Using a surface force apparatus, we have measured the normal and friction forces between layers of the human glycoprotein lubricin, the major boundary lubricant in articular joints, adsorbed from buffered saline solution on various hydrophilic and hydrophobic surfaces: i), negatively charged mica, ii), positively charged poly-lysine and aminothiol, and iii), hydrophobic alkanethiol monolayers. On all these surfaces lubricin forms dense adsorbed layers of thickness 60-100 nm. The normal force between two surfaces is always repulsive and resembles the steric entropic force measured between layers of end-grafted polymer brushes. This is the microscopic mechanism behind the antiadhesive properties showed by lubricin in clinical tests. For pressures up to approximately 6 atm, lubricin lubricates hydrophilic surfaces, in particular negatively charged mica (friction coefficient mu = 0.02-0.04), much better than hydrophobic surfaces (mu > 0.3). At higher pressures, the friction coefficient is higher (mu > 0.2) for all surfaces considered and the lubricin layers rearrange under shear. However, the glycoprotein still protects the underlying substrate from damage up to much higher pressures. These results support recent suggestions that boundary lubrication and wear protection in articular joints are due to the presence of a biological polyelectrolyte on the cartilage surfaces.

We review the historical and modern understanding of the most basic equation of friction, Amontons' law, which describes phenomena that were already understood and studied by Leonardo da Vinci 500 years ago. This law states that for any two materials the (lateral) friction force is directly proportional to the (normal) applied load, with a constant of proportionality, the friction coefficient, that is constant and independent of the contact area, the surface roughness, and the sliding velocity. No theory has yet satisfactorily explained this surprisingly general law; all attempts have been model or system dependent. We review the experimental evidence and find, for example, that the same friction coefficient is often measured for the same system of materials with junctions whose areas differ by more than 6 orders of magnitude. The trends obtained through molecular dynamics (MD) simulations agree with recent and past experiments and with Amontons' law, and they suggest that the local energy-dissipating mechanisms are not merely "mechanical", as assumed in most models, but "thermodynamic" in nature, like miniature irreversible compression-decompression cycles of the trapped molecules between the surface asperities as they pass over each other. The MD analysis reveals that, for such dynamic, nonequilibrium, energy-dissipating processes, a proper statistical description can be formulated through the use of the Weibull distribution of the local friction forces, which may be regarded to serve in this context a similar purpose as the Boltzmann distribution for classical systems at equilibrium. Another important conclusion is that the concept of the "real" area of contact is a nonfundamental quantity, whether at the nano-, micro-, or macroscale. However, it may serve as a convenient scaling parameter for describing the really fundamental parameters, which are the number density of atoms, molecules, or bonds involved in an adhesive or frictional interaction. Brief History of the Concept of the "Coefficient of Friction"

Polyelectrolyte brushes provide wear protection and lubrication in many technical, medical, physiological, and biological applications. Wear resistance and low friction are attributed to counterion osmotic pressure and the hydration layer surrounding the charged polymer segments. However, the presence of multivalent counterions in solution can strongly affect the interchain interactions and structural properties of brush layers. We evaluated the lubrication properties of polystyrene sulfonate brush layers sliding against each other in aqueous solutions containing increasing concentrations of counterions. The presence of multivalent ions (Y, Ca, Ba), even at minute concentrations, markedly increases the friction forces between brush layers owing to electrostatic bridging and brush collapse. Our results suggest that the lubricating properties of polyelectrolyte brushes in multivalent solution are hindered relative to those in monovalent solution.

Polymer chain bridging by multivalent ions and solvophobic attractions drives structure formation in charged polymer brushes.

The surface forces apparatus technique was used to measure the normal forces, thin film viscosity, and lateral (frictional) forces between two surfaces interacting across 4′-n-octyl-4-cyanobiphenyl (8CB), to determine the effects of confinement (film thickness) and shear (sliding velocity) on the ordering of the smectic-A and the nematic phases. The surface roughness and hydrophobicity were altered by different adsorbed surfactant monolayers to study the effects on the orientation and anchoring of 8CB. The positional order increases as the surface separation decreases, and the orientational ordering increases with increasing shear rate. The friction force in the planarly oriented nematic phase resembles results for alkanes, while the better ordered smectic-A phase exhibits lower friction forces. The liquid crystal orients perpendicularly to surfactant-coated surfaces. On a loose-packed surfactant layer, it becomes strongly anchored, which increases the resistance to sliding, while on a close-packed monolayer the friction force is low, but the liquid crystal is easily removed by the applied load or pressure.

Due to its perfect cleavage that provides large areas of molecularly smooth, chemically inert surfaces, mica is the most commonly used natural substrate in measurements with the surface forces apparatus (SFA), in atomic force microscopy (AFM), and in many adsorption studies. However, preparing mica surfaces that are truly clean is not easy since mica is a high-energy surface that readily adsorbs water, organic contaminants, and gases from the atmosphere. Mica can also become charged on cleaving, which makes it prone to picking up oppositely charged particles or mica flakes from the surroundings. High refractive index particles, such as metals, will adhere to mica through van der Waals forces. Recent articles have demonstrated that particle contamination is obtained when inappropriate cutting and handling procedures for the mica are used. In this paper, we show that both particle and other critical contamination is easy to detect and provide proper steps to take during the sample preparation process.

The boundary friction of three aromatic thiol monolayers with different packing densities on gold was studied with friction force microscopy. The friction of each monolayer was measured against unfunctionalized silicon tips and thiol-functionalized gold-covered tips. The experiments were done in ethanol to significantly reduce the tip-substrate adhesion and therefore the dependence of the friction force on contact area. This allowed a direct, quantitative comparison of load-and velocity-dependent friction with tips of different radii. In close-packed systems, the friction force was lower when both surfaces were covered with a monolayer, and plateaus in the friction force vs velocity curves appeared at higher velocity, suggesting a more fluidlike sliding. A transition in the monolayers at high loads was found to be dependent on the tip radius and appeared at higher average pressure for more close-packed systems but did not depend on whether the system contained one or two monolayers.

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