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.

For the friction measurements with AFM/FFM, usually the relative values of friction signal can be obtained. In order to compare the micro-tribological properties of different samples, mica is often used as an reference sample for friction measurement. However, due to the friction force of new cleaved mica surface is unstable, it is urged to systematically investigate the tribological properties of mica to design the experimental rules of the reference sample mica for friction measurements. Experimental results show that the friction of mica varies with the cleaving time, humidity and surface state of tip. The friction measured with different tips on mica varies in the range of

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 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.

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.