A cationic polyelectrolyte was adsorbed on mica from highly concentrated solutions. The friction and surface force behaviors of the adsorbed layers in aqueous media were studied using a new homemade surface force apparatus (SFA). The long-range repulsions produced by the pure cationic polymer at low salt concentration indicate that the chains are in an extended conformation. The addition of anionic surfactant or of salt condenses the cationic polymer chains as evidenced by the much shorter range of the repulsions. These forces are, for both conformations, a combination of steric and double-layer forces. During sliding, the friction forces produced by the adsorbed layers increase monotonically with the load. A strong dependence of these forces on the sliding speed is noticeable for the extended conformations, while the dependence vanishes in the coiled conformations. This study shows the important role of the conformational state of adsorbed polymer chains on their tribological properties.

In this paper, we present our experimental results on the process of the in situ chemical modification of the silicon nitride atomic force microscopy/frictional force microscopy tip by n-octadecyltrimethoxysilane (OTE)/mica, OTE/SiO2, and SiO2. The modified tips have different friction and adhesion properties against mica reference samples compared to those before modification. The resultant tip modification depends not only on OTE self-assembled monolayer (SAM) but also on the substrates the OTE SAM is prepared on. In the case of OTE/mica, the friction of the modified tip against mica reference is greatly reduced; in the case of OTE/SiO2, the friction of the modified tip against mica reference is greatly increased. It is surprising that bare SiO2 can also chemically modify the Si3N4 tip to increase the friction against mica reference. In the case of OTE modification, it was found that the tips could be cleaned by repetitive friction scans on mica. However, a tip modified by SiO2 cannot be mechanically cleaned. Moreover, it was found that humidity and load could also affect the tip chemical modification. Ourresults are important for interpreting tribological data since the actual contact chemistry was often overlooked in the atomic force microscopy experiments in the past.

A device to study the friction of two molecularly smooth surfaces separated by an ultrathin liquid film is presented along with its design, calibration, and performance. The apparatus can move one of the surfaces and measure the friction force on the other one bidimensionally for both processes. A high mechanical impedance system (104N/m) measures continuous friction forces where only stick-slip was previously observed. The frequency and travel distance of the movement can be varied over a wide range (frequency from 10-4 to 7Hz and distance from1 to 800μm) to provide variations of the shear rate over seven orders of magnitude. The actual movement provided by piezoelectric bimorph drive can be affected by the friction forces and is measured by strain gauges. The friction forces are measured with an accuracy of ±2μN with a capacitance sensor. The mechanical design prevents the surfaces from rolling under force. The apparatus is tested with hexadecane. The potential applications of this apparatus and its limitations are discussed.

Poly-12-hydroxystearic acid (PHSA) is widely used as a coating on colloidal spheres to provide a "hardsphere-type" interaction. These hard spheres have been widely used in fundamental studies of nucleation, crystallization, and glass formation. Most authors escribe the interaction as "nearly" hard sphere. In this paper we directly measure this interaction, using layers of PHSA adsorbed onto mica sheets in a surfaces force apparatus. We find that the layers, in appropriate solvents, have no long-range interaction. When the solvent is decahydronaphthalene (decalin), the repulsion rises from zero to the maximum measurable over a distance range of 15-20nm. The data is converted to equivalent forces between spheres of different diameters, and modeled using a hard core potential. Using zeroth-order perturbation theory and computer simulation, we demonstrate that the equation of state does not deviate from that of a perfect hard-sphere system under any relevant experimental conditions.

Friction forces measurements between smooth surfaces across two layers of linear alkanes over five decades of speeds are presented. A maximum friction dissipation is observed at a characteristic speed. This behaviour is described by a new approach: the formation and destruction of molecular bridges between confined alkane layers. These bridges which interdigitate between the layers exhibit a thermally activated resistance to shear. An analytical model involving activation barriers accounts for the overall behaviour of the speed dependence of the forces over four decades. This first simple semi-quantitative description sheds new light on the subtle mechanisms of friction at the nanoscale level and shows how the molecular length influences the tribological properties of the liquid.