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.

Pure organic molecules exhibiting a suitable concave rigid shape are expected to give porous glasses in the solid state. Such a feature opens new opportunities to avoid crystallization and to improve molecular solubility in relation to the high internal energy of these solid phases. To quantitatively explore the latter strategy, a series of rigid tetrahedral conjugated molecules nC and the corresponding models nR have been synthesized. Related to the present purpose, several properties have been investigated using UV absorption, steady-state fluorescence emission, differential scanning calorimetry, 1H NMR translational self-diffusion, magic angle spinning 13C NMR, and multiple-beam interferometry experiments. The present tetrahedral crosses are up to 8 orders of magnitude more soluble than the corresponding model compounds after normalization to the same molecular length. In addition, they give concentrated monomeric solutions that can be used to cover surfaces with homogeneous films whose thickness goes down to the nanometer range. Such attractive features make cross-like molecular architectures promising for many applications.

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.

Due to the strong capillary condensation, the adhesion force between a Si3N4 atomic force microscope (AFM) tip and silicon oxide was observed to first increase and then decrease with an increase of humidity. In contrast, due to weak capillary condensation, the adhesion force between the AFM tip and the N-octadecyltrimethoxysilane (OTE,CH3(CH2)17Si(OCH2CH3)3) self-assembled monolayer (SAM) was found to be almost independent of humidity. It was found that the formulation commonly used for acroscopic objects fails to explain our data. Using an accurate formulation and an assumed tip shape, we can explain the observed decrease of adhesion for SiO2 at high humidity as being due to the decreased capillary pressure force when the dimension of the meniscus becomes omparable with the tip size. However, theobserved late onset of adhesion of SiO2 cannot be understood within the framework of the classical continuum theory. We attribute this late onset to the properties of an ultrathin water film at molecular thickness. The new formulation predicts a vanishing water meniscus between the AFM tip and OTE over the entire humidity range and can also fully account for the humidity-independent adhesion results for OTE.

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.