Adhesion and stable low friction provided by a subnanometer-thick monolayer of a natural polysaccharide

Using a surface forces apparatus, we have investigated the adhesive and lubrication forces of mica surfaces separated by a molecularly thin, subnanometer film of a high-molecular-weight (2.3 MDa) anionic polysaccharide from the algae Porphyridium sp. adsorbed from aqueous solution. The adhesion and friction forces of the confined biopolymer were monitored as a function of time, shearing distance, and driving velocity under a large range of compressive loads (pressures). Although the thickness of the dilute polysaccharide was <1 nm, the friction was low (coefficient of friction = 0.015), and no wear was ever observed even at a pressure of 110 atm over 3 decades of velocity, so long as the shearing distances were less than twice the contact diameter. Atomic force microscopy in solution shows that the biopolymer is able to adsorb to the mica surface but remains mobile and easily dragged upon shearing. The adhesion (adsorption) of this polysaccharide even to negatively charged surfaces, its stable low friction, its robustness (high-load carrying capacity and good wear protection), and the weak (logarithmic) dependence of the friction force on the sliding velocity make this class of polyelectrolytes excellent candidates for use in water-based lubricant fluids and as potential additives to synovial fluid in joints and other biolubricating fluids. The physical reasons for the remarkable tribological properties of the ultrathin polysaccharide monolayer are discussed and appear to be quite different from those of other polyelectrolytes and proteins that act as thick "polymer brush" layers.     

Adhesion and friction properties of molecularly thin perfluoropolyether liquid films on solid surface

The adhesion and friction properties of molecularly thin perfluoropolyether (PFPE) lubricant films dip-coated on a diamond-like carbon (DLC) overcoat of magnetic disks were studied using a pin-on-disk-type micro-tribotester that we developed. The load and friction forces were simultaneously measured on a rotating disk surface under an increasing/decreasing load cycle and slow sliding conditions. Experiments were performed using two types of PFPE lubricants: Fomblin Z-tetraol2000S with functional end-groups and Fomblin Z-03 without any end-group. The curves of the friction force as a function of the applied load agree with the curves estimated using the Johnson-Kendall-Roberts (JKR) model. The friction forces on the Z-03 films having different thicknesses were not found to decrease drastically; however, the friction forces on the Z-tetraol film were found to decrease drastically when the film thickness is more than ~1.2 nm. This drastic change in the case of the Z-tetraol film is estimated to be affected by the coverage of the lubricant film.     

Nanotribology of surface-grafted PEG layers in an aqueous environment

The lubrication properties of adsorbed poly(L-lysine)-graft-poly(ethylene glycol) in aqueous buffer solution were studied with the surface forces apparatus. In general, the polymer brushes revealed extremely low friction forces. Two distinct regimes could be identified. In response to lateral shear, the friction forces of intact polymer films at moderate loads were below the detection limit. At high loads, when the films were compressed to about 10% of the original equilibrium film thickness, the friction showed a reversible increase with load. Under certain conditions, film destruction was observed, immediately followed by a dramatic increase in the frictional force and an expansion of the adsorbed brush layer. By the addition of free polymer to the buffer solution, the resistance of the polymer brushes to abrasion was dramatically increased by readsorption of the polymer following friction-induced desorption. This self-healing capacity and the extremely low friction of the adsorbed copolymer films contribute to their excellent properties as lubricant additives for water-based lubrication under boundary conditions.     

Normal and friction forces between mucin and mucin-chitosan layers in absence and presence of SDS

Employing the colloidal probe AFM technique we have investigated normal and friction forces between flat mica surfaces and silica particles coated with mucin and combined mucin-chitosan layers in presence and absence of anionic surfactant, SDS, in 30 mM NaCl solution. We have shown that the normal interactions between mucin coated mica and silica surfaces are dominated by long-range steric repulsion on both compression and decompression. Friction forces between such mucin layers are characterized by a low effective friction coefficient, mu(eff)=0.03+/-0.02, which is lower than the value of 0.13+/-0.02 observed when chitosan layers were adsorbed. Forces between combined mucin-chitosan layers have also been measured. Adsorption of chitosan on mucin results in considerable compaction of the layer, and development of attractive forces detectable on separation. Friction between mucin-chitosan layers in 30 mM NaCl solution is high, with mu(eff) approximately 0.4. Adsorption of additional mucin to this layer results in no improvement with respect to lubrication as compared to the mucin-chitosan layer, and mu(eff) approximately 0.4 is observed. We argue that the layers containing both mucin and chitosan are not strictly layered but rather strongly entangled. As a result attractive interactions between oppositely charged moieties of sialic acid residues from mucin and amine groups from chitosan residing on the opposing surfaces contribute to the increased friction. The effects of SDS on normal and friction forces between combined mucin-chitosan layers were also investigated. The relation between surface interactions and friction properties is discussed.     

Reinforcement of a surfactant boundary lubricant film by a hydrophilic-hydrophilic diblock copolymer

The coadsorption from aqueous solutions of an anionic-neutral hydrophilic-hydrophilic diblock copolymer onto a mica-suported surfactant bilayer of a cationic oligomeric surfactant has been investigated. By using an atomic force microscope and a surface forces apparatus nanotribometer, we studied the resulting film morphology, the interactions between two coated surfaces, and the frictional properties of the boundary film. When the coated surfaces were compressed while being fully immersed in an aqueous surfactant solution, the hemifusion of the adsorbed surfactant bilayers could be easily induced. Noticeable friction forces could then be measured between the monolayer-coated surfaces. Coadsorbing poly(acrylic acid)-poly(acrylamide) diblock copolymer with the cationic surfactant changes the cohesion of the adsorbed layers. When the copolymer concentration is sufficiently high, the hemifusion instability of the adsorbed layers can be inhibited, considerably improving its lubricant properties.     

疏水蛋白在云母表面的吸附

采用分子动力学模拟方法研究了疏水蛋白(HFBI)在亲水云母表面的吸附过程.通过6组平行的分子动力学模拟得到了2种不同的稳定吸附结构,即通过N端和通过亲水的α螺旋与表面吸附,得到了5种吸附残基.进一步用自适应偏置力方法计算了所有吸附残基与表面的结合自由能.结果表明,残基Lys是吸附过程的关键残基,即当HFBI通过含有Lys残基的α螺旋与云母表面作用时,其吸附构象最稳定.静电作用是吸附过程的主要驱动力.在该吸附结构中,HFBI的疏水面暴露在溶液中,有效降低了云母表面的润湿性.     

Interfacial friction of thin PDMS network films

This study focuses on developing dry, surface‐tethered polymeric lubricant coatings capable of significantly decreasing friction and wear of nano‐ and micrometer scale machines. Vinyl‐terminated polydimethylsiloxane chains are spin‐coated with a crosslinking agent and platinum catalyst onto silicon wafers functionalized with a self‐assembling monolayer containing reactive vinyl groups. Lateral force microscopy (LFM) measurements employing a bead probe are used to quantify the coefficient of friction (COF) and adhesion characteristics of the PDMS‐SAM surface tethered networks. The combined polymer network and SAM layer manifest extremely low friction coefficients, μ = 4 × 10^?3, which is nearly one order of magnitude lower than the friction coefficient of the bare silicon substrate. The lowest friction forces are measured using silicon substrates covered with nanometer thick, peroxide crosslinked PDMS networks; though poorly crosslinked, these networks display COFs as much as ten‐times lower than a solitary SAM coating layer. Micrometer thick end‐linked optimal networks also manifest attractive interfacial friction properties, with COFs approximately three times larger than the thinner, imperfect networks. These observations are discussed in terms of the structure of the polymer networks and the role of adhesion forces on interfacial friction. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1773–1787, 2008     

Friction of fatty acids in nanometer-sized contacts of different adhesive strength

The effects of adhesion, contact area, and pressure on the lubricating properties of self-assembled monolayers on steel have been investigated with friction force microscopy. The adsorbed molecules were fatty acids with varying degrees of unsaturation (0-2 double bonds; stearic, oleic, and linoleic acid) and a rosin acid (dehydroabietic acid), adsorbed from n-hexadecane solution. The friction of these loose-packed monolayers was studied in dry N2 gas and in ethanol. Low adhesion (in ethanol) resulted in a linear increase in friction force at low loads, that is, F = muL, whereas higher adhesion (in N2 gas) gave an apparent area-dependence at low loads of the form F = S(c)A, where S(c) is the critical shear stress. A recent model for the contact mechanics of a compliant elastic film confined between stiffer substrates was applied to the data obtained in dry N2. Using this approach, we obtained interfacial energies of the compliant monolayers in good agreement with van der Waals-Lifshitz theory. With a low monolayer elastic modulus of E'(1)=0.2 GPa, we obtained a slightly higher value of Sc for stearic acid than that established for more close-packed stearic acid monolayers. An increase of mu and S(c) was found with increasing degree of unsaturation of the fatty acid.     

The electrochemistry of proteins and related substances part III. Bovine serum albumin

The electroreduction of bovine serum albumin (BSA) in pH 7.4 solution was studied at the hanging mercury drop electrode by double potential step chronocoulometry and cyclic voltammetry, and at a mercury pool electrode by controlled potential coulometry. The proposed mechanism involves reduction of an adsorbed monolayer of BSA (maximum coverage 3.3 μC^?2); at short times 3 or 4 disulfide bonds are reduced, the product remains adsorbed and may be reoxidized. On the coulometric time scale (hours) 8 or 9 of the total 17 disulfide bonds are broken and an insoluble product which cannot be oxidized is formed.     

Driving Forces for Enzyme Adsorption at Solid-Liquid Interfaces: 2. The Fungal Lipase Lipolase

Lipolase is the trade name for a fungal lipase that can catalyze the hydrolysis of ester bonds in, for example, triacylglycerol molecules. An important characteristic of this enzyme is that it is water-soluble whereas its substrate is water-insoluble. The adsorption of Lipolase was studied on several polystyrene latices and glass in order to determine the effect of the nature of the solid phase and to determine the interactions which are of importance for the adsorption. Electrostatic interaction and dehydration of hydrophobic surfaces are the main driving forces for Lipolase adsorption. Under attractive electrostatic conditions between the surface and the enzyme, the plateau value of the isotherm corresponds to saturated monolayer coverage. Under conditions where dehydration of the hydrophobic surface is almost compensated for by electrostatic repulsion the lateral repulsion between the adsorbed enzymes becomes also important and contributes to the surface coverage. The adsorption mechanism of Lipolase is similar to that of the protein Savinase. However, Lipolase adsorbs much less on hydrophobic interfaces under electrostatic repulsive conditions than proteins examined in the literature, indicating that the dehydrated contact area between enzyme and surface is relatively small and that consequently the enzyme does not unfold significantly upon adsorption.     

Superior biolubricant from a species of red microalga

The rheological properties of the sulfated polysaccharide of the red microalga Porphyridium sp., a heteropolymer with a molecular weight of 3-5 x 10(6) Da, indicated that this material might be an excellent candidate for lubrication applications: the viscosity of the polysaccharide is stable over a range of temperatures, pH values, and salinities. In this study, various rheological and lubricant properties of the polysaccharide were evaluated in comparison with those of a widely used biolubricant, hyaluronic acid. The viscosity of the Porphyridium sp. polysaccharide remained essentially unchanged in a temperature range of 25-70 degrees C. In tribology tests on a ball-on-flat ceramic pair, the values for the friction coefficient and wear rate for the pair lubricated with polysaccharide were remarkably lower than those for hyaluronic acid, especially at high loads. In a test on a steel ring/ultrahigh-molecular-weight polyethylene (UHMWPE) block pair, the wear tracks on the surface of the UHMWPE were more pronounced for hyaluronic acid than for the polysaccharide. Atomic force microscopy showed that the polysaccharide was effectively adsorbed onto mica surfaces, forming ultrathin coating layers in the nanometer range. As is required for biolubricant applications, the polysaccharide was not degraded by hyaluronidase. The stability of the Porphyridium sp. polysaccharide to heat and to hyaluronidase combined with its ability to reduce friction and wear indicate its potential as an advantageous biolubricant.     

Normal Forces between Cellulose Surfaces Measured with Colloidal Probe Microscopy

Colloidal probe microscopy was employed to study interactions between cellulose surfaces in aqueous solutions. Hydrodynamic forces must be accounted for in data analysis. Long-range interactions betweeen cellulose surfaces are governed by double-layer forces and, once surfaces contact, by osmotic repulsive forces and viscoelasticity. Increasing the ionic strength decreases surface potentials and increases adhesive forces. Polyelectrolytes cause strong steric repulsion at high surface coverage, where interactions are sensitive to probe velocity. Polymer bridging occurs at low coverage. The conformation of adsorbed polyelectrolytes depends on the polymer concentration. Copyright 2000 Academic Press.     

Lubrication, adsorption, and rheology of aqueous polysaccharide solutions

Aqueous lubrication is currently at the forefront of tribological research due to the desire to learn and potentially mimic how nature lubricates biotribological contacts. We focus here on understanding the lubrication properties of naturally occurring polysaccharides in aqueous solution using a combination of tribology, adsorption, and rheology. The polysaccharides include pectin, xanthan gum, gellan, and locus bean gum that are all widely used in food and nonfood applications. They form rheologically complex fluids in aqueous solution that are both shear thinning and elastic, and their normal stress differences at high shear rates are found to be characteristic of semiflexible/rigid molecules. Lubrication is studied using a ball-on-disk tribometer with hydrophobic elastomer surfaces, mimicking biotribological contacts, and the friction coefficient is measured as a function of speed across the boundary, mixed, and hydrodynamic lubrication regimes. The hydrodynamic regime, where the friction coefficient increases with increasing lubricant entrainment speed, is found to depend on the viscosity of the polysaccharide solutions at shear rates of around 10(4) s(-1). The boundary regime, which occurs at the lowest entrainment speeds, depends on the adsorption of polymer to the substrate. In this regime, the friction coefficient for a rough substrate (400 nm rms roughness) is dependent on the dry mass of polymer adsorbed to the surface (obtained from surface plasmon resonance), while for a smooth substrate (10 nm rms roughness) the friction coefficient is strongly dependent on the hydrated wet mass of adsorbed polymer (obtained from quartz crystal microbalance, QCM-D). The mixed regime is dependent on both the adsorbed film properties and lubricant's viscosity at high shear rates. In addition, the entrainment speed where the friction coefficient is a minimum, which corresponds to the transition between the hydrodynamic and mixed regime, correlates linearly with the ratio of the wet mass and viscosity at ～10(4) s(-1) for the smooth surface. These findings are independent of the different polysaccharides used in the study and their different viscoelastic flow properties.     

Inhibition of cold-welding and adhesive wear occurring on surface of the 6061 aluminum alloy by graphene oxide/polyethylene glycol composite water-based lubricant

In this paper, graphene oxide/polyethylene glycol (GO/PEG) composite water-based lubricant was prepared by an ultrasonic dispersion method, and characterized and analyzed by Fourier transform infrared (FT-IR), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The suspension performance of GO/PEG composite water-based lubricant in water was verified by static sedimentation and centrifugation, and then, the prepared GO/PEG composite water-based lubricant was added into 304 stainless steel and 6061 aluminum alloy, and the coefficient of friction (COF) curve, average COF value, average wear rate, corresponding photomicrographs of balls and disks after wear, and energy-dispersive spectrometer (EDS) elemental analysis were used to illustrate the lubrication effect and lubrication mechanism. The results show that the GO/PEG composite water-based lubricant possesses excellent suspension ability in water, and the average COF value and wear rate of GO/PEG composite water-based lubricant are reduced by 78.8% and 88.8%, respectively, compared with water lubrication. The excellent lubrication effect of GO/PEG composite water-based lubricant can effectively reduce the cold-welding and adhesive wear phenomenon, mainly because GO/PEG composite water-based lubricant first fills the uneven surface of friction mating to form a high-quality lubricating film and then because of the special space structure of GO and the low shear between GO layers and the synergistic lubrication effect of GO/PEG.     

Enzymatic activity and catalytic hydrogen evolution in reduced and oxidized urease at mercury surfaces

It was originally shown [10] that urease retains its enzymatic activity when adsorbed at bare mercury and solid amalgam surfaces. However the opinion later prevailed that, when adsorbed at bare metal electrodes, proteins are irreversibly denatured. Here we confirm that urease is enzymatically active at a bare solid amalgam surface as found by Santhanam et al., and we show that this enzyme is equally active at a thiol-modified amalgam surface. We also show that it is the reduced form of urease, which is enzymatically active at Hg surfaces. Oxidation of the protein, resulting in formation of disulfide bonds, strongly decreases the enzyme activity. Using constant current chronopotentiometric stripping (CPS) we show that the exposure of surface-attached urease to negative potentials results in the protein unfolding. The extent of the unfolding depends upon the amount of time for which the protein is exposed to negative potentials, and at very short times this unfolding can be avoided. At thiol-modified Hg surfaces the protein is less vulnerable to the effects of the electric field. We conclude that the loss of enzymatic activity, resulting from a 10 min exposure of the protein to −0.58 V, is not due to reduction of the disulfide bonds as suggested by Santhanam et al. This loss is probably a result of protein reorientation, due to reduction of the Hg-S bonds (formed by accessible cysteines), followed by prolonged electric field effect on the surface-attached protein.     

Biomimetic Bottlebrush Polymer Coatings for Fabrication of Ultralow Fouling Surfaces

Demand for long‐lasting antifouling surfaces has steered the development of accessible, novel, biocompatible and environmentally friendly materials. Inspired by lubricin (LUB), a component of mammalian synovial fluid with excellent antifouling properties, three block polymers offering stability, efficacy, and ease of use were designed. The bottlebrush‐structured polymers adsorbed strongly on silica surfaces in less than 10 minutes by a simple drop casting or online exposure method and were extremely stable in high‐salinity solutions and across a wide pH range. Antifouling properties against proteins and bacteria were evaluated with different techniques and ultralow fouling properties demonstrated. With serum albumin and lysozyme adsorption <0.2 ng cm^?2, the polymers were 50 and 25 times more effective than LUB and known ultralow fouling coatings. The antifouling properties were also tested under MPa compression pressures by direct force measurements using surface forces apparatus. The findings suggest that these polymers are among the most robust and efficient antifouling agents currently known.     

Interfacial friction and adhesion of polymer brushes

A bead-probe lateral force microscopy (LFM) technique is used to characterize the interfacial friction and adhesion properties of polymer brushes. Our measurements attempt to relate the physical structure and chemical characteristics of the brush to their properties as thin-film, tethered lubricants. Brushes are synthesized at several chain lengths and surface coverages from polymer chains of polydimethylsiloxane (PDMS), polystyrene (PS), and a poly(propylene glycol)-poly(ethylene glycol) block copolymer (PPG/PEG). At high surface coverage, PDMS brushes manifest friction coefficients (COFs) that are among the lowest recorded for a dry lubricant film (μ ≈ 0.0024) and close to 1 order of magnitude lower than the COF of a bare silicon surface. Brushes synthesized from higher molar mass chains exhibit higher friction forces than those created using lower molar mass polymers. Increased grafting density of chains in the brush significantly reduces the COF by creating a uniform surface of stretched chains with a decreased surface viscosity. Brushes with lower surface tension and interfacial shear stresses manifest the lowest COF. In particular, PDMS chains exhibit COFs lower than PS by a factor of 3.7 and lower than PPG/PEG by a factor of 4.7. A scaling analysis conducted on the surface coverage (σ) in relation to the fraction (ε) of the friction force developing from adhesion predicts a universal relation ε ~ σ(4/3), which is supported by our experimental data.     

AFM-based nanotribological and electrical characterization of ultrathin wear-resistant ionic liquid films

Ionic liquids (ILs) are considered as lubricants for micro/nanoelectromechanical systems (MEMS/NEMS) due to their excellent thermal and electrical conductivity. So far, only macroscale friction and wear tests have been conducted on these materials. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a crucial step to understand how these novel materials can efficiently lubricate MEMS/NEMS devices. To this end, the adhesion, friction and wear properties of two ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM-OctSO4), applied on Si(100), are investigated for the first time using atomic force microscopy (AFM). Data is compared to the perfluoropolyether lubricant Z-TETRAOL, which has high thermal stability and extremely low vapor pressure. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated using AFM-based surface potential and contact resistance techniques. Thermally treated coatings containing a mobile lubricant fraction (i.e., partially bonded) were better able to protect the Si substrate from wear compared to the fully bonded coatings, and this enhanced protection is attributed to lubricant replenishment.     

Chemisorption of (CHx and C2Hy) hydrocarbons on Pt(111) clusters and surfaces from DFT studies

We used the B3LYP flavor of density functional theory (DFT) to study the chemisorption of all CH(x) and C(2)H(y) intermediates on the Pt(111) surface. The surface was modeled with the 35 atom Pt(14.13.8) cluster, which was found to be reliable for describing all adsorption sites. We find that these hydrocarbons all bind covalently (sigma-bonds) to the surface, in agreement with the studies by Kua and Goddard on small Pt clusters. In nearly every case the structure of the adsorbed hydrocarbon achieves a saturated configuration in which each C is almost tetrahedral with the missing H atoms replaced by covalent bonds to the surface Pt atoms. Thus, (Pt(3))CH prefers a mu(3) hollow site (fcc), (Pt(2))CH(2) prefers a mu(2) bridge site, and PtCH(3) prefers mu(1) on-top sites. Vinyl leads to (Pt(2))CH-CH(2)(Pt), which prefers a mu(3) hollow site (fcc). The only exceptions to this model are ethynyl (CCH), which binds as (Pt(2))C=CH(Pt), retaining a CC pi-bond while binding at a mu(3) hollow site (fcc), and HCCH, which binds as (Pt)HC=CH(Pt), retaining a pi bond that coordinates to a third atom of a mu(3) hollow site (fcc) to form an off center structure. These structures are in good agreement with available experimental data. For all species we calculated heats of formation (DeltaH(f)) to be used for considering various reaction pathways on Pt(111). For conditions of low coverage, the most strongly bound CH(x) species is methylidyne (CH, BE = 146.61 kcal/mol), and ethylidyne (CCH(3), BE = 134.83 kcal/mol) among the C(2)H(y) molecules. We find that the net bond energy is nearly proportional to the number of C-Pt bonds (48.80 kcal/mol per bond) with the average bond energy decreasing slightly with the number of C ligands.     

Conformational and adsorptive characteristics of albumin affect interfacial protein boundary lubrication: From experimental to molecular dynamics simulation approaches

The lifetime of artificial joints is mainly determined by their biotribological properties. Synovial fluid which consists of various biological molecules acts as the lubricant. Among the compositions of synovial fluid, albumin is the most abundant protein. Under high load and low sliding speed articulation of artificial joint, it is believed the lubricants form protective layers on the sliding surfaces under the boundary lubrication mechanism. The protective molecular layer keeps two surfaces from direct collision and thus decreases the possibility of wear damage. However, the lubricating ability of the molecular layer may vary due to the conformational change of albumin in the process. In this study, we investigated the influence of albumin conformation on the adsorption behaviors on the articulating surfaces and discuss the relationship between adsorbed albumin and its tribological behaviors. We performed the friction tests to study the effects of albumin unfolding on the frictional behaviors. The novelty of this research is to further carry out molecular dynamics simulation, and protein adsorption experiments to investigate the mechanisms of the albumin-mediated boundary lubrication of arthroplastic materials. It was observed that the thermal processes induce the loss of secondary structure of albumin. The compactness of the unfolded structure leads to a higher adsorption rate onto the articulating material surface and results in the increase of friction coefficient.