surface have been investigated using a modified surface force apparatus. This method allows forces to be detected in two regimes: at long-range, where the bubble is distorted slightly by the surface forces, and at short-range, where the force is manifest as disjoining pressure in a thin wetting film separating the bubble from the solid. A long-range double-layer repulsion was detected, which indicates that there is a negative surface charge at the air–water interface. However, when the bubble is pressed closer to the mica, charge regulation effects cause a reversal of sign and the double-layer force becomes attractive. This causes an “adhesion” of the bubble to the mica, even when a wetting film remains between them, and a finite force is required to pull the bubble from the mica.

Capillary Condensation from Vapors of n-Hexane/Perfluoro-n-hexane Mixtures

We have used a modified surface force apparatus to study capillary condensation of near-saturated vapors of n-hexane/perfluoro-n-hexane mixtures in a model pore formed by the gap between two mica surfaces. The measured compositions and mean meniscus radii of curvature of the capillary condensates are in broad agreement with the predictions of the binary Kelvin equations. Our results also show that n-hexane is preferentially adsorbed at the mica/liquid interface of the condensed mixtures, with the extent of adsorption depending on the position in the liquid−liquid phase diagram of the mixture. In addition, liquid−liquid-phase separation was observed in condensates formed from vapors of bulk perfluoro-n-hexane-rich mixtures at, or very close to phase-coexistence. This results in the formation of novel two-phase or “double” condensates, consisting of an inner condensate of a n-hexane-rich phase within an outer condensate of perfluoro-n-hexane-rich phase. Capillary condensation from vapors coexisting with phase-separated bulk mixtures gives rise to condensates of the n-hexane-rich phase. Finally, we have observed that the composition of a mixed condensate can show a significant time-dependence in the initial stages of condensation.

Apparent Slip of Newtonian Fluids Past Adsorbed Polymer Layers

We show unexpected drag reduction as solvent flows past polymer-coated surfaces in solution. Adsorbed polymer layers of two types, poly(vinyl alcohol) (PVA) homopolymers in deionized water and poly(vinylpyridine)−polybutadiene diblock copolymers (PVP−PB) in tetradecane, were placed at variable spacings larger than twice the layer thickness within a modified surface forces apparatus, and the hydrodynamic forces owing to flow of solvent past these layers were measured as a function of surface spacing, pumping frequency, and pumping velocity (product of frequency and amplitude). When the flow rate was below a critical level, which depended on the system, the findings agreed with a simple hydrodynamic picture in which the solvent appeared to flow past surfaces of defined spacing, the solid−solid spacing less twice the “hydrodyamic radius” (RH). The value of RH was not always constant, however. In the PVA system it tripled as frequency was raised from 1 to 50 Hz though it was frequency-independent in the PVP−P...

Adsorption from pure and mixed vapours of n-hexane and n-perfluorohexane

We have used a modified surface force apparatus (SFA) to study adsorption onto mica surfaces from near-saturated vapours of n-hexane and n-perfluorohexane, and mixtures thereof. For relative vapour pressures in the range 0.9-0.998 the films adsorbed from vapours of the pure liquids range in thickness from 1 to 4 nm, in crude agreement with the predictions of non-retarded van der Waals-Lifshitz theory. The observed deviations from theory show a qualitative difference between the two liquids, which may reflect differences in the significance of structural contributions to the disjoining pressure. Under the same experimental conditions, adsorption from vapours of (one-phase) liquid mixtures gives rise to films which are significantly thicker, over a broad range of intermediate compositions, than those adsorbed from the pure vapours, with a broad maximum in thickness observed near the critical composition of the bulk liquid mixture.

Transition from static to kinetic friction in a model lubricated system

Molecularly thin confined fluids were deformed in shear faster than structural relaxations in response to shear could be accomplished, such that with increasing deformation the systems passed from the rest state to sliding. The response of these systems—two atomically smooth mica sheets separated by a fluid comprised of globularly shaped molecules [octamethylcyclotetrasiloxane]—was studied as a function of film thickness of the fluid (from 80 to 10 Å, i.e, from ∼8 to ∼1 molecular dimensions), as a function of normal pressure, and as a function of deformation rate, using a modified surface forces apparatus. Whereas the linear response was always liquid-like provided that the deformation rate was sufficiently slow, a “stick-slip” transition from the rest state to sliding was observed when the deformation rate was large, provided that the oscillatory frequency sufficiently exceeded the inverse intrinsic relaxation time of the confined fluid. This transition was monotonic and reversible without hysteresis for relatively thick films but for thinner films was discontinuous with hysteresis. For films thicker than 3 molecular layers (ML), two length scales in deformation were observed; the films showed nonlinear force-deformation response beginning at a deformation amplitude of 3 Å but in general showed stick slip only when the deformation was larger than this. The critical deformation at the point of stick slip decreased from 9 to 3 Å with increasing normal pressure, indicating diminished plasticity of the confined structures with increasing normal pressure. The critical film thickness of 3 ML correlates with the possibility of one rather than more slip planes. The thinnest films under the highest compressive pressures showed moderate increase of the viscous shear force with increasing effective sliding velocity, but in general the viscous force reached a plateau in which force showed almost no dependence on sliding rate. In interpreting the results in the context of friction, static friction was identified with the elastic stress at rupture and kinetic friction was identified with the limiting maximum observed level of viscous force. After normalizing friction and normal forces by the contact area, the static friction coefficient was found to be 0.44 and the kinetic friction coefficient to be 0.14. In other words, as the normal pressure increased, the elastic force needed to rupture the system increased more rapidly than the limiting shear stress. The magnitude of the limiting shear stress increased exponentially with decreasing film thickness with a decay length of 1 molecular dimension. This decay length correlates well with the known exponential decay of oscillations in the static force–distance profile. The critical shear amplitude of 3 Å, relative to the molecular dimension of ≈9 Å, is reminiscent of early estimates by Frenkel of the point of instability when planes of atoms slide over one another.

Rate-Dependent Adhesion between Opposed Perfluoropoly(alkyl ether) Layers: Dependence on Chain-End Functionality and Chain Length

120 Im/s of a detachment device with spring constant 1.1 10 4 N/m) using a modified surface forces apparatus with piezoelectric actuators supporting one surface. Rate dependence resulting in an increase of the adhesion by a factor up to 3 was observed when the unloading rate exceeded a critical value that was found to depend not only on the polymer sample but also on the contact time between opposed layers. At a given unloading rate, the adhesion was larger, the longer the surfaces were left in stationary contact before separation. No history dependence was observed when the unloading rate was raised and subsequently lowered. For relatively short contact times (0.01-1 s), the critical unloading rate was larger for the polymer that contained polar end groups than for unfunctionalized polymer of similar chain length, indicating a lower mobility in the former case, and thus slower initial interdigitation of the chains, since the polar end group had a preferential affinity for the underlying surface. This effect vanished for contact times of 100-500 s. For polymer chains without polar end groups, the critical value of the unloading rate was smaller, the larger the chain length, and the increase in adhesion with unloading rate was more pronounced, presumably reflecting slower relaxation of the longer chains. The rate dependence is analyzed quantitatively.

Thin Film Rheology and Tribology of Confined Polymer Melts: Contrasts with Bulk Properties

A modified Surface Forces Apparatus was used for making rheological and tribological measurements of thin fluid films, and polybutadiene (PBD) of Mw ≈ 7000 was studied in detail. At low shear rates, PBD exhibits bulklike properties in films thicker than about 200 nm. In thinner films (200−20 nm), the shear viscosity ηeff and moduli G‘ and G‘‘ become quantitatively modified from those of the bulk. However, a sinusoidal input still produces a sinusoidal output, and a modified WLF representation remains applicable to such films. On entering the tribological regime (film thickness <30 nm) polybutadiene exhibits highly nonlinear behavior and yield points, indicative of phase transitions to “glassy” or “solidlike” states. Other features are (i) a shift of the slip plane, (ii) an additional normal force component, and (iii) the emergence of new scaling relationships for ηeff, G‘, G‘‘, and the friction force F as a function of frequency, sliding speed, and other system parameters.