Publications

Hydrogels with pure hydrophilic network have received much attention due to their excellent low frictional behavior. However, the lubrication performance of hydrogels is not satisfied under high-speed condition due to the energy dissipation caused by adsorbed polymer chains as well as the failure of lubricating mechanisms accompanied by the transition of lubrication regime. In this work, interpenetrating double-network organohydrogels were constructed by combining hydrophilic and oleophilic polymer networks to modify the physiochemical properties of surface polymer chains, especially the chain mobility. The oleophilic polymer network spatially restricting the mobility of the swollen hydrophilic network in water, resulted in a low coefficient of friction (ca. 0.01) compared with conventional hydrogels at high speed (0.1\u2005m\u2009s−1). Meanwhile, the organohydrogels had superior wear resistance, with almost no wear observed on the sliding track after 5\u2005k\u2005cycles of rubbing at high speed. The design concept of organohydrogels can be extended to a variety of low-wear, highly-lubricating materials.

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.

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.

ConspectusIn the course of evolution, nature has achieved remarkably lubricated surfaces, with healthy articular cartilage in the major (synovial) joints being the prime example, that can last a lifetime as they slide past each other with ultralow friction (friction coefficient μ = the force to slide surfaces past each other/load compressing the surfaces

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\u201310-4 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\u201360 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.

The original version of this Article contained an error throughout in which an incorrect symbol was used for the diffusion coefficient: it should be cambria math, italicized, and not bold. These have been corrected in both the PDF and HTML versions of the Article.

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.

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 interferometric 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 (∼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 (μ ≈ 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\u2010exposing phosphatidylcholine lipid bilayer vesicles (PC\u2010liposomes) were shown to be very efficient boundary lubricants in aqueous media by virtue of the highly\u2010hydrated phophocholine groups, we examined interactions between surfaces bearing phosphocholinated polystyrene nanoparticles (pc\u2010PS\u2010NPs). We synthesized such particles by incorporation of alkyl chains terminated with phosphocholine groups in the PS\u2010NP, 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\u2009>\u20099\u2009atm) the pc\u2010PS\u2010NPs were squeezed out, leaving a single layer between the surfaces. Shear of the surfaces revealed a large frictional dissipation, with a friction coefficient μ\u2009≈\u20090.2, in contrast to our expectations that such phosphocholinated NPs would provide highly lubricating analogues of PC\u2010liposomes (for which μ\u2009≈\u200910−3 \u2013 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\u2010NP surfaces. In particular, at higher compressions, sliding results in energy\u2010dissipating 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\u2010PS\u2010NPs.

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 μ 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 μ, 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.

Understanding the structure of solid supported lipid multilayers is crucial to their application as a platform for novel materials. Conventionally, they are prepared from drop casting or spin coating of lipids dissolved in organic solvents, and lipid multilayers prepared from aqueous media and their structural characterisation have not been reported previously, due to their extremely low lipid solubility (i.e. ∼10^-9 M) in water. Herein, using X-ray reflectivity (XRR) facilitated by a "bending mica" method, we have studied the structural characteristics of dioleoylphosphatidylcholine (DOPC) multilayers prepared via drop casting aqueous small unilamellar and multilamellar vesicle or liposome (i.e. SUV and MLV) dispersions on different surfaces, including mica, positively charged polyethylenimine (PEI) coated mica, and stearic trimethylammonium iodide (STAI) coated mica which exposes a monolayer of hydrocarbon tails. We suggest that DOPC liposomes served both as a delivery matrix where an appreciable lipid concentration in water (∼25 mg mL^-1 or 14 mM) was feasible, and as a structural precursor where the lamellar structure was readily retained on the rupture of the vesicles at the solid surface upon solvent evaporation to facilitate rapid multilayer formation. We find that multilayers on mica from MLVs exhibited polymorphism, whereas the SUV multilayers were well ordered and showed stronger stability against water. The influence of substrate chemistry (i.e. polymer coating, charge and hydrophobicity) on the multilayer structure is discussed in terms of lipid-substrate molecular interactions determining the bilayer packing proximal to the solid-liquid interface, which then had a templating effect on the structure of the bilayers distal from the interface, resulting in the overall different multilayer structural characteristics on different substrates. Such a fundamental understanding of the correlation between the physical parameters that characterise liposomes and substrate chemistry, and the structure of lipid multilayers underpins the potential development of a simple method via an aqueous liposome dispersion route for the inclusion of hydrophilic functional additives (e.g. drugs or nanoparticles) into lipid multilayer based hybrid materials, where tailored structural characteristics are an important consideration.

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 (μ -4) up to pressures of 70-90 atm. Similarly, POPC SUVs also formed bilayers which exhibited low friction (μ -4) up to pressures as high as 160 atm. DPPC and DSPC SUVs also provided good lubrication, but with slightly higher friction coefficients (μ = 10^-3-10^-4). We believe these differences originate from fast self-healing of the softer surface layers (which are in their liquid disordered phase, POPC, or close to it, DMPC), which renders the robustness of the DPPC or DSPC (both in their solid ordered phase) less important in these conditions. Under these circumstances, the enhanced hydration of the less densely packed POPC and DMPC surface layers is now believed to play an important role, and allows enhanced lubrication via the hydration lubrication mechanism. Our findings may have implications for the understanding of complex biological systems such us biolubrication of synovial joints.

Jee et al. assert (1) that the effect we measure\u2014what happens to a lubricant film thickness in individual slip events during intermittent stick\u2013slip sliding (2)\u2014was already measured long ago (3, 4), and with better resolution. We are bemused by this assertion because in the works cited (3, 4) not only is intermittent stick\u2013slip friction, with its characteristic saw-tooth pattern, not shown or measured, but their inability to measure what happens during the individual, fleeting slip events\u2014the key finding of our own study (2)\u2014is explicitly stated in the papers themselves (below).

Intermittent sliding (stick-slip motion) between solids is commonplace (e.g., squeaking hinges), even in the presence of lubricants, and is believed to occur by shear-induced fluidization of the lubricant film (slip), followed by its resolidification (stick). Using a surface force balance, we measure how the thickness of molecularly thin, model lubricant films (octamethylcyclotetrasiloxane) varies in stick-slip sliding between atomically smooth surfaces during the fleeting (ca. 20 ms) individual slip events. Shear fluidization of a film of five to six molecular layers during an individual slip event should result in film dilation of 0.4-0.5 nm, but our results show that, within our resolution of ca. 0.1 nm, slip of the surfaces is not correlated with any dilation of the intersurface gap. This reveals that, unlike what is commonly supposed, slip does not occur by such shear melting, and indicates that other mechanisms, such as intralayer slip within the lubricant film, or at its interface with the confining surfaces, may be the dominant dissipation modes.

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.

Using a surface force balance (SFB), we measured the boundary friction and the normal forces between mica surfaces immersed in a series of alkyltrimethylammonium chloride (TAC) surfactant solutions well above the critical micelle concentration (CMC). The surfactants that were used-C ₁₄TAC, C₁₆TAC, and C₁₈TAC-varied by the length of the alkyl chain. The structures of the adsorbed layers on the mica were obtained using AFM imaging and ranged from flat bilayers to rodlike micelles. Despite the difference in alkyl chain, all the surfactant solutions reduce the friction between the two mica surfaces enormously relative to immersion in water, and have similar friction coefficients (μ -0.001). The pressure at which such lubrication breaks down is higher for the surfactants with longer chain lengths and indicates that an important role of the chain length is to provide a more robust structure of the adsorbed layers which maintains its integrity to higher pressures.

Using a surface force balance, we have measured the forces between bare (hydrophilic) mica surfaces, and between hydrophobized mica surfaces, in each case coated with the amphiphilic protein hydrophobin (HFBI) from Trichoderma reesei. We additionally characterized these surfaces by contact angle measurements and AFM. The results are consistent with the formation of hydrophobic surfaces exposed by HFBI adsorbed on the hydrophilic substrate, and hydrophilic surfaces exposed by HFBI adsorbed on the hydrophobic substrate. In particular, friction between HFBI surfaces on hydrophobized mica, exposing hydrophilic surfaces, is an order of magnitude lower than friction between HFBI-coated hydrophilic surfaces, which expose the hydrophobic side of the protein to the water interface. This result can be readily understood in terms of the greater hydration level of the exposed outer surfaces in the former case.

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 a surface force balance, normal and shear interactions have been measured between two atomically smooth surfaces coated with hyaluronan (HA), and with HA/aggrecan (Agg) complexes stabilized by cartilage link protein (LP). Such HA/Agg/LP complexes are the most abundant mobile macromolecular species permeating articular cartilage in synovial joints and have been conjectured to be present as boundary lubricants at its surface. The aim of the present study is to gain insight into the extremely efficient lubrication when two cartilage surfaces slide past each other in healthy joints, and in particular to elucidate the possible role in this of the HA/Agg/LP complexes. Within the range of our parameters, our results reveal that the HA/Agg/LP macromolecular surface complexes are much better boundary lubricants than HA alone, likely because of the higher level of hydration, due to the higher charge density, of the HA/Agg/LP layers with respect to the HA alone. However, the friction coefficients (μ) associated with the mutual interactions and sliding of opposing HA/Agg/LP layers (μ ≈ 0.01 up to pressure P of ca. 12 atm, increasing sharply at higher P) suggest that such complexes by themselves cannot account for the remarkable boundary lubrication observed in mammalian joints (up to P > 50 atm).

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 μ ~ 0.24 and μ ~ 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.

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 0.15 M NaNO ₃. 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 × 10 ^-5 in pure water system or 6 × 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.

The roles of macromolecules in living systems as information storage systems (as DNA) and in biochemical synthesis have been much studied and are relatively well understood. Far less is known about their physical behavior at biological surfaces and interfaces. This review considers in particular the roles of polymers in biological lubrication and its relation both to diseases such as osteoarthritis and to remedies such as tissue engineering. The lubricating behavior of common bio-interfacial macromolecules including mucins, hyaluronan, lubricin, and aggrecan are described, and insights into the mechanism of biolubrication are examined in the light of the recently revealed role of hydration lubrication in water-based (including living) systems.

Human salivary statherin was purified from parotid saliva and adsorbed to bare hydrophilic (HP) mica and STAI-coated hydrophobic (HB) mica in a series of Surface Force Balance experiments that measured the normal (F_n) and friction forces (F_s*) between statherin-coated mica substrata. Readings were taken both in the presence of statherin solution (HP and HB mica) and after rinsing (HP mica). F_n measurements showed, for both substrata, monotonic steric repulsion that set on at a surface separation D ~ 20 nm, indicating an adsorbed layer whose unperturbed thickness was ca 10 nm. An additional longer-ranged repulsion, probably of electrostatic double-layer origin, was observed for rinsed surfaces under pure water. Under applied pressures of ~ 1 MPa, each surface layer was compressed to a thickness of ca 2 nm on both types of substratum, comparable with earlier estimates of the size of the statherin molecule. Friction measurements, in contrast with F_n observations, were markedly different on the two different substrata: friction coefficients, μ ≡ ∂ F_s*/∂F_n, on the HB substratum (μ ≈ 0.88) were almost an order of magnitude higher than on the HP substratum (μ ≈ 9 and 0.12 for unrinsed and rinsed, respectively), and on the HB mica there was a lower dependence of friction on sliding speed than on the HP mica. The observations were attributed to statherin adsorbing to the mica in multimer aggregates, with internal re-arrangement of the protein molecules within the aggregate dependent on the substratum to which the aggregate adsorbed. This internal rearrangement permitted aggregates to be of similar size on HP and HB mica but to have different internal molecular orientations, thus exposing different moieties to the solution in each case and accounting for the very different friction behaviour.

Using a surface force balance, we measured normal and shear interactions as a function of surface separation between layers of hydrogenated soy phosphatidylcholine (HSPC) small unilamellar vesicles (SUVs) adsorbed from dispersion at physiologically high salt concentrations (0.15 M NaNO ₃). Cryo-scanning electron microscopy shows that each surface is coated by a close-packed HSPC-SUV layer with an overlayer of liposomes on top. A clear attractive interaction between the liposome layers is seen upon approach and separation, followed by a steric repulsion upon further compression. The shear forces reveal low friction coefficients (μ = 0.008-0.0006) up to contact pressures of at least 6 MPa, comparable to those observed in the major joints. The spread in μ-values may be qualitatively accounted for by different local liposome structure at different contact points, suggesting that the intrinsic friction of the HSPC-SUV layers at this salt concentration is closer to the lower limit (μ = ∼0.0006). This low friction is attributed to the hydration lubrication mechanism arising from rubbing of the 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.

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

This paper presents measurements, using the surface force balance (SFB), of the normal and shear forces in aqueous solutions between polyelectrolyte layers grown directly on mica substrates (grafted-from). The graftingfrom was via surface-initiated atom transfer radical polymerization (surface-initiated ATRP) using a positively charged methacrylate monomer. X-ray reflectometry measurements confirm the successful formation of polyelectrolyte layers by this method. Surface-inititated ATRP has the advantages that the polymer chains can be strongly grafted to the substrate, and that high grafting densities should be achievable. Measured normal forces in water showed a long-range repulsion arising from an electrical double layer that extended beyond the polyelectrolyte layers, and a stronger, shorter-range repulsion when the polyelectrolyte brushes were in contact. Swollen layer thicknesses were in the range 15-40 nm. Upon addition of ̃10-2-10-1 M sodium nitrate, screening effects reduced the electrical double layer force to an undetectable level. Shear force measurements in pure water were performed, and the measured friction may arise from polymer chains bridging between the surfaces.

The importance of understanding the interactions between nanoscale materials and living matter has now begun to be appreciated by an extraordinaryly large range of stakeholders, including researchers, industry, governments and society, all of whom appreciate both the opportunities presented by and challenges raised by this arena of research. Not only does it open up new directions in nanomedicine and nanodiagnostics, but it also offers the chance to implement nanotechnology across all industry in a safe and responsible manner. The underlying reasons for this arena as a new scientific paradigm are real and durable. Less than 100 nm nanoparticles can enter cells, less that 40 nm they can enter cell nucleus, and less that 35 nm they can pass through the blood brain barrier. These are fundamental length scales of biological relevance that will ensure that engineered nanoscience will impinge on biology and medicine for many decades to come. One important issue is the current lack of reproducibility of the outcomes of many experiments in this arena. Differences are likely a consequence of such things as uncontrolled nanoparticle aggregation leading to unpredictable doses being presented to cells, interference of the nanoparticles themselves with many of the tests being applied, differences in the degree of confluency of the cells used, and a host of other factors. NanoInteract has shown how careful control of all aspects of the test system, combined with round robin type approaches, can help resolve these issues and begin to ensure that the field can become a quantitative science. The basic principle of NanoInteract is that given identical nanomaterials, cells and biological materials, and using a common protocol, experiments must yield identical answers. Thus, any deviations result from errors in (applying) the protocol which can be tracked and eliminated, until quantitatively reproducible results are obtained by any researcher in any location. This paper outlines the NanoInteract programme, illustrates key advances, and highlights early successes. (www.nanointeract.net)

We use a surface force balance to study shear interactions between adsorbed poly(ethylene oxide) layers (M_w = 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 σ_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).

The interactions of hyaluronan (HA), a high-molecular-weight linear polysaccharide present in many pericellular coatings, with different facets of chiral calcium tartrate (CT) crystal surfaces are investigated using a molecular force probe. Forces between {011} and {110} facets of (R,R) and (S,S) CT crystals and a HA-bearing surface have been measured in saturated CT solutions. It has been observed that hyaluronan binds most strongly to the {011} facet of the (R,R) crystal, compared with the other facets examined, which is consistent with earlier observations of the adhesion of HA-coated cells to chiral CT crystals. The variation of binding strength among the facets studied is tentatively attributed to the surface structure difference between the {011} and {110} facets as well as to the preferential matching of the local hyaluronan H-bond network to the -OH groups on the {011} facet of the (R,R) enantiomer.

We briefly review the model that correlates friction between two surfaces in adhesive contact with the loading-unloading adhesion hysteresis between them. We then examine in light of this model the observed low friction between two mica surfaces coated with a double-chained quaternary ammonium surfactant in intimate adhesive contact in water. This enables us to propose a mechanism for surfactant boundary lubrication in water that is rather different from the classic boundary lubrication in air: in this mechanism, adhesion takes place at the interface between the opposing surfactant hydrocarbon tails, whereas frictional sliding takes place at the interface between the hydrated surfactant headgroups and mica. The implications of our findings to biolubrication processes are discussed.

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.

Boundary lubrication, in which the rubbing surfaces are coated with molecular monolayers, has been studied extensively for over half a century. 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, though few systematic studies on friction between surfactant layers in aqueous environments have been carried out. 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.

Using surface force balance measurements we have established that polystyrene chains bearing three zwitterionic groups have a higher end-group sticking energy than equivalent chains bearing a single zwitterionic group. In a good solvent, polystyrene chains end-functionalized with three zwitterionic groups form brushes of a higher surface coverage than those bearing a single zwitterion. The increase in surface coverage is slow compared with the initial formation of the brush. Measurements of the refractive index allow us to directly quantify the variation of surface coverage, permitting comparison with models for the kinetics of brush formation based on scaling theory and an analytical self-consistent field. We find qualitative support for associating the kinetic barrier with the energy required for an incoming chain to stretch as it penetrates the existing brush.

The persistant droplet motion in liquid-liquid dewetting was discussed. It was found that the droplet continued to move in all direction of the dewetting front for extended periods with an initial droplet velocity varying linearly with the droplet size. It was observed that the persistant motion was due to a transient surface tension gradient on the substrate liquid surface trailing the dewetting front. A model to calculate the absolute values of the spontaneous droplet displacement and its variation with time was also presented.

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 × 10⁵, 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 μ_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 layers. Refractive index measurements revealed an adsorbance of 1.2 ± 0.4 mg/m² of the polymer on each mica surface, consistent with expectations for cationic polymers adsorbed on solid surfaces.

Recent studies have revealed that, in contrast to non-associating liquids such as oils or organic solvents, salt-free water retains a viscosity close to its bulk value even when confined to films thinner than some 3 nm, indeed down to only one or two monolayers thick. For the case of high concentration aqueous salt solution compressed down to subnanometer films between charged surfaces, the trapped hydrated ions serve to act as molecular ball-bearings, sustaining a large load while remaining very fluid under shear. This behaviour is attributed to the tenacity of the hydration sheaths together with their rapid relaxation time. Finally, a very recent study has shown that when charged polymer brushes in aqueous media are compressed and slid past each other, they provide a lubrication that is considerably superior to that afforded by neutral brushes: This is attributed on the one hand to the resistance to mutual interpenetration of the chains due to entropic barriers in the good-solvent conditions, and, on the other hand, to the hydration-sheaths on the charged polymer segments which can act - as noted above - as molecular ball-bearings.

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.

We have measured the shear forces between solid surfaces sliding past each other across aqueous salt solutions, at pressures and concentrations typical of naturally occurring systems. In such systems the surface-attached hydration layers keep the compressed surfaces apart as a result of strongly repulsive hydration forces. We find, however, that the bound water molecules retain a shear fluidity characteristic of the bulk liquid, even when compressed down to films 1.0 ± 0.3 nanometer thick. We attribute this to the ready exchange (as opposed to loss) of water molecules within the hydration layers as they rub past each other under strong compression.

An investigation of the time-dependent behavior of the normal forces between two smooth mica surfaces immersed in salt-free water was presented, using a surface force balance (SFB). An equilibrium surface-charge density σ = σ₀ was indicated by a long-ranged repulsion on initial approach to adhesive contact. The results showed that subsequent force measurements at times τ following separation of the surfaces revealed drop in σ, followed by contact and separation, but then relaxed back to σ₀ with a characteristic time τ_r = 11 ± 2 minutes.

We have used a new design of the mica surface force balance (SFB), with extreme sensitivity in measuring normal and particularly shear or frictional forces between two surfaces sliding past each other, to measure the forces between two mica surfaces across a confined 4-cyano-4\u2032-hexabiphenyl nematogen (6CB). In an earlier study (Langmuir 1997, 13, 4466, paper 1 of the series) we investigated the normal force-distance profiles and the orientation of the confined liquid crystal (LC). Here we extend this to Study the shear forces F_s between the sliding mica surfaces across the 6CB nematogen as a function of orientation of the confined LC, the applied normal load F_n, the separation D of the mica surfaces, the shear velocity v_s, and the relative shear direction of the confining surfaces (which may be varied using the new SFB design). Our results, where the shear forces are measured down to levels that are some orders of magnitude more sensitive than in earlier studies, show that the highly confined nematogen (D in the range from 16 to ca. 100 Å) behaves under shear in a quasi-solidlike fashion for all three orientations studied: planar, planar twisted, and homeotropic. There is a linear relation between F_s and F_n for each of the three orientations, with the effective friction coefficient (dF_s/dF_n) largest for the homeotropic orientation and lowest for the planar twisted one. Intriguingly, for the planar orientation a clear increase in the friction could be observed when the initial shear direction was changed by 90°. We attribute this to the effect of the initial shear in orienting the confined nematogen layer in the initial direction, so that subsequent sliding motion at right angles to this encounters greater resistance. We also find that within a range of some 40-fold in v_s, there was little change in the shear force, in line with what is generally observed for solid-solid friction, and consistent with the fact that little relaxation in F_s is observed over macroscopic times on applying a step strain to the confined LC.

The forces between layers of poly(ethylene oxide) (PEO), of molecular weights M = 37 × 10³ (PEO37) and M = 112 × 10³ (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,ν_s) between them as they slide past each other at velocity ν_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. 10⁵ N m^-2), corresponding to effective friction coefficients μ_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 μ_eff saturates at the highest loads (for the case of μ_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 ν_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(ν_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.

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 olefinic 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.

The interactions between two mica surfaces bearing fifth-generation amino acid-modified poly(propyleneimine) dendrimers (the dendritic box that exposes methyl groups at their 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 indicated that dendrimers adsorb from dilute toluene solution (ca. 5×10^-5-3×10^-4 w/w) as a monolayer on the surfaces. Two interacting dendrimer monolayer-bearing surfaces experience a van der Waals attraction followed by steric repulsion on compression. The dendrimer bilayer could be compressed reversibly, yielding a measure of the compressibility of the molecules. Frictional force versus normal load profiles were measured at different shear velocities, and reveal both solidlike and liquidlike behavior of the confined dendrimers, consistent with NMR measurements on the dendritic box. The results show that the yield stress increases with compression of the layers. Observation of the relaxation behavior of sheared dendrimer layers - for adsorption from dilute solution - suggest that, within the parameters of our experiments, the relaxation times are insensitive to the compression. For the case of surface interactions after incubation in more concentrated dendrimer/toluene solutions (ca. 10^-3 w/w), the results of both normal and shear force measurements suggested aggregation of much thicker, loose dendrimer layers on the mica surfaces. These layers resulted in hysteretic and longer-ranged monotonic repulsion, and much weaker frictional forces - at comparable loads and shear velocities - than in the case of the monolayers adsorbed from dilute solutions.

Based on segregation data, determined with profiling techniques, we analyze surface phase diagram of binary liquid mixtures composed of random olefinic copolymers. Blends with extended- and small- critical point wetting regimes are discussed. First observation of wetting transition is presented. Surface enrichment-depletion duality, a phenomenon prerequisite to the 2nd order wetting transition, is described.

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 φ_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 φ_bulkbulk > 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.

We have examined the effect of deuterium labeling on surface interactions in mixtures of random olefinic copolymers [C₄H₈]_1-x[C₂H₃(C ₂H₅)_x. Based on surface segregation data we have determined a surface energy difference χ_s between pure blend constituents. In each binary mixture components have different fractions x₁, x₂ 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 (dx₁/hx₂, hx₁/dx₂). For each pair the surface energy parameter χ_s increases when the component with higher fraction x is deuterated, i.e., χ_s(dx₁/hx₂) > χ_s(hx₁/dx₂) for x₁ > x₂. A similar pattern has been found previously for the bulk interaction parameter χ. This is explained by the solubility parameter formalism aided by the lattice theory relating the surface excess to missing-neighbor effect. χ_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 χ_s seem to depend on the extent of chemical mismatch between blend components.

We have measured the surface segregation towards the vacuum in films of a binary mixture of deuterated E₄₈EE₅₂ (d52) and hydrogenous E₃₈EE₆₂ (h66) - random olefinic copolymers. Here E and EE are the linear ethylene and branched ethyl ethylene groups (C₄H₈) and [C₂H₃(C₂H₅)], respectively. The d52 copolymer is enriched at the surface when d52 is the minority component in the blend. On the contrary, the surface is depleted in the d52 chains, when they constitute a majority of the mixture. We have determined two branches of the segregation isotherm corresponding to the enrichment and the depletion. A mean-field Cahn model describes consistently these two isotherm branches. The observed enrichment-depletion duality modifies a common viewpoint on surface segregation.

Using a surface force balance with high sensitivity in measuring shear forces we investigated the mechanical properties of thin layers of cyclohexane and octamethylcyclotetrasiloxane (OMCTS) in the gap between two smooth solid surfaces at discrete thicknesses n= 6-3 molecular layers. At these layer thicknesses the films have undergone solidification due to their confinement (see preceding paper) and are capable of sustaining a finite yield stress upon being sheared. The sliding of the confining surfaces at mean velocity ν_s across the films is characterized by a shear or frictional force F_s which varies with a characteristic stick-slip pattern. We investigate comprehensively the dependence of F_s on n, ν_s, and on the applied normal forces F across the films. We find that transitions in film thickness from n→(n-1), with a consequent increase in F_s, may occur spontaneously during sliding with no change in F, corresponding to a multivalued friction force between the surfaces for a given load. The critical yield stress S for sliding at a given film thickness n increases monotonically with applied normal pressure P as S = S₀+ CP where S₀ is a constant of order 10⁵ Pa (depending on n) and C is roughly constant and of order 1. A simple model for friction across such films which can account semiquantitatively for this behavior is introduced, based on a shear-melting mechanism using the Lindemann criterion. We find that the characteristic stick-slip behavior persists over the range of film thicknesses and the entire (large) range of mean shear velocities studied, and that over most of this range the mean shear forces are independent of ν_s.

Wetting properties of thin films of a polymer melt on top of a network formed by irradiation cross-linking of an identical melt were studied. The permeability of the network and its wettability as a function of the extent of cross-linking were characterized by nuclear reaction analysis, atomic force microscopy. contact-mechanics measurements of the adhesion energy, and Light microscopy. As the degree of cross-linking increased, we observed a crossover from complete to partial wetting, followed by a second crossover back to complete wetting at higher cross-link densities. We suggest that the first crossover is a manifestation of entropy-induced autophobicity arising from the brush-like nature of the cross-linked surface, while the second originates From surface roughening induced by the cross-linking.

Thin liquid oligostyrene films on top of a surface-anchored, polystyrene brush-solvated by the oligostyrene itself - roughen spontaneously. Using optical phase-interference microscopy and nuclear reaction analysis we have characterized the topography of such roughened films. We see that the undulations formed reach down at most to approximately the top of the solvated brush, but not to the underlying solid substrate on which the brush is anchored. Possible origins for this behaviour are discussed in terms of a picture wherein the solvated brush is regarded as a quasi-solid substrate for the liquid film on top.

Surface segregation in thin films of binary liquid mixtures consisting of random olefine copolymers was studied by nuclear reaction analysis over a wide temperature and composition range. A divergence of the surface excess Γ was indicated as the binodal of each mixture was approached from the one-phase region, even at temperatures 100 °C below the critical point T_c, and interpreted as the advent of complete wetting behavior. A consistent description of the adsorption isotherms in terms of a mean field approach assuming a short-ranged surface potential f_s is feasible, but requires an unexpected temperature dependence of f_s. This dependence causes the wetting transition temperature to be located lower than expected on the basis of present models.

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 -(C₄H₈)-, EE is the ethylethylene group -[C₂H₃(C₂H₅)]-, and one of the copolymers is partially deuterated. The components in each binary mixture have different values x₁,x₂ of the EE fraction. Using a simple Flory-Huggins mixing model, our results enable us to extract an interaction parameter of the form χ(x₁,x₂,T)=A(x₁,x₂)/T, where for given x₁,x₂, A is a constant. Calculated binodals using this form fit our measured coexistence curves well, while allowing χ 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.

Stability of thin films of non-volatile liquids is a key issue in a variety of applications. Often a film is forced to spread on a substrate which is not wetted by the liquid. The film then ruptures within minutes and dewets. Common methods for achieving stability include the introduction of surface-active low molecular weight agents, or modification of the chemistry of the substrate. We describe here a mechanism for suppressing the rupture of the films by surface-attached polymers together with trace amounts of free polymers in the bulk of the film. The effect may have a kinetic origin, which is related to the entanglement of free chains and surface-attached polymer chains, or it may be due to a modification of the thermodynamic interactions.

Interfacial adsorption from a binary mixture of short and long polystyrene\u2014polyisoprene diblock copolymers (PS-PI or its deuterated analogue dPS-PI) to interfaces of a PS(M = 3 × 106) homopolymer was studied using nuclear reaction analysis. Short symmetric diblocks dPS(M = 10⁴)-PI(M = 10⁴) (designated dS), or a similar protonated analogue (hS), and a highly asymmetric, long diblock dPS(M = 10⁶)\u2013PI(M = 10⁴) (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\u2010 and 1,4\u2010olefinic 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 D₀ [defined by D = D₀(N_e/N²) for each copolymer, where N is the backbone length and N_e the entanglement spacing] decreases monotonically with increasing vinyl content x. Over the range of microstructures and temperatures T (−14−40°C) investigated we find log(D₀/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 D₀ 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. ©1995 John Wiley & 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 weee 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.

Using nuclear reaction analysis, we have measured the rate of surface segregation of an asymmetric diblock polyisoprene-polystyrene (PIPS) copolymer to the free surface of polystyrene (PS) homopolymers within which it is incorporated at low bulk concentrations. We have also measured the tracer diffusion coefficient of PIPS in PS under similar conditions, and shown it to equal, within error, the self-diffusion coefficient of PS of a similar length. The equilibrium surface excess Gamma of the diblock is low (

It has been shown by surface force studies that the properties of grafted polymer layers formed from functionalized polystyrene (polystyrene terminated by a zwitterionic group at the end) can be changed dramatically by surface exchange: the replacement of longer molecules attached to the surface via the end group by the shorter amphiphile terminated with the same group - behaviour opposite to that shown by adsorbed homopolymers. This exchange requires a certain minimum concentration of the shorter polymer in the solution, but the substantial replacement of long chains by short chains can occur even when the monomer concentration of the shorter polymer is relatively very small, i.e. only 1/500 that of the longer polymers.

We have measured simultaneously both the normal forces F_⊥(D) and the shear forces F_∥(D) that act between compressed polymer-bearing mica surfaces, a distance D apart, as they slide past each other. We find that for surface-attached polystyrene (PS) brushes in the good solvent toluene the shear forces are extremely weak, over a wide range of shear velocities and at normal loads that correspond to 2L ∥/F_⊥) by two-three orders of magnitude. For mica surfaces bearing adsorbed PS layers in the near-θ-solvent cyclopentane, the forces required to slide the surfaces are very much larger than for the PS brushes in toluene, at similar normal loads and shear velocities. The very different behaviour in the two cases is attributed to the different extents of mutual interpenetration and entanglement of the compressed polymer layers.

A thin bilayer of two coexisting polymer phases, in contact with a surface which favors and is partially wetted by one of them, was studied by nuclear reaction analysis and phase contrast microscopy. We find that when the less-favored phase is initially in contact with this surface, it is entirely replaced by the partially wetting phase. This inversion evolves with time very differently than in the case of complete wetting. The results suggest that a microscopic surface layer enriched in the surface-preferred component may not be a stable signature of partial wetting in small binary mixtures.

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 ≈ L_H. When the layers are compressed (D

Nuclear reaction analysis (NRA) was used to study the segregation of an asymmetric diblock copolymer, consisting of polyisoprene (PI, molecular weight M = 10⁴)/deuterated polystyrene (dPS, M = 10⁵) blocks, to the interfaces formed by polystyrene (PS) homopolymer with various phases. PS/vacuum, PS/silicon, and PS/PI homopolymer interfaces were investigated for different M values of the PS matrix (M = 1.7 × 10³−330 × 10³), and segregation isotherms were established as a function of temperature and of diblock concentration within the PS homopolymer. In all cases the diblocks attach to the interfaces by their PI moieties alone, to form brushlike structures of end-attached PS tails. The high spatial resolution of the NRA technique enabled studies of the brush conformation as a function of the attachment density at the PS/vacuum surface and was used to characterize the extent of penetration of the PS matrix chains into the diblock brushes. Detailed analysis of the brush conformation and of the segregation isotherms, mainly in terms of a Flory-type mean field model based on those due to de Gennes and to Leibler, provided a consistent description of our data; it enabled the extraction of the PI/PS segmental interaction parameter χ_PIPS, yielding values in accord with scattering studies, and of the attachment energies of the PI diblock moiety to the PS/air and PS/silicon interfaces. The values of χ_PIPS extracted from our data using this Flory-type model were found to increase at lower M values of the PS matrix, in qualitative accord with previous results.

Coexisting polymer phases are characterized by very small interfacial energies, even well below their critical solution temperature. This situation should readily lead to the exclusion of one of the phases from any interface that favors the other. Such complete wetting behavior from a binary mixture of statistical olefinic copolymers is reported. By means of a self-regulating geometry, it is found that the thickness of a wetting layer of one of the phases at the polymer-air interface, growing from the other coexisting phase, attains macroscopic dimensions, increasing logarithmically with time. These results indicate that binary polymer mixtures could be attractive models for the study of wetting phenomena.

The use of optical second harmonic generation (SHG) to probe, in situ and in real time, the physical grafting of polymers from dilute toluene solution onto mica crystal surfaces is reported. Despite using polymers with no hyperpolarizable groups, and the very low surface density of end-attached species, we show how changes in the SHG signal of the substrate may be monitored during polymer grafting. Characteristic attachment rates are obtained by this method.

We describe a method for directly determining the composition profile of deuterated polymer chains in polymer mixtures. Our technique, nuclear reaction analysis, is based on the ²H(³He, ⁴He)¹H nuclear reaction. By detecting ⁴He in a forward geometry, we achieve a spatial resolution of 14 nm (FWHM). We use this technique to probe the broadening of the interface between two partially miscible polymers. We found that such a system attains a finite interfacial width in equilibrium. For short times, we monitor the dynamics of interface formation. We found that the interfacial width increases significantly slower than in the case of free diffusion.

ADSORBED or grafted polymers are used to control phenomena such as colloidal stability, fluid flow and the tribological properties of surfaces. Forces between polymer-bearing surfaces have been studied comprehensively over the past decade^1-10, but little is known about such forces in shear. These are intimately related to the dynamic as well as the equilibrium properties of the surface-attached chains. We have constructed a device that measures directly the forces that act between surfaces bearing polymer layers in a liquid medium as they slide past each other. For the case of mica sheets bearing end-grafted chains of polystyrene in toluene (a good solvent), we find that, as the surfaces move parallel to each other, there is a marked change in the normal forces between them. These become increasingly repulsive at higher velocities. The effect occurs only above certain critical shear rates, probably related to relaxation dynamics of the end-grafted chains themselves. Our findings have direct implications for the properties of polymeric lubricants, and for the rheological behaviour both of stabilized dispersions and of multi-phase polymeric systems.

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.

We describe a method based on nuclear reaction analysis, using the reaction ²H(³He, ⁴He)¹H, (Q=18.352 MeV) to determine composition profiles of deuterated polymer chains in thin films. By detecting the emitted α particles (⁴He) at forward angles (30°) we are able to achieve a spatial resolution of 7 nm half width at half maximum (HWHM) at the deuterated sample surface, and 15 nm HWHM at a depth of some 130 nm. We use our method to probe initial diffusional broadening at the interface between deuterated and protonated polystyrene films. Our measured profiles are in close agreement with earlier measurements (over larger spatial scales) and with mean field models for the diffusional process in this system.

The adsorption of polymers onto a surface from a θ solvent is treated in terms of a mean- field model based on a Cahn-de Gennes approach. In particular, we calculate the forces between two such polymer bearing surfaces and compare our results in some detail with available experimental data on force profiles between mica surfaces bearing adsorbed polymer near the θ temperature. All parameters in our model may be estimated from experiment. The predicted force profiles are in very good qualitative agreement with the experimental data, though the absolute spatial and energy scales are smaller for the calculated profiles by factors of ca. 4 and 2.5 relative to experiment. The causes of this discrepancy and possible improvements to the model are considered.

We have directly measured the interface formation and diffusion between two partially miscible polymers, SCPE and PMMA, using Rutherford backscattering spectrometry, as the temperature was varied from the glass transition into the two-phase region. As the temperature was increased, the interfacial width for a given diffusion time increased sharply, attaining a maximum value approximately 25 °C above the lower critical solution temperature and then dropped abruptly. The results are briefly discussed within the framework of the extended Flory-Huggins theory wherein nonlocal terms are included in the free energy.

The variation of the effective adsorbed layer thickness δ_effwith molecular weight M for a polymer adsorbed at a mica-solvent interface under good solvent conditions, as revealed by force measurements, is analyzed. The observed variation, δ_eff∝M⁰-⁴³, is shown to be consistent with a scaling form for the extension of the polymer from the surface. The corresponding effective thickness of adsorbed layers in poor solvents is briefly considered.

Forces between polystyrene layers absorbed on mica and immersed in cyclohexane have been measured. The measurements were made on two different molecular weights (6 X 10⁵ and 9 X 10⁵) in two different laboratories and therefore support the quantitative reliability of the results. We have reproduced previous results of this type of measurement below the θ temperature and extended the force measurement to (a) adsorbed layers at lower surface coverage and (b) temperatures above the bulk polystyrene-cyclohexane θ temperature. At a coverage of about 1.1 mg/m² of polystyrene, which is on the order of 20-30% of saturation, we found strongly attractive forces below T_θ, detectable at separations of about 300 Å between the bare mica surfaces. The forces can be measured very accurately and precisely in this situation. The force reaches a minimum at 46 ± 2 Å and becomes strongly repulsive closer in. The long-range attractive portion of the force curve is very nearly exactly exponential, with a decay length of 45 ± 2 Å. For saturated surfaces with about 4.5 ± 1 mg/m² the force is detectably attractive at larger distances (600-1200 Å) both below (23 and 26 °C) and above (37 ± 2 °C) the temperature (34.5 °C). For each molecular weight the positions of the minimum in the F(D) curve and of the short-range repulsive barrier are at smaller separations at T > T_θ. The magnitudes of the minima are smaller at T > T_θ as well. Both of these new results suggest strongly that the forces, especially the attractive components, between the polymer surfaces are influenced by effects in addition to the usual segment-segment interactions which determine bulk thermodynamic properties.

The forces acting between atomically smooth mica surfaces immersed in both organic and aqueous liquid media have been determined in the range of surface separations 0 - 300 nm, both in the absence and the presence of adsorbed polymer layers. In this way the interactions between the adsorbed macromolecular layers themselves were determined. We present results for the following cases: i) Poor solvent, ii) θ - solvent, iii) good solvent, iv) polyelectrolytes in aqueous media. Our results show that interactions between the adsorbed layers may be either attractive or repulsive, depending on the nature of the solvent and the extent of adsorption. For the polyelectrolyte case the interactions are a combination of field-type (electrostatic) and steric forces.

The forces between two molecularly smooth curved mica surfaces a distance D apart immersed in a liquid have been measured, both in the absence and in the presence of macromolecules adsorbed from the liquid. The forces between bare mica surfaces in 0.2 mol dm^-3 KNO₃ aqueous solution are repulsive for D ≲ 5 nm and correspond reasonably well to electrostatic double-layer (DLVO) theory. Following adsorption of soluble monomeric collagen onto the surfaces from the solution, repulsion commences at D ≲ 600-700 nm and increases monotonically with decreasing D, suggesting that the collagen monomers are adsorbed in an upright configuration normal to each surface. The forces between bare mica surfaces in cyclohexane are monotonically attractive for D ≲ 10 nm, and appear to obey a van der Waals-like potential acting across the non-polar medium. Following adsorption of polystyrene (of two molecular weights) onto the mica from the cyclohexane, interactions between the surfaces are found to commence at D ≲ 2.5 R_g (unperturbed radius of gyration of the respective polymers) and are initially attractive. For D ≲ R_g the attraction decreases, and on further reducing D strong repulsion between the surfaces is observed. These results may be understood in terms of the phase equilibrium of the polystyrene-cyclohexane system (which under the experimental conditions is below its critical temperature), and are in good accord with the predictions of a recent theory for interaction between adsorbed polymer layers in a poor solvent.