Publications

Transient electric fields across cell bilayer membranes can lead to electroporation, as well as to cell fusion, and have been extensively studied. We find that transmembrane electric fields similar to those in cells can lead to a massive, reversible modulation--by up to 200-fold--of the interfacial energy dissipation between surfaces sliding across the lipid bilayer membranes. Atomistic simulations reveal that this arises from (fully reversible) electroporation of the interfacially-confined bilayers, and formation of bilayer bridges analogous to stalks preceding intermembrane fusion. These cell-membrane-mimicking effects topologically-force the slip to partially-revert from the low-dissipation, hydrated lipid-headgroups plane to the intra-bilayer, high-dissipation acyl tail interface. Our results demonstrate that lipid bilayers under transmembrane electric fields can have striking materials-modification properties, and shed new light on membrane hemifusion.

It is well known that lipid membranes respond to a threshold transmembrane electric field through a reversible mechanism called electroporation, where hydrophilic water pores form across the membrane, an effect widely used in biological systems. The effect of such fields on interfacially-confined (stacked or supported) lipid membranes, on the other hand, which may strongly modulate interfacial properties, has not to our knowledge been previously studied. Motivated by recent surface forces experiments showing a striking effect of electric fields on lubrication by confined lipid bilayers, we carried out all-atom molecular dynamics simulations of such membranes under transverse electric fields. We find that in addition to electroporation, a new feature emerges of locally merged bilayers which act to bridge the confining interfaces. These features shed light on the remodelling of confined lipid membrane stacks by electric fields, and provides insight into how such fields may modulate frictional and more generally surface interactions in the presence of lipid-based boundary layers.

Drug delivery via nanovehicles is successfully employed in several clinical settings, yet bacterial infections, forming microbial communities in the form of biofilms, present a strong challenge to therapeutic treatment due to resistance to conventional antimicrobial therapies. Liposomes can provide a versatile drug-vector strategy for biofilm treatment, but are limited by the need to balance colloidal stability with biofilm penetration. We have discovered a liposomic functionalization strategy, using membrane-embedded moieties of poly[2-(methacryloyloxy)ethyl phosphorylcholine], pMPC, that overcomes this limitation. Such pMPCylation results in liposomic stability equivalent to current functionalization strategies (mostly PEGylation, the present gold-standard), but with strikingly improved cellular uptake and cargo conveyance. Fluorimetry, cryo-electron, and fluorescence microscopies reveal a far-enhanced antibiotic delivery to model Pseudomonas aeruginosa biofilms by pMPC-liposomes, followed by faster cytosolic cargo release, resulting in significantly greater biofilm eradication than either PEGylation or free drug. Moreover, this combination of techniques uncovers the molecular mechanism underlying the enhanced interaction with bacteria, indicating it arises from bridging by divalent ions of the zwitterionic groups on the pMPC moieties to the negatively charged lipopolysaccharide chains emanating from the bacterial membranes. Our results point to pMPCylation as a transformative strategy for liposomal functionalization, leading to next-generation delivery systems for biofilm treatment.

The hydration layer surrounding the phosphocholine headgroups of single-component phosphatidylcholine (PC) lipids, or of lipid-mixtures, assembled at an interface greatly modifies the interfacial properties and interactions. As water molecules within the hydration layer are held tightly by the headgroup but are nonetheless very fluid upon shear, the boundary lipid layers, exposing the highly hydrated headgroup arrays, can provide efficient boundary lubrication when sliding against an opposing surface, at physiologically-high contact pressures. Additionally, any free lipids in the surrounding liquid can heal defects which may form during sliding on the boundary PC layer. Similar boundary lipid layers contribute to the lubricating, pressure-bearing, and wear-protection functions of healthy articular joints. This review presents a survey of the relationship between the molecular composition of the interfacial complex and the lubrication behavior of the lipid-based boundary layers, which could be beneficial for designing boundary lubricants for intra-articular injection for the treatment of early OA.

The viscoelectric effect concerns the increase in viscosity of a polar liquid in an electric field due to its interaction with the dipolar molecules, and was first determined for polar organic liquids more than 80 years ago. For the case of water, however, the most common polar liquid, direct measurement of the viscoelectric effect is challenging and has not to date been carried out, despite its importance in a wide range of electrokinetic and flow effects. In consequence, estimates of its magnitude for water vary by more than 3 orders of magnitude. Here we measure the viscoelectric effect in water directly using a sur-face force balance (SFB), by measuring the dynamic approach of two molecularly-smooth surfaces with a controlled, uniform electric field between them across highly-purified water. As the water is squeezed out of the gap between the approaching surfaces, viscous damping dominates the approach dynamics; this is modulated by the viscoelectric effect under the uniform transverse electric field across the water, enabling its magnitude to be directly determined as a function of the field. We find a value for this mag-nitude which differs respectively by 1 and by 2 orders of magnitude from its highest and lowest previ-ously estimated values.

Healthy articular cartilage, covering the ends of bones in major joints such as hips and knees, presents the most efficiently-lubricated surface known in nature, with friction coefficients as low as 0.001 up to physiologically high pressures. Such low friction is indeed essential for its well-being. It minimizes wear-and-tear and hence the cartilage degradation associated with osteoarthritis, the most common joint disease, and, by reducing shear stress on the mechanotransductive, cartilage-embedded chondrocytes (the only cell type in the cartilage), it regulates their function to maintain homeostasis. Understanding the origins of such low friction of the articular cartilage, therefore, is of major importance in order to alleviate disease symptoms, and slow or even reverse its breakdown. This progress report considers the relation between frictional behavior and the cellular mechanical environment in the cartilage, then reviews the mechanism of lubrication in the joints, in particular focusing on boundary lubrication. Following recent advances based on hydration lubrication, a proposed synergy between different molecular components of the synovial joints, acting together in enabling the low friction, has been proposed. Additionally, recent development of natural and bio-inspired lubricants is reviewed.

Duncan Dowson, whom this issue commemorates, was a world leader in the field of biotribology, with prolific contributions both in fluid-based and boundary lubrication of biological tissues, in particular articular cartilage, a central issue in biotribology due to its importance for joint homeostasis. Here we explore further the issue of cartilage boundary lubrication, which has been attributed to phospholipid (PL)-exposing layers at the cartilage surface in part. A surface force balance (SFB) with unique sensitivity is used to investigate the normal and frictional interactions of the boundary layers formed by PLs extracted from osteoarthritic (OA) human synovial fluid (hSF). Our results reveal that vesicles of the OA-hSF lipids rupture spontaneously to form bilayers on the mica substrate (which, like the in-vivo articular cartilage surface in synovial joints, is negatively-charged) which then undergo hemifusion at quite low pressures in the SFB, attributed to the large heterogeneity of the hSF lipids. Nanometric friction measurements reveal friction coefficients μ ≈ 0.03 across the hemifused bilayer of these lipids, indicating residual hydration lubrication at the lipid-headgroup/substrate interface. Addition of calcium ions causes an increase in friction to μ ≈ 0.2, attributed either to calcium-bridging attraction of lipid headgroups to the negatively-charged substrate, or a shift of the slip plane to the more dissipative hydrophobic-tail/hydrophobic-tail interface. Our results suggest that the heterogeneity and composition of the OA-hSF lipids may be associated with higher friction at the cartilage boundary layers – and thus a connection with greater wear and degradation – due to hemifusion of the exposed lipid bilayers.

Osteoarthritic joints contain lower molecular weight (MW) hyaluronan (hyaluronic acid, HA) than healthy joints. To understand the relevance of this HA-size effect for joint lubrication, the friction and surface structure of cartilage-emulating surfaces with HA of different MW were studied using a surface force balance (SFB), and atomic force microscopy (AFM) . Gelatin (gel)-covered mica surfaces were coated with either high MW HA (HHA), medium MW HA (MHA) or low MW HA (LHA), and lipids of Hydrogenated Soy L-α-phosphatidylcholine (HSPC) in the form of small unilamellar vesicles, using a layer-by-layer assembly method. SFB results indicate that the gel-HHA-HSPC boundary layer provides very efficient lubrication, attributed to hydration lubrication at the phosphocholine head-groups exposed by the HA-attached lipids, with friction coefficients (COF) as low as l0-3–10-4 at contact stresses at least up to P = 120 atm. However, for the gel-MHA-HSPC and gel-LHA-HSPC surfaces, the friction, initially low, increases sharply at much lower pressures (up to 30–60 atm at most). This higher friction with the shorter chains may be due to their weaker total adhesion energy to the gelatin, where the attraction between the negatively-charged HA and the weakly-positively charged gelatin is attributed largely to counterion-release entropy. Thus the complexes of LHA and MHA with the lubricating HSPC lipids are more easily removed by shear during sliding, especially at high stresses, than is the HHA-HSPC complex which is strongly adhered to the gelatin. This is ultimately the reason for lower-pressure lubrication breakdown with the shorter polysaccharides. Our results provide molecular level insight into why the decrease in HA molecular weight in osteoarthritic joints may be associated with higher friction at the articular cartilage surface, and may have relevance for treatments of osteoarthritis involving intra-articular HA injections.

Hydration lubrication has emerged as a new paradigm for lubrication in aqueous and biological media, accounting especially for the extremely low friction (friction coefficients down to 0.001) of articular cartilage lubrication in joints. Among the ensemble of molecules acting in the joint, phosphatidylcholine (PC) lipids have been proposed as the key molecules forming, in a complex with other molecules including hyaluronic acid (HA), a robust layer on the outer surface of the cartilage. HA, ubiquitous in synovial joints, is not in itself a good boundary lubricant, but binds the PC lipids at the cartilage surface; these, in turn, massively reduce the friction via hydration lubrication at their exposed, highly hydrated phosphocholine headgroups. An important unresolved issue in this scenario is why the free HA molecules in the synovial fluid do not suppress the lubricity by adsorbing simultaneously to the opposing lipid layers, i.e., forming an adhesive, dissipative bridge between them, as they slide past each other during joint articulation. To address this question, we directly examined the friction between two hydrogenated soy PC (HSPC) lipid layers (in the form of liposomes) immersed in HA solution or two palmitoyl-oleoyl PC (POPC) lipid layers across HA-POPC solution using a surface force balance (SFB). The results show, clearly and surprisingly, that HA addition does not affect the outstanding lubrication provided by the PC lipid layers. A possible mechanism indicated by our data that may account for this is that multiple lipid layers form on each cartilage surface, so that the slip plane may move from the midplane between the opposing surfaces, which is bridged by the HA, to an HA-free interface within a multilayer, where hydration lubrication is freely active. Another possibility suggested by our model experiments is that lipids in synovial fluid may complex with HA, thereby inhibiting the HA molecules from adhering to the lipids on the cartilage surfaces.

Poly(ethylene oxide), PEO, is widely exploited in biomedical applications, while phosphatidylcholine (PC) lipids (in the form of bilayers or liposomes) have been identified as very efficient boundary lubricants in aqueous media. Here we examine, using a surface force balance (SFB), the interactions between surface-adsorbed layers of PEO complexed with small unilamellar vesicles (SUVs, i.e. liposomes) or with bilayers of PC lipids, both well below and a little above their main gel-toliquid phase-transition temperatures T-M. The morphology of PEO layers (adsorbed onto mica), to which liposomes were added, was examined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). Our results reveal that the PC lipids could attach to the PEO either as vesicles or as bilayers, depending on whether they were above or below T-M. Under water (no added salt), excellent lubrication, with friction coefficients down to 10(-3)-10(-4), up to contact stresses of 6.5 MPa (comparable to those in the major joints) was observed between two surfaces bearing such PEO-PC complexes. At 0.1 M KNO3 salt concentration (comparable to physiological salt levels), the friction between such surfaces was considerably higher, attributed to bridging by the polymer chains. Remarkably, such bridging could be suppressed and the friction could be restored to its previous low value if the KNO3 was replaced with NaNO3, as a result of the different PEO-mica ligation properties of Na+ compared to those of K+ Our results provide insight into the properties of PEO-PC complexes in potential applications, and large interfacial effects that can result from the seemingly innocuous replacement of K+ by Na+ ions.

A wide range of phosphatidylcholine (PC)
lipids with different degrees of unsaturation has been
identified in the human synovial fluid and on the cartilage
surface. The outstanding lubricity of the articular cartilage
surface has been attributed to boundary layers comprising
complexes of such lipids, though to date, only lubrication by
single-component PC-lipid-based boundary layers has been
investigated. As distinguishable lubrication behavior has been
found to be related to the PC structures, we herein examined
the surface morphology (on mica) and the lubrication ability
of binary PC lipid mixtures, 1,2-dipalmitoyl-sn-glycero-3-
phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero3-phosphocholine (POPC), using atomic force microscopy (AFM) and a surface force balance (SFB). These two PC lipids are
among the most abundant saturated and unsaturated PC components in synovial joints. Small unilamellar vesicles (SUVs)
prepared from DPPC-POPC mixtures (8:2, 5:5, and 2:8, molar ratios) ruptured and formed bilayers on mica. The normal and
shear forces between two DPPC-POPC bilayer-coated mica surfaces across the corresponding SUV dispersions show good
boundary lubrication (friction coefficients ≤ ca. 10−4
) up to contact stresses of 8.3 ± 2.2 MPa for 8:2 DPPC-POPC and 5.0 ±
1.7 MPa for the others. Hemifusion induced at high normal pressures was observed, probably because of the height mismatch of
two components. Reproducible successive approaches after hemifusion indicate rapid self-healing of the mica-supported bilayers
in the presence of the SUVs reservoir. This work is a first step to provide insight concerning the lubrication, wear, and healing of
the PC-based boundary layers, which must consist of multicomponent lipid mixtures, on the articular cartilage surface.

We have prepared phosphatidylcholine (PC) vesicles (liposomes) incorporating a novel lipid/poly-phosphocholine conjugate. This both stabilizes the liposomes against aggregation (for example, during storage or when being delivered) and allows them to act as very efficient lubricating elements readily attaining superlubric performance (defined as coefficient of friction mu

Using viscosity and dynamic light scattering (DLS) measurements, we monitored the changes in the properties of dispersions of chitosan (a cationic polysaccharide) in acidic solution over a period of up to 700 h. Different polymer concentrations, weight average molecular weights, and degrees of deacetylation were examined. We found that the solution rheology and chitosan aggregates continue to change even up to 700 h. It was observed, remarkably, using both capillary and cone and plate viscometry that the viscosity decreased significantly during the storage period of the chitosan dispersions, with a rapid initial decrease and a slow approach to the steady state value. DLS measurements over this period could be interpreted in terms of a gradual decrease in the size of the chitosan aggregates in the dispersion. This behavior is puzzling, insofar as one expects the dissolution of compact polymer aggregates with time into individual polymer chains to increase the viscosity rather than decrease it as observed: We attribute this apparently anomalous behavior to the fact that the chitosan aggregates are rigid crystalline rod-like entities, which dissolved with time from dispersion of overlapping rods (with high viscosity) into solution of individual random coils (with lower viscosity). A detailed model comparing the hydrodynamic behavior of the initial overlapping rod-like aggregates with the subsequent free coils in solution is in semi-quantitative agreement with our observation. Published by AIP Publishing.

Poly[2-(methacryloyloxy)ethylphosphorylcholine] (pMPC) brushes provide extremely low friction coefficients up to high compressions, but their use as boundary lubricating layers is limited by the challenge of surface grafting of the chains. Coating by thin layers of cross-linked pMPC hydrogels may provide an attractive alternative. Here we use a surface force balance (SFB) and other techniques to examine the effect of light cross-linking (0.1% cross-linker) on the surface interactions and frictional behavior of grafted-from pMPC brushes on mica, up to physiologically high contact pressures. Atomic force microscopy, X-ray photoelectron spectroscopy, and interferoinetric surface-excess measurements show little difference between the non-cross-linked (linear) pMPC brushes and the cross-linked brushes prepared under otherwise identical conditions. Normal force-distance profiles between the polymer-bearing surfaces, however, reveal a marked compaction of the unperturbed thickness of the cross-linked brushes relative to linear ones, attributed to the cross-linking which limits chain swelling. The cross-linked pMPC layers exhibit very low friction (friction coefficients of order 10(-3)-10(-4), depending on sliding velocities), similar to the corresponding linear brushes and due to hydration lubrication by the highly hydrated phosphocholine monomer structure. Within the range of our parameters, however, there is a marked qualitative difference in the dependence of friction on sliding velocity v(s). While for the linear brushes friction is only very weakly v(s)-dependent (over 3 orders of magnitude in v(s)), due to a self-regulating brush interpenetration, for the cross-linked brushes friction increases markedly with v(s) (similar to v(s)(1/2)), an effect attributed to suppression of interpenetration by the cross-links.

The lubrication properties of saturated PC lipid vesicles containing high cholesterol content under high loads were examined by detailed surface force balance measurements of normal and shear forces between two surface-attached lipid layers. Forces between two opposing mica surfaces bearing distearoylphosphatidylcholine (PC) (DSPC) small unilamellar vesicles (SUVs, or liposomes), or bilayers, with varying cholesterol content were measured across water, whereas dimyristoyl PC (DMPC), dipalmitoyl PC (DPPC), and DSPC SUVs containing 40% cholesterol were measured across liposome dispersions of SUVs of the same lipid composition as in the adsorbed layers. The results clearly demonstrate decreased stability and resistance to normal load with the increase in cholesterol content of DSPC SUVs. Friction coefficients between two 10% cholesterol PC-bilayers were in the same range as for 40% cholesterol bilayers (mu approximate to 10(-3)), indicating that cholesterol has a more substantial effect on the mechanical properties of a bilayer than on its lubrication performance. We further find that the lubrication efficiency of DMPC and DPPC with 40% cholesterol is superior to that of DSPC 40% cholesterol, most likely because of enhanced hydration-lubrication in these systems. We previously found that when experiments are performed in the presence of a lipid reservoir, layers can self-heal and therefore their robustness is less important under such conditions. We conclude that the effect of cholesterol in decreasing the stability is more pronounced than its effect on hydration, but the stability is, in turn, less important when a lipid reservoir is present. This study complements our previous work and sheds light on the effect of cholesterol, a prominent and important physiological lipid, on the mechanical and lubrication properties of gel-phase lipid layers.

Extending earlier studies where phosphocholinated brushes and phosphocholine‐exposing phosphatidylcholine lipid bilayer vesicles (PC‐liposomes) were shown to be very efficient boundary lubricants in aqueous media by virtue of the highly‐hydrated phophocholine groups, we examined interactions between surfaces bearing phosphocholinated polystyrene nanoparticles (pc‐PS‐NPs). We synthesized such particles by incorporation of alkyl chains terminated with phosphocholine groups in the PS‐NP, with the hydrophilic phosphocholines exposed at the NP surface. These were then allowed to adsorb onto mica surfaces, and the normal and shear interactions between them were examined in a surface force balance. On moderate compressions (contact pressures > 9 atm) the pc‐PS‐NPs were squeezed out, leaving a single layer between the surfaces. Shear of the surfaces revealed a large frictional dissipation, with a friction coefficient μ ≈ 0.2, in contrast to our expectations that such phosphocholinated NPs would provide highly lubricating analogues of PC‐liposomes (for which μ ≈ 10−3 – 10−4). This is attributed to a number of factors, including the inherent roughness of the NP multilayers at low compressions and the relatively low areal density of the phosphocholine groups on the PS‐NP surfaces. In particular, at higher compressions, sliding results in energy‐dissipating breaking and reforming of phosphocholine/mica (dipole/charge) bonds at the mica surfaces (and thus high friction), because of bridging of the surfaces by a single layer of pc‐PS‐NPs.

Highly efficient lubricating boundary layers at biosurfaces such as cartilage have been proposed to comprise phospholipids complexed with biomacromolecules exposed at the surfaces. To gain insight into this, a systematic study on the normal and frictional forces between surfaces bearing a sequentially deposited model alginate-on-chitosan bilayer, bearing different adsorbed phosphatidylcholine (PC) liposomes, was carried out using a surface force balance. Structures of the resulting surface complexes were determined using atomic force microscopy (AFM) and cryo-scanning electron microscopy (cryo-SEM). The liposome/lipid-polymer complexes could maintain their integrity up to high pressures in terms of both normal and shear interactions between the surfaces, which were repeatable, reproducible, and revealed very low friction (coefficient of friction mu down to 10(-3)-10(-4), depending on the PC used) up to pressures of hundreds of atm. We attribute this remarkable lubrication capability ultimately to hydration lubrication acting at the hydrated phosphocholine headgroups of the PC lipids, either exposed at the liposome surfaces or through complexation with the polyelectrolyte bilayer. Values of mu, while low, were roughly an order of magnitude higher than for the same PC vesicles adsorbed on bare mica, a difference attributed to their lower density on the bilayer; the bilayer, however, stabilized the PC-vesicles far better than bare mica against rupture and shear at high compressions and sliding.

Frictional energy dissipation between sliding solid surfaces in aqueous media may proceed by different pathways. Using a surface force balance (SFB), we have examined systematically how such dissipation is mediated by the series of hydrated cations M+ = Li+, Na+, and K+ that are trapped between two atomically smooth, negatively charged, mica surfaces sliding across the ionic solutions over many orders of magnitude:loading. By working at local contact pressures up to ca. 30 MPa (similar to 300 atm), up to 2 orders of magnitude higher than earlier studies, we could show that the frictional dissipation at constant sliding velocity, represented by the coefficient of sliding friction mu(M+), decreased as mu(Li+) > mu(Na+) greater than or similar to mu(K+), This result contrasts with the expectation (in conceptual analogy with the Hofmeister series) that the lubrication would improve with the extent of ionic hydration, since that would have led to the opposite mu(M+) sequence. It suggests, rather, that frictional forces, even in such simple systems, can be dominated by rate-activated pathways where the size of the hydration shell becomes a dissipative liability, rather than by the hydration-shell dissipation expected via the hydration lubrication mechanism.

Measurements of normal and shear (frictional) forces between mica surfaces across small unilamellar vesicle (SUV) dispersions of the phosphatidylcholine (PC) lipids DMPC (14 : 0), DPPC (16 : 0) and DSPC (18 : 0) and POPC (16 : 0, 18 : 1), at physiologically high pressures, are reported. We have previously studied the normal and shear forces between two opposing surfaces bearing PC vesicles across pure water and showed that liposome lubrication ability improved with increasing acyl chain length, and correlated strongly with the SUV structural integrity on the substrate surface (DSPC > DPPC > DMPC). In the current study, surprisingly, we discovered that this trend is reversed when the measurements are conducted in SUV dispersions, instead of pure water. In their corresponding SUV dispersion, DMPC SUVs ruptured and formed bilayers, which were able to provide reversible and reproducible lubrication with extremely low friction (mu

Macromolecules, which adsorb or intrinsically form boundary layers at surfaces sliding past each other in aqueous media, are ubiquitous both in technology and in biological systems and can form effective boundary lubricants. Over the past decade or so, hydration layers - robustly bound water molecules that surround charges or zwitterionic groups of different macromolecular species - have been identified as remarkable lubricating elements, sustaining high loads while exhibiting a fluid-like response to shear with extremely low friction. This modification of frictional forces in aqueous systems, based on the behavior of water molecules confined to hydration shells, is the central idea behind the hydration lubrication mechanism, which is presented and discussed in detail in the current Perspective. We describe the nature of hydration under confinement and the underlying experiments revealing this mechanism, focusing in particular on synthetic and biological macromolecules attached to surfaces and on phospholipid assemblies. We also emphasize these recent findings in relation to physiological environments and functions of the human body, such as cartilage lubrication, in which hydration lubrication is believed to play an important role.

Hyaluronan, lubricin and phospholipids, molecules ubiquitous in synovial joints, such as hips and knees, have separately been invoked as the lubricants responsible for the remarkable lubrication of articular cartilage; but alone, these molecules cannot explain the extremely low friction at the high pressures of such joints. We find that surface-anchored hyaluronan molecules complex synergistically with phosphatidylcholine lipids present in joints to form a boundary lubricating layer, which, with coefficient of friction mu approximate to 0.001 at pressures to over 100 atm, has a frictional behaviour resembling that of articular cartilage in the major joints. Our findings point to a scenario where each of the molecules has a different role but must act together with the others: hyaluronan, anchored at the outer surface of articular cartilage by lubricin molecules, complexes with joint phosphatidylcholines to provide the extreme lubrication of synovial joints via the hydration-lubrication mechanism.

Why is friction in healthy hips and knees so low? Hydration lubrication, according to which hydration shells surrounding charges act as lubricating elements in boundary layers (including those coating cartilage in joints), has been invoked to account for the extremely low sliding friction between surfaces in aqueous media, but not well understood. Here we report the direct determination of energy dissipation within such sheared hydration shells. By trapping hydrated ions in a 0.4-1 nm gap between atomically smooth charged surfaces as they slide past each other, we are able to separate the dissipation modes of the friction and, in particular, identify the viscous losses in the subnanometre hydration shells. Our results shed light on the origins of hydration lubrication, with potential implications both for aqueous boundary lubricants and for biolubrication.

We review here two recent works on boundary lubrication by macromolecules with the aim of obtaining an insight into the extremely efficient lubrication of mammalian joints. The first work emulates the structure of the cartilage superficial layer by reconstructing films of hyaluronan (HA) and of HA/aggrecan complexes stabilized by cartilage link protein and studies normal and shear interactions between two atomically smooth mica surfaces coated with such molecules using a surface force balance. These molecules have been conjectured to act as boundary lubricants while sliding past each other at the interface between the articular cartilage surfaces and the synovial fluid. It was discovered that the HA/aggrecan/link protein macromolecular complexes provide better boundary lubrication compared with HA alone, most likely as a result of the higher charge density allowing a more efficient hydration lubrication effect. However, the results suggest that such complexes by themselves cannot be responsible for the very efficient lubrication of synovial joints. On the other hand, in the second work, phosphatidylcholine vesicles were shown to provide extremely efficient lubrication at physiological pressures as long as the robustness of the layer is maintained. Normal and shear interactions between two surfaces bearing different phosphatidylcholine vesicles, having the same head group but varying acyl chain length, were studied systematically. In addition to hydration, robustness of the surface layer plays a major role in the lubrication efficiency of the system. Implications of the results of these studies to the lubrication mechanism of synovial joints are suggested. Copyright (c) 2014 John Wiley & Sons, Ltd.

Phosphatidylcholine (PC) vesicles have been shown to have remarkable boundary lubricating properties under physiologically-high pressures. Here we carry out a systematic study, using a surface force balance, of the normal and shear (frictional) forces between two opposing surfaces bearing different PC vesicles across water, to elucidate the origin of these properties. Small unilamellar vesicles (SUVs, diameters

Using the surface force balance (SFB), we recorded the changes with time of the adhesion, normal, and shear interactions between a monolayer of cetyltrimethylammonium bromide (CTAB) on mica and a bare mica surface across surfactant-free water. In this asymmetric case, the bare mica acts a stable probe of the interactions between the two surfaces as the CTAB-coated one undergoes changes with time. As previously demonstrated, when a CTAB monolayer on mica is immersed in water, it reorganized to form bilayer patches, exposing the bare mica surface, followed by a gradual release of free surfactants to the bulk. We probe how this degradation with time affects both the normal force vs distance interaction profiles, and adhesion between the CTAB-coated surface and a bare mica surface. We demonstrate that the CTAB layer leads to a reduction in the sliding friction relative to that between bare mica surfaces, which is reversed only at advanced degradation levels, whereupon an abrupt increase in the friction occurs. This change is ascribed to adhesion between exposed bare mica surfaces, which sets on when the density of CTAB patches is low. The reproducibility of the normal force profiles and of adhesion forces on sequential approaches at the same contact spot indicates that there is no substantial transfer of materials between the surfaces while they are in adhesive contact.

In this paper we review recent work (Goldberg et al., 2011a,b) on a new use for phosphatidylcholine liposomes: as ultra-efficient boundary lubricants at up to the highest physiological pressures. Using a surface force balance, we have measured the normal and shear interactions as a function of surface separation between layers of hydrogenated soy phophatidylcholine (HSPC) small unilamellar vesicles (SUVs) adsorbed from dispersion, at both pure water and physiologically high salt concentrations of 0.15M NaNO3. Cryo-Scanning Electron Microscopy shows each surface to be coated by a close-packed HSPC-SUV layer with an over-layer of liposomes on top. The shear forces reveal strikingly low friction coefficients down to 2 x 10(-5) in pure water system or 6 x 10(-4) in the 150 mM salt system, up to contact pressures of at least 12 MPa (pure water) or 6 MPa (high salt), comparable with those in the major joints. This low friction is attributed to the hydration lubrication mechanism arising from rubbing of the highly hydrated phosphocholine-headgroup layers exposed at the outer surface of each liposome, and provides support for the conjecture that phospholipids may play a significant role in biological lubrication. (C) 2011 Elsevier Ireland Ltd. All rights reserved.

We have used neutron reflectometry to investigate the behavior of a strong polyelectrolyte brush on a sapphire substrate, grown by atom-transfer radical polymerization (ATRP) from a silane-anchored initiator layer. The initiator layer was deposited from vapor, following treatment of the substrate with an Ar/H2O plasma to improve surface reactivity. The deposition process was characterized using X-ray reflectometry, indicating the formation of a complete, cross-linked layer. The brush was grown from the monomer [2-(methacryloyloxy)ethyl]trimethyl-ammonium chloride (METAC), which carries a strong positive charge. The neutron reflectivity profile of the swollen brush in pure water (D2O) showed that it adopted a two-region structure, consisting of a dense surface region similar to 100 angstrom thick, in combination with a diffuse brush region extending to around 1000 angstrom from the surface. The existence of the diffuse brush region may be attributed to electrostatic repulsion from the positively charged surface region, while the surface region itself most probably forms due to polyelectrolyte adsorption to the hydrophobic initiator layer. The importance of electrostatic interactions in maintaining the brush region is confirmed by measurements at high (1 M). added 1:1 electrolyte, which show a substantial transfer of polymer from the brush to the surface region, together with a strong reduction in brush height. On addition of 10(-4) M oppositely charged surfactant (sodium dodecyl sulfate), the brush undergoes a dramatic collapse, forming a single dense layer about 200 angstrom in thickness, which may be attributed to the neutralization of the monomers by adsorbed dodecyl sulfate ions in combination with hydrophobic interactions between these dodecyl chains. Subsequent increases in surfactant concentration result in slow increases in brush height, which may be caused by stiffening of the polyelectrolyte chains due to further dodecyl sulfate adsorption.

In a series of Surface Force Balance experiments, material from human whole saliva was adsorbed to molecularly smooth mica substrata (to form an 'adsorbed salivary film'). Measurements were taken of normal (load bearing, F-n) and shear (frictional, F-s*) forces between two interacting surfaces. One investigation involved a salivary film formed by overnight adsorption from undiluted, centrifuged saliva, with the adsorbed film rinsed with pure water before measurement. Measurements were taken under pure water and 70 mM NaNO3. In a second investigation, a film was formed from and measured under a solution of 7% filtered saliva in 10 mM NaNO3. F-n results for both systems showed purely repulsive layers, with an uncompressed thickness of 35-70 nm for the diluted saliva investigation and, prior to the application of shear, 11 nm for the rinsed system. F-s* was essentially proportional to F-n for all systems and independent of shear speed (in the range 100-2000 nm s(-1)), with coefficients of friction mu similar to 0.24 and mu similar to 0.46 for the unrinsed and rinsed systems, respectively. All properties of the rinsed system remained similar when the pure water measurement environment was changed to 70 mM NaNO3. For all systems studied, shear gave rise to an approximately threefold increase in the range of normal forces, attributed to the ploughing up of adsorbed material during shear to form debris that stood proud of the adsorbed layer. The results provide a microscopic demonstration of the wear process for a salivary film under shear and may be of particular interest for understanding the implications for in vivo oral lubrication under conditions such as rinsing of the mouth cavity. The work is interpreted in light of earlier studies that showed a structural collapse and increase in friction for an adsorbed salivary film in an environment of low ionic strength.

Liposomes of hydrogenated soy phosphatidylcholine (PC) lipids spontaneously self-assemble in close-packed layers on solid surfaces to form boundary lubricants that reduce the coefficient p of sliding friction between them down to mu approximate to 10(-4) - 2 x 10(-5), at pressures up to ca. 12 MPa. This strikingly low value is attributed to hydration-lubrication by the PC headgroups exposed at the surfaces of the gel-phase vesicles.

A surface force balance was used to measure the normal and shear, forces between two mica surfaces each bearing an adsorbed layer of porcine gastric mucin ("Orthana" mucin), genetically similar to. human MUC6. This mucin is a highly purified, 546 kDa, weakly negative, polyampholytic molecule with a "dumbbell" structure. Both bare (HP) and hydrophobized (HB) mica substrates were used, and forces were measured under 1 and 30 mg/mL mucin solutions, under pure (no-added-salt) water, and under 0.1 M aqueous Na(+) solution. Normal surface forces were monotonically repulsive in all cases, with onset of repulsion occurring at smaller surface separations, D, in the 0.1 M salt solutions (similar to 20 nm, compared with similar to 40 nm for no added salt). Repulsion on RP mica was greater on surface compression. than decompression, an effect, attributed to bridging and slow-relaxing additional adsorption on compression, not seen on HB mica, a difference attributed to the denser coverage of mucin hydrophobic moieties on the HB surface, Friction forces increased with compression in all cases, showing hysteretic behavior on HP but not on HB mica, commensurate with the hysteresis observed in the normal measurements. Low friction coefficients mu (= partial derivative F(s)/partial derivative F(n)

Using a surface force balance with fast video analysis, we have measured directly the attractive forces between oppositely charged solid surfaces (charge densities sigma(+), sigma(-)) across water over the entire range of interaction, in particular, at surface separations D below the Debye screening length lambda(S). At very low salt concentration we find a long-ranged attraction between the surfaces (onset ca. 100 nm), whose variation at D

The very low sliding friction at natural synovial joints, which have friction coefficients of mu

Prompted by the recent discovery that water and aqueous monovalent Na(+) solutions remain fluid-like when confined to films of a few molecular layers between mica surfaces,[Raviv et al., Nature, 2001, 413. 51-54; and Raviv and Klein, Science, 2002. 297, 1540-1543] we now extend the previous study by comparing the shear- and normal-force properties of 0.1 M Na(+), Cs(+) and Ni(2+) salt solutions which demonstrate a diverse range of behaviors under confinement. In the case of hydrated Na(+) we extend the previous study to higher pressures, up to similar to 10 atmospheres, and record similar fluidity of the hydration layers at these elevated pressures. Aqueous Cs(+) films under confinement between mica sheets have been found to be unable to support an applied load-that is to say they do not demonstrate any hydration repulsion regime as a result of their lower hydration energy [see Goldberg et al., Phys. Chem. Chem. Phys., 2008. 10, 4939-4945] which contrasts with the properties of Na(+). We show that 0.1 M Ni(2+) solution remains close to its bulk viscosity down to nanometre thin films, but does not demonstrate a hydration repulsion. By comparing the properties of this range of cations, with differing valency and hydration, we aim to examine the conditions under which ions serve as a effective lubricants and what we call the 'hydration lubrication' mechanism.

Articular joints in human body are uniquely efficient lubrication systems.. While the cartilage surfaces slide past each other under physiological working conditions (pressure of tens of atmospheres and shear rates up to 10(6)-10(7) Hz), the friction coefficient (mu) achieves extremely low values (down to 0.001) never successfully reached by mechanical prosthetic devices. Friction studies on polymer brushes attached to surfaces have recently demonstrated (17) their ability to reduce friction between the rubbing surfaces to extremely low values by means of the hydrated ions and the charges on the polymer chains We propose that the extremely efficient lubrication observed in living joints arises from the presence of a brush-like phase of charged macromolecules at the surface of the cartilage superficial zone: hydration layers which surround the charges on the cartilage macromolecules might provide a lubricating ball-bearing-like effect as demonstrated for the synthetic polyelectrolytes (17). In this work macromolecules of the cartilage superficial zone (aggrecans) are extracted from human femoral heads and purified using well developed biochemical techniques (20). The extracted molecules are then characterized with atomic force microscope (AFM). By means of a surface force balance (SFB) normal and shear interactions between mica surfaces coated with these molecules are examined focusing on the frictional forces between such surfaces at normal stresses similar to those in human joints.

We use a surface force balance to study shear interactions between adsorbed poly(ethylene oxide) layers (M. = 150 000 or 170 000) adsorbed onto mica in 0.1 M KNO3 aqueous solution. The shear forces increase with increasing compression of the layers or with increasing shear rates (at a constant compression), though at high compressions or shear rates the shear stress sigma(s) appears to saturate. This behavior is attributed to frictional effects that are dominated by viscous dissipation (and possible weak monomer-monomer complexing) between the mutually sliding layers at low compressions or shear rates, and by a substrate-slip mechanism at the highest compressions and shear rates. The shear behavior of adsorbed PEO layers in aqueous electrolyte at the higher compressions or shear rates can be well understood in terms of the attachment mechanism of PEO segments, via ion ligands to the charged substrate (as elucidated earlier, J. Am. Chem. Soc. 2005, 127, 1104).

Using a surface force balance, we have measured the normal and shear forces between mica surfaces across aqueous caesium salt solutions (CsNO3 and CsCl) up to 100 mM concentrations. In contrast to all other alkali metal ions at these concentrations, we find no evidence of hydration repulsion between the mica surfaces on close approach: the surfaces appear to be largely neutralized by condensation of the Cs ions onto the charged lattice sites, and are attracted on approach into adhesive contact. The contact separation at adhesion indicates that the condensed Cs ions protrude by 0.3 +/- 0.2 nm from each surface, an observation supported both by the relatively weak adhesion energies between the surfaces, and the relatively weak frictional yield stress when they are made to slide past each other. These observations show directly that the hydration shells about the Cs+ ions are removed as the ions condense into the charged surface lattice. This effect is attributed to the low energies-resulting from their large ionic radius-required for dehydration of these ions.

Using large-area (cm(2)) single-crystal mica sheets as the templating substrate, we have created correspondingly large template-stripped (TS) gold films (thickness 82 +/- 2 nm) that appear smooth to within 0.2 nm rms roughness over their entire area. These gold films, created without the use of any releasing solvent, are characterized using AFM, X-ray diffraction, multiple beam interferometric fringes of equal chromatic order (FECO), and contact angle measurements. Being molecularly smooth over large areas and (adjustably) semitransparent, these films are especially suitable for use in the surface force balance (SFB), as shown by measurements of the normal force (F) versus distance (D) profiles between such a flat gold surface and a bare mica surface in water. The F(D) profiles are in good agreement with DLVO theory down to molecular contact and indicate that the gold surface is negatively charged under water.

Analysis of friction versus time traces for stick-slip friction of mica surfaces across solidified films confined between them, reveals that most of the frictional dissipation occurs via viscous heating of the shear melted film during the part of the cycle where the surfaces slide past each other (slip). A much smaller part of the energy dissipation results from the loss of residual kinetic energy at the abrupt end of the slip.

The time variation of the frictional force between two surfaces, undergoing stick-slip sliding across a molecularly thin film of a confined model liquid, was examined at high time and force resolution, showing clearly that dissipation of energy occurs both during the slip, and at the instant of stick (via transfer of residual momentum). Detailed analysis indicates that, in marked contrast to earlier suggestions, of order 90% or more of the dissipation occurs by viscous heating of the confined shear-melted film during the slip, and only a small fraction of the energy is dissipated at the instant of stick.

Boundary lubrication, in which the rubbing surfaces are coated with molecular monolayers, has been studied extensively for over half a century(1-7). Such monolayers generally consist of amphiphilic surfactants anchored by their polar headgroups; sliding occurs at the interface between the layers, greatly reducing friction and especially wear of the underlying substrates. This process, widespread in engineering applications, is also predicted to occur in biological lubrication via phospholipid films(8,9), though few systematic studies on friction between surfactant layers in aqueous environments have been carried out(5,10). Here we show that the frictional stress between two sliding surfaces bearing surfactant monolayers may decrease, when immersed in water, to as little as one per cent or less of its value in air ( or oil). We attribute this to the shift of the slip plane from between the surfactant layers, to the surfactant/ substrate interface. The low friction would then be due to the fluid hydration layers surrounding the polar head groups attached to the substrate. These results may have implications for future technological and biomedical applications.

We use a surface force balance (SFB) to study the normal interactions between polymer brushes, which are self-assembled from solution. They consist of polystyrene (PS) chains in toluene (neutral chains in a good solvent) anchored on the interacting mica surfaces via sulfozwitterionic end groups. The properties of the brush depend on the length,N, of the chain, and the energy, alphak(B)T, with which the end group adsorbs on the surface. In contrast to earlier studies where N was varied, we attempt to vary the sticking energy by using polymer chains with one, two, and three zwitterions attached to their ends. We use the theory of Alexander and de Gennes to predict how the normal force profile should vary with a and N, finding, for example, that the brush height L-0 obeys L-0 = a . N-3/5. alpha(2/5). Surprisingly, our measurements show that the grafting density does not vary significantly between polymers with 1, 2, and 3 end groups. This could be attributed to dipole-dipole interactions between the zwitterions themselves. It is also possible that the effect could be kinetic: that the brush is unable to reach its equilibrium state because successive polymer chains are hindered from attaching to the surface by those already in the brush. Measurements on longer time scales will be necessary to determine whether kinetic effects are important.

When a nonvolatile liquid film dewets from a partly compatible liquid substrate, the advancing dewetting front leaves behind droplets formed through a Rayleigh instability mechanism at its rim. We have found that these droplets continue to move in the direction of the dewetting front for extended periods (of order one day) with an initial droplet velocity varying linearly with the droplet size, and a displacement varying logarithmically with time. We attribute this persistent motion to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front.

Using a mica surface force balance, we have measured the interactions between mica surfaces bearing chitosan (a common, naturally occurring, cationic polysaccharide; the average molecular weight of our sample was 6 x 10(5), degree of deacetylation 85%) adsorbed from acetic acid solution. We also introduced a polyionic cross-linking agent in order to cause gelation of the adsorbed layer. Both normal and shear interactions (the lateral forces acting between the surfaces as they slide past each other under compression) were measured in the two cases (freely adsorbed and cross-linked chitosan layers). Normal interactions between the adsorbed non-cross-linked chitosan layers were similar to previous reports of interactions between such layers; their shear interactions revealed a very low effective friction coefficient mu(eff) = ca. 0.003 at low compressions, increasing to ca. 0.07 at pressures of some atmospheres. We attribute the low friction to the weak interpenetration between the layers arising from steric effects and counterion osmotic pressures, together with the presence of hydration sheaths about the charged polyelectrolyte segments, which are known to provide efficient local lubrication. The higher friction on strong compressions is attributed to bridging effects. For the case of the cross-linked layer, normal interactions revealed longer ranged and more repulsive forces, due probably to the network formation resulting in a higher effective modulus of the layers. Frictional forces between the rubbing cross-linked layer were much higher than for the non-cross-linked chitosan, an effect we attribute to increased segmental friction arising from attractive interactions between the cross-linking points; this is also consistent with our observation that for the cross-linked layers significant hysteresis was observed when measuring normal interactions on approach and separation of the layers, an effect that was absent for the case of the non-cross-linked adsorbed la

Langmuir-Blodgett monolayers from end-functionalized polyisoprene (PI-X) were studied in a surface force apparatus (SFA) as a model of a highly stretched brush melt. After deposition on a freshly cleaved mica, two identical brush monolayers (with surface area per molecule of about 170 Angstrom(2)) were brought into adhesive contact in the SFA; then kinetic changes in the film thickness and the topography of the contact were continuously monitored. We observed spontaneous thinning of the brush melt bilayer. This effect can be attributed to the enhanced lateral motion of the sticking end-groups under the "contact induced pressure." The possible model describing kinetic changes in the film thickness is presented. The behavior of the two opposing brush melts and a single brush monolayer in contact with two mica surfaces was compared. Molecular mechanisms involved in the rearrangements of brush melts are proposed for both systems.

We have studied the properties of layers of end-functionalized poly(ethylene glycol) (PEG) grafted by one end to mica surfaces, and, using a surface force balance, the interaction between two such layers. PEG of molecular weight 3.4 kg/mol, functionalized at both ends with an N-hydroxysuccinimide carbamate (NHS) group, to form NHS-PEG-NHS, does not adsorb on mica. When a trimethylammonium group ((CH(3))(3)N(+)) replaced one of the NHS groups to form ((CH(3))(3)N(+))-PEG-NHS, a surface layer formed on the mica from its aqueous solution, showing that the functionalized PEG had been attached by its (CH(3))(3)N(+) end. The properties of the ((CH(3))(3)N(+))-PEG-NHS layer were examined primarily by surface force measurements as well as by atomic force microscopy, contact angle measurements, and confocal fluorescence microscopy. The layer was found to be very smooth, and the NHS group was found to retain its reactivity also when attached to the surface, demonstrating that the end-functionalized PEG chains can be used as flexible surface-active spacers for binding amino-containing molecules to the surface. Refractive index measurements yield the adsorbance and hence mean interanchor spacings of the PEG chains, while force profiles revealed the role of both electrostatic double layers and steric components of the normal interactions at different surface separations D. Shear forces between the sliding surfaces were first observed at compression ratios (2L(0)/D) similar to 1.2, increasing markedly for (2L(0)/D) > 3, where L(0) is the unperturbed PEG-brush thickness. This may be attributed both to the relatively low (L(0)/S) ratio, which allows high interpenetration and therefore high segmental density and viscous shear dissipation between opposite layers, and to bridging effects.

A surface force balance has been used to investigate the viscosity of salt-free (conductivity) water confined between hydrophilic and between hydrophobic surfaces. We examine the process of jump-in, across the last few nanometres of thin water films, to adhesive contact between the surfaces. We analyse the flow of water out of the gap under slip and no-slip boundary conditions at the confining surfaces. In both cases we find that the effective viscosity of water remains comparable to its bulk value even when it is confined to sub-nanometre thin films.

A surface force balance has been used to investigate the time-dependent behavior of the normal forces between two smooth mica surfaces immersed in salt-free (conductivity) water. A long-ranged repulsion on initial approach to adhesive contact indicates an equilibrium surface-charge density sigma=sigma(0) due to the loss of K+ ions from the mica surface into the water. Subsequent force measurements at times tau following separation of the surfaces reveal that sigma drops markedly following contact and separation, but then relaxes back to sigma(0) with a characteristic time tau(r)=11+/-2 min. This behavior is attributed to condensation of the counterions into the surfaces when they come into contact, and to their subsequent slow release into the water. The energy barrier associated with the counterion release was estimated from the value of tau(r) to be 33+/-4 k(B)T. (C) 2002 American Institute of Physics.

The fluidity of water in confined geometries is relevant to processes ranging from tribology to protein folding, and its molecular mobility in pores and slits has been extensively studied using a variety of approaches(1-6). Studies in which liquid flow is measured directly suggest that the viscosity of aqueous electrolytes confined to films of thickness greater than about 2-3 nm remains close to that in the bulk(7-9); this behaviour is similar to that of non-associative organic liquids confined to films thicker than about 7-8 molecular layers(8,10,11). Here we observe that the effective viscosity of water remains within a factor of three of its bulk value, even when it is confined to films in the thickness range 3.5 +/- 1 to 0.0 +/- 0.4 nm. This contrasts markedly with the behaviour of organic solvents, whose viscosity diverges when confined to films thinner than about 5-8 molecular layers(10-15). We attribute this to the fundamentally different mechanisms of solidification in the two cases. For non-associative liquids, confinement promotes solidification by suppressing translational freedom of the molecules(11,15-18); however, in the case of water, confinement seems primarily to suppress the formation of the highly directional hydrogen-bonded networks associated with freezing(1,3).

The forces between layers of poly(ethylene oxide) (PEO), of molecular weights M = 37 x 10(3) (PEO37) and M = 112 x 10(3) (PEO112) adsorbed onto smooth, curved solid (mica) surfaces across the good solvent toluene have been determined using a surface force balance (SFB). The SFB used is capable of measuring both normal interactions F-n(D) as a function of surface separation D and, with extreme sensitivity, shear or frictional forces F-s(D,nu (s)) between them as they slide past each other at velocity nu (s). The F-n(D) profiles are closely similar to those measured in earlier studies between adsorbed PEO layers. The shear or frictional forces between the sliding PEO-bearing surfaces are very low up to moderate compressions of the adsorbed layers (local pressures up to ca. 10(5) N m(-2)), corresponding to effective friction coefficients mu (eff) = (F-s/F-n) of order 0.003 or less. This is attributed to the fluid interfacial layer between the adsorbed layers resulting from their weak mutual interpenetration. At higher loads F-s increases markedly, and two forms of behavior are found depending on the PEO molecular weight. For PEO37, a sharp increase in F-s is followed by removal of polymer from within the intersurface gap during sliding, high friction, and adhesion between the surfaces. For the longer PEO112, the initial increase in F-s and in mu (eff) saturates at the highest loads (for the case of mu (eff) even decreasing), indicating that the slip plane has moved from the polymer/polymer midplane to the polymer/ solid interface. The dependence of F-s on the sliding velocity in the high-friction regime is weak, suggesting that at low compressions there is a thinning of the mutual adsorbed-layer-interpenetration region at high nu (s) that offsets the higher viscous dissipation in that region. At the highest loads, when the slip plane has shifted to the mica surface, the weak F-s(nu (s)) dependence is characteristic of sliding friction at solid substrates.

We have confirmed the existence of the BeC62- dianion produced in a Cs-sputter negative ion source with a cathode of mixed Be metal and graphite powder. Using our laser-accelerator mass spectrometry (AMS) system, we have studied the photoelectric interaction of the dianion and show that it is bound by more than 1.165 eV relative to any photodetachment or dissociation channel. The total cross-section leading to either of the channels for 2.33 eV photons is 6 Mb. (C) 2000 Elsevier Science B.V. All rights reserved.

The interactions between two mica surfaces bearing a fourth-generation carbosilane dendrimer (modified to expose -OH groups on its outer surface) were studied across a toluene medium, using a surface force balance capable of measuring shear as well as normal forces. Normal force measurements indicate that the dendrimers adsorb from dilute toluene solution (ca. 5 x 10(-4) w/w) as a monolayer on each surface. Two such interacting surfaces experience a longer-ranged van der Waals attraction followed by strong short-range adhesion (probably of dipolar origin) as the adsorbed dendrimers come into contact. Within the range of our parameters, the dendrimer layers were incompressible normal to the surfaces. Friction versus load profiles were measured at different shear velocities, revealing marked stick-slip sliding, whereas the magnitude of the yield stress increased with longer times of contact and with normal pressure. This suggests that over time scales comparable with the experimental times the interacting layers rearrange to optimize their interfacial shear strength. The behavior of these -OH-exposing carbosilane dendrimers differs qualitatively from that of CH3-exposing poly(propyleneimine) dendrimers studied earlier, a difference attributable to the much more polar nature of the hydroxyl groups.

We have used composition depth profiling, based on nuclear-reaction analysis and secondary ion mass spectroscopy, to study segregation at the free surface of a partly miscible binary mixture consisting of random oleifinic copolymers. The equilibrium surface excess data, analysed within a mean-field Cahn approach, point to a wetting transition. The surface phase diagram obtained was confirmed by the observed dynamics of the segregation from a coexistence composition: the monotonic and halted growth of the surface layer was observed at temperatures above and below the predicted wetting point, respectively.

We compare segmental interaction parameters chi(AB) in binary polyolefin mixtures extracted from a fit to the binodal-as deduced from composition-depth profiling of thin films-with values extracted from small-angle neutron scattering (SANS), on the self-same blends. We find good quantitative agreement as regards both absolute Values and compositional dependence of chi(AB). We also compare values of chi(AB) obtained from composition-depth profiles of surface segregation in isotopic polystyrene mixtures with values deduced from SANS from these mixtures, and again we find good agreement between them. These results point to the use of thin-film composition-depth profiling, which is potentially a more accessible technique and requires far smaller quantities of material, as a convenient, accurate, and widely applicable alternative to SANS for measuring segmental interactions in binary polymer mixtures.

According to a common viewpoint the surface of the binary polymer mixture A/B could be enriched only in one component, say A, regardless of the value of its bulk volume fraction phi(bulk). From recent theoretical analyses and Monte Carlo simulations one can expect however that some mixtures can exhibit surface segregation in the minority blend component, i.e. enrichment in the component A for phi(bulk) 50%- Using composition-depth profiling techniques, we have observed both types of the segregation with only one- or both-blend component(s) preferred at the surface. These results were obtained for model mixtures composed of partly deuterated (dx) and protonated (hx) random olefinic copolymers of the structure -[C4H8](1-x)[C2H3(C2H5)](x)-. A simple mean-field model is presented to explain both situations. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.

The capillary broadening of a 2-phase interface is investigated both experimentally and theoretically. When a binary mixture in a thin film with thickness D segregates into two coexisting phases the interface between the two phases mag form parallel to the substrate due to preferential surface attraction of one of the components. We show that the interfacial profile (of intrinsic width w(o)) is broadened due to capillary waves, which lead to fluctuations, of correlation length xi(parallel to) of the local interface positions in the directions parallel to the confining walls. We postulate that xi(parallel to) acts as an upper cutoff for the spectrum of capillary waves on the interface, so that title effective mean square interfacial width w varies as w(2) proportional to ln xi(parallel to). In the limit of large D this yields w(2) proportional to D or w(2) proportional to ln D respectively for the case of short- or long-range forces between walls and the interface. We used the Nuclear Reaction Analysis depth profiling technique, to investigate this broadening effect directly in two binary polymer mixtures. Our results reveal that the interfacial width indeed increases with film thickness D, through the observed interfacial width is lower than the predicted w. This is probably due to surface tension effects imposed by the confining surfaces which are not taken into account in our model.

We have examined the effect of deuterium labeling on surface interactions in mixtures of random olefinic copolymers [C4H8](1-x)[C2H3(C2H5)](x). Based on surface segregation data we have determined a surface energy difference chi(s) between pure blend constituents. In each binary mixture components have different fractions x(1), x(2) of the group C2H3(C2H5), and one component is labeled by deuterium (dx) while the other is hydrogenous (hx). The mixtures are grouped in four pairs of structurally identical blends with swapped labeled constituent (dx(1)/hx(2), hx(1)/dx(2)). For each pair the surface energy parameter chi(s) increases when the component with higher fraction x is deuterated, i.e., chi(s)(dx(1)/hx(2)) > chi(s)(hx(1)/dx(2)) for x(1) > x(2). A similar pattern has been found previously for the bulk interaction parameter chi. This is explained by the solubility parameter formalism aided by the lattice theory relating the surface excess to missing-neighbor effect. chi(s) has also an additional contribution, insensitive to deuterium swapping effect, and related to entropically driven surface enrichment in a more stiff blend component with a lower fraction x. Both enthalpic and entropic contributions to chi(s) seem to depend on the extent of chemical mismatch between blend components. (C) 1998 John Wiley & Sons, Inc.

A surface force balance with extremely high resolution in measuring shear forces has been used to study the properties of films of the simple organic solvents cyclohexane, octamethylcyclotetrasiloxane, and toluene, confined in a gap between smooth solid surfaces. We were able to probe in detail the transition between liquidlike and solidlike behavior of the films as the gap thickness decreased. Our results reveal that in such confined layers the liquids are fluid down to a film thickness of few molecular layers (typically seven, depending on the particular liquid examined). On further decreasing the gap thickness by a single molecular layer, the films undergo an abrupt transition to become solidlike in the sense that they are able to sustain a finite sheer stress for macroscopic times. At the transition, the effective rigidity of the films, quantified in terms of an effective creep viscosity, increases by at least seven orders of magnitude. This sharp transition is reversible and occurs as a function of the confinement alone: it does not require external applied pressure. Following the transition the confined films behave under shear in a manner resembling ductile solids. (C) 1998 American Institute of Physics.

A detailed analysis of the experimental implications of the scaling theory of polymer adsorption in the good solvent regime is presented. A number of new results that simplify the task of comparing theoretical predictions and experimental data are given: these include a new simple approximate expression for the force vs distance profile, which is close to the exact solutions over an appreciable range of the intersurface separations. The theoretical predictions are compared to available experimental data for the forces arising between mica plates exposed to poly(ethylene oxide)-water solutions. Good quantitative agreement between theory and experiment is found, with no need for adjustable parameters.

Using a surface force balance, the orientation of a nematic liquid crystal (6CB) confined between mica surfaces, and its response to shear under extreme confinement was examined. Slow transitions (over a period of days) from a planar twisted to a planar orientation were observed in certain conditions. We propose that these are associated with anchoring transitions.

We describe a novel approach based on atomic force microscopy to determine the dihedral contact angle of a liquid polymer on top of a solid substrate. By cross-linking the polymer in an ion beam, the polymer surface becomes amenable to AFM imaging. This sample preparation enables us to take topography scans of the polymer in a solid state. The cross section of the contact area of a polymer drop and the substrate can then be utilized to determine the contact angle between polymer and substrate. Extremely low contact angles in the range 2-8 degrees are reproducibly measured. Control measurements carried out with optical phase modulated interference microscopy gave contact angles well comparable to the AFM results. We use our method in a study of polymer dewetting on top of a network of itself.

Using a surface force balance, we have examined the orientation of a nematic liquid crystal (6CB) confined between mica surfaces. By simultaneous measuring of the force and the refractive index profile, we were able to distinguish between planar, planar twisted, and homeotropic orientation of the nematic. When the mica was exposed to air for short periods (

We have investigated the wetting behaviour of a polymer melt on top of a cross-linked network of itself. For substrate films that were not cross-linked at all (or at very low cross-link densities) the melt completely wets the underlying layer. At intermediate cross-linking densities we observe dewetting, which we suggest is due to the brush-like surface of the network. At higher cross-linking densities the melt again completely wets the network, due, we believe, to increased roughening of the surface of the cross-linked substrate.

We have measured the growth with time t of a wetting layer (of thickness I(t)) at the surface of a thin film of a binary liquid (polymer) mixture. Over a wide range of experimental parameters, our data is well described by a model of diffusion limited wetting which takes into account the finite film thickness. In this model, l(t) is a function of time which sensitively depends on the nature of the interfacial potential: a detailed comparison shows that long range van-der-Waals forces provide the main driving force for the build-up of the wetting layer.

When a film of thickness D of a binary mixture on a substrate segregates into two coexisting phases, an interface between the phases parallel to the substrate may form due to preferential surface attraction of one of them. It is argued then that the correlation length xi(parallel to) for interfacial fluctuations parallel to this interface (ln xi(parallel to) proportional to D) leads to a size-dependent interfacial width w proportional to root D. Nuclear reaction depth profiling experiments on polymer mixtures as well as Ising model simulations both support this prediction.

We have used composition depth profiling of polymer bilayers, based on nuclear reaction analysis, to determine miscibility, phase coexistence, and critical temperatures in mixtures of random olefinic copolymers of mean composition E(1-x)/EE(x); here E is the ethylene group -(C4H,)-, EE is the ethylethylene group -[C2H3(C2H5)]-, and one of the copolymers is partially deuterated. The components in each binary mixture have different values x(1),x(2) of the EE fraction. Using a simple Flory-Huggins mixing model, our results enable us to extract an interaction parameter of the form chi(x(1),x(2),T)=A(x(1),x(2))/T, where for given x(1),x(2), A is a constant. Calculated binodals using this form fit our measured coexistence curves well, while allowing chi a weak composition dependence improves the fit further. Within the range of our parameters, our results suggest that in such binary polyolefin mixtures the interaction parameter increases roughly linearly with the extent of chemical mismatch expressed as the difference in degree of ethyl branching between the two components. (C) 1996 American Institute of Physics.

Interfacial adsorption from a binary mixture of short and long polystyrene -polyisoprene diblock copolymers (PS-PI or its deuterated analogue dPS-PI) to interfaces of a PS(M = 3 x 10(6)) homopolymer was studied using nuclear reaction analysis. Short symmetric diblocks dPS(M = 10(4))-PI(M = 10(4)) (designated dS), or a similar protonated analogue (hS), and a highly asymmetric, long diblock dPS(M = 10(6))-PI(M = 10(4)) (L) were used in the binary mixtures. We found that the shorter diblocks adsorb preferentially at the interfaces: the isotopic contrast between dS and hS in the mixtures with L enables us to extract detailed information on the surface segregation of the short and long species. Our data are analyzed in terms of a modified Flory-type mean field model due to de Gennes and Leibler. Interfacial segregation from the binary diblock mixture is well predicted by this model with no adjustable parameters, using the adsorption characteristics determined earlier for systems with a single diblock component.

We have used nuclear reaction analysis to measure diffusion coefficients D in couples consisting of hydrogenated polybutadienes of structure (C2H3(C2H5))(x)(C4H8)(1-x) and their partly deuterated counterparts. The 1,2- and 1,4-olefinic isomers are randomly distributed along the chains and the mean vinyl fraction x varies between 0.38 and 0.94. We find that the effective monomeric mobility D-0 [defined by D = D-0(N-e/N-2) for each copolymer, where N is the backbone length and N, the entanglement spacing] decreases monotonically with increasing vinyl content x. Over the range of microstructures and temperatures T (-14-40 degrees C) investigated we find log(D-0/T) varies smoothly with (T - T-g), where T-g is the glass transition temperature of the respective melts. An analysis of our data in terms of a simple activated rate process model suggests that D-0 is controlled by thermally activated hopping of segments whose effective volume is close to that of the respective statistical segment lengths of the copolymeric chains. (C) 1995 John Wiley and Sons, Inc.

Thin films of a low molecular weight, nonvolatile liquid which are forced to spread on silicon wafers, rupture within minutes and dewet. Addition of long polystyrene chains (for which the liquid is a good solvent) up to a concentration of 10% does not change this behavior qualitatively. In contrast we find that the unbroken uniformity of the films may be preserved for many months or longer by a polystyrene brush attached to the silicon, together with some unattached polystyrene in the liquid. By varying the molecular weights and concentrations of the unattached chains within the film, we were able to establish a stability diagram in this system which shows that suppression of rupture is only observed at free-polymer concentrations which exceed the overlap concentration. This suggests that the effect may arise from the formation of an entanglement network between the free chains (within the liquid film) and the surface-tethered brush molecules.

The diffusion process in dPS (M(w) = 1.06 x 10(6) g mol-1)/hPS (M(w) = 2.89 x 10(6) g mol-1) mixtures at constant temperature below the critical temperature slows down approaching the coexistence curve from the homogeneous single fluid phase region of the phase diagram. This effect can be understood in terms of the composition dependence of the thermodynamic factor partial derivativemu(dPS)/partial derivativePHI(dPS)T,P in combination with the Flory-Huggins theory of polymer melts. The theoretically expected decrease of the mutual diffusion coefficient with increasing temperature along the coexistence curve approaching the critical temperature is not observed in the temperature range studied (5 K

Critical point wetting generally occurs whenever a binary fluid system in the demixed region of phase space, in contact with a surface, approaches its critical temperature. For the case of polymeric mixtures the segmental interaction parameter, which determines the interfacial tension between the coexisting phases, can be very low even far from the critical point. This suggests that polymer blends should be likely candidates to exhibit complete wetting even far below the critical point. Using nuclear reaction analysis, we have now observed such complete wetting behaviour from two different classes of binary polymer mixtures. These are mixtures of statistical olefinic copolymers, with slightly different chemical microstructures; and an isotopic pair of deuterated and protonated polymers. In the former case we have been able to follow the growth with time t of the wetting layer thickness l; our results indicate l approximately log t.

Stabilization against the rupture and breakup of thin, nonwetting liquid films spread on surfaces is generally sought by modification of equilibrium interfacial properties. A mechanism for suppressing rupture in such films that uses surface-attached polymers together with free chains in the bulk of the film is reported. Films of an oligostyrene liquid, which rupture within several minutes when spread on a silicon wafer, may be stabilized for many months by a polystyrene brush attached to the substrate, together with some free polystyrene in the liquid. The effect may arise from entanglements of the free chains with the immobilized brush.

Keywords: Crystallography; Polymer Science

We have studied both the equilibrium and the hydrodynamic lubrication forces that act in the normal direction between mica surfaces in toluene when bearing polystyrene chains anchored to each surface by a zwitterionic group at one end. The quasistatic force-distance profiles reveal the long-ranged repulsion characteristic of steric interactions between the extended brush-like layers in the good solvent and provide a measure of the brush thickness L in close agreement with earlier studies of such brushes. Dynamic measurements of the surface forces, where the surface separation D is varied sinusoidally in time, show two regimes: at D > 2.5L, the hydrodynamic forces are characteristic of Newtonian flow of liquid with the viscosity of bulk toluene, but with a shear plane shifted a distance L(H) from each mica surface (qualitatively similar to earlier observations with adsorbed homopolymer layers). We find that L almost-equal-to L(H). When the layers are compressed (D

The surfaces of thin, liquid films can be unstable due to thinning van der Waals interactions, leading to the formation of holes in the initially uniform film. These instabilities can be greatly retarded in viscoelastic materials (and completely inhibited in elastic materials) even when the finite frequency shear modulus, E, is small compared to the infinite frequency modulus, G. This occurs when E/G much greater than (a/h0)5 much less than 1 where a is a molecular size and h0 is the film thickness. We relate the growth rate of the instability to the dynamic viscosity, eta (omega), with examples for the cases of a polymer brush, an elastic fluid (gel), and a transient polymer network, described by reptation dynamics.

Polymers that attach at solid-liquid interfaces can considerably modify the long-ranged forces acting between surfaces. Over the past decade or so these forces have been studied directly using a surface-force-balance approach, and the equilibrium force laws between two polymer-bearing surface interacting across a liquid medium are reasonably well understood. Recently we have extended these studies to the case of surfaces with adsorbed and with grafted layers as they shear part each other. Our results suggest that the lateral forces depend on the extent of interpenetration of the opposing layers and on their relative shear velocities; they reveal a marked coupling between the normal forces and the shear velocity, beyond a certain critical value of the latter which is probably related to relaxation rates of the end-tethered chains. We find that while non-adsorbing but end-anchored polymer layers are stable with respect to shear and compression, they can be rapidly replaced by addition of shorter polymers with the same adhering end-group.

We have measured the composition-distance profile across a film consisting of two thin layers (200-600 nm) of a model binary isotopic mixture of deuterated polystyrene (dPS) and protonated polystyrene (hPS), coexisting with each other near their equilibrium compositions below the critical temperature for phase demixing for this pair. Profiles were determined normal to the silicon wafer on which the bilayer is mounted using nuclear reaction analysis, both for an uncoated silicon surface and for one coated with a gold layer. Measurements reveal that when both layers are thick relative to the characteristic width w (approximately 100 nm) of the interfacial region between them, the coexisting compositions about the interface are close to their bulk values as determined earlier for this system. When the dimensions of the layers are made comparable with w, however, interactions with the confining surfaces may significantly modify the composition profile of the coexisting layers about the interface. This effect is marked at the polymer/silicon interface as a result of its interactions with one of the components (dPS), but is absent for a gold-coated surface in an identical geometry due to the much weaker influence of the surface. Our results are discussed in detail in terms of mean-field models of mixing in polymeric mixtures, and enable quantitative determination (using a Cahn construction approach) of the interaction parameters both at the polymer-air and polymer-silicon interfaces. Though we are not able to calculate in a completely a priori fashion the coexistence profiles as a function of the film thickness, we propose an approximate approach which provides good agreement of calculated composition profiles with those determined experimentally over the range of parameters in our experiments.

We have measured directly the phase coexistence characteristics in a high-molecular-weight binary isotopic polymer mixture over a range of temperatures around the critical point, using nuclear-reaction analysis. Our results reveal an equilibrium phase diagram with an upper critical solution temperature, in fair quantitative accord with the simplest form of the Flory-Huggins mean-field model. Deviations may be due to a mild compositional dependence of the monomeric interaction parameter.

The mutual mixing, and hence the development with time of the interface between two chemically different polymeric species, may be dominated by enthalpic rather than entropic considerations. Two extreme cases are described. The first is accelerated interdiffusion, that occurs when the polymers have a large mutual attraction, and results in remarkable concentration profiles across the interface. At the other extreme is suppressed interdiffusion, where two partially miscible polymers are in contact. The interface in this case tends to an equilibrium width, that varies with time quite differently than for the case of free interdiffusion.

In this lecture we briefly review our understanding of surface forces with adsorbed, neutral flexible polymer chains, and describe some recent studies of surface forces with a range of end-grafted chains. In the latter case we find the polymer layers to be approximately twice the thickness of adsorbed layers of comparable size and surface-exess; we find that the interactions are monotonically repulsive and rate-independent at all surface coverages. Our results are compared with both scaling and meanfield models. and are in very good qualitative and quantitative agreement with both theoretical approaches.