Molecular-level insight into the frictional properties of fluorinated self-assembled monolayers (SAMs) was achieved by combining two recently developed techniques that operate at the subnanometer scale:  control of the interfacial composition through molecular self-assembly and tribological measurements performed with the atomic force microscope. To explore the origin of frictional forces in fluorinated films, the frictional properties of two classes of alkanethiols adsorbed on single crystal gold were measured and compared. In these studies, films of equivalent chain length, packing density and packing energy, but different termination (methyl vs trifluoromethyl), were characterized and investigated. For these films, in which the only detectable difference was the outermost chemical structure/composition, a factor of 3 increase in the frictional response was observed in going from the hydrogenated to the fluorinated film. These results support the conclusion that chemical structure/composition alone plays an integral role in determining the frictional properties of an interface. We propose that the difference in friction arises predominantly from the difference in size of the methyl and trifluoromethyl groups.

The origin of frictional forces in self-assembled monolayers (SAMs) was investigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy was used to probe the frictional properties of the SAMs formed by the adsorption of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(111) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiols were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-packed backbone structure. For these films, which differed only in the nature of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluoromethyl-terminal groups and for mixed monolayers containing small concentrations of the trifluoromethyl-terminated component. These results strongly support the model that the difference in friction in such systems arises predominantly from the difference in the size of the terminal groups. Larger terminal groups in films of the same lattice spacing give rise to increased steric interactions that provide pathways for energy dissipation during sliding.

We report measurements of the interaction force between functionalized probe tips and oligo(ethylene glycol) (OEG)-terminated monolayers. Force profiles obtained with the interfacial force microscope indicate the presence of an interphase water layer adjacent to the OEG-terminated monolayer that generates a repulsive hydration force. This interphase layer extends up to 5 nm from the surface and has a viscosity that is 6 orders of magnitude greater than that of bulk water. The possible role this interphase layer plays in the exceptional protein resistance of such systems is discussed.

We have used interfacial force microscopy to study the adhesion, friction, and mechanical properties of molecular monolayers self-assembled on Au surfaces. This quantitative and stable scanning-probe technique permits detailed studies of these factors. By systematic variation of the chemical nature of the end groups on the monolayers and utilization of standard and intuitive contact-mechanics models, quantitative results are presented of inter- and intrafilm bonding strength as well as the relationship between mechanical behavior and the lateral friction force.

We have used optical microscopy, atomic force microscopy, and ellipsometry to study the dewetting of films of a perfluoropolyether polymer on silicon substrates. The disjoining pressure of these films is determined, for the first time for a dewetting system, by using noncontact atomic force microscopy to measure the dimensions of the liquid dewetting droplets. The determined disjoining pressure explains the different dewetting processes observed for different initial film thicknesses and is dominated by structural forces and by the inability of the polymer to spread on its own monolayer.[S0031-9007 (99)09002-X] PACS numbers: 68.15. + e, 68.45.Gd, 68.55. -aThin polymer films have a wide variety of applications including protective coatings, dielectric layers, and lubrication films. As the trend for miniaturizing devices continues, so will the demand for increasingly thinner, more uniform polymer films. Unfortunately, for many combinations of polymer and substrate materials, dewetting is observed when the polymer is in the liquid state. Consequently, the physics and chemistry of wetting and dewetting phenomena are topics of great current interest [1][2][3][4][5][6]. In these previous studies, which primarily use apolar liquid films such as polystyrene on silicon, the dewetting process commonly occurs in three successive phases: rupture of the film to form holes, growth of the holes to form a polygonal network of straight liquid rims, and then decay of the rims via a Rayleigh instability to form droplets. The rupturing process, which initiates dewetting, is believed to result either by the formation of holes nucleated at defects or from spinodal decomposition of the unstable film.The physics of why a liquid film wets or dewets is found in the negative derivative of the free energy with respect to film thickness, called the disjoining pressure, which arises from the interaction energies of molecules in a film being different from that in the bulk. Following Derjaguin and Churaev [7], the disjoining pressure P can be written as P P w 1 P e 1 P s , (1) where P w is the pressure component arising from van der Waals forces acting between the film and the substrate, P e is the component from the electronic or polar interactions, and P s is the component from the molecules in the film having a structure different from the bulk liquid. If the interactions between the molecules in the film and the solid substrate are more attractive than the interactions between molecules in the bulk liquid, P . 0. Consequently, a liquid film with a thickness in a range where dP͞dh . 0 can lower its free energy by becoming thicker in some areas while thinning in others, i.e., by dewetting. When dP͞dh , 0, wetting or spreading occurs.In this Letter, we report the first experimental determination of disjoining pressure for a dewetting system. This is accomplished by using noncontact atomic force microscopy (AFM) to measure the curvature of the liquid surface at the top of the dewetting droplets to determine the capillary (or Laplace) pressure ...