Forces have been measured between silica surfaces with adsorbed surfactants by means of a bimorph surface force apparatus. The surfactants used are the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) and the nonionic surfactant hexakis(ethylene glycol) mono-n-tetradecyl ether (C14E6) as well as mixtures of these two surfactants. The measurements were made at elevated pH, and the effect of salt was studied. At high pH the glass surface is highly charged, which increases the adsorption of TTAB. Despite the low adsorption generally seen for nonionic surfactants on silica at high pH, addition Of C14E6 has a considerable effect on the surface forces between two glass surfaces in a TTAB solution. The barrier force is hardly affected, but the adhesion is reduced remarkably. Also, addition of salt decreases the adhesion, but increases the barrier force. In the presence of salt, addition Of C14E6 also increases the thickness of the adsorbed layer. The force barrier height is also shown to be related to literature values for surface pressure data in these systems.

The forces and friction between cellulose spheres have been measured in the absence and presence of xyloglucan using an atomic force microscope. The forces between cellulose are monotonically repulsive with negligible adhesion after contact is achieved. The friction coefficient is observed to be unusually high in comparison with other nanotribological systems. We have confirmed that xyloglucan adsorbs strongly to cellulose, which results in a much stronger adhesion, which is dependent on the time the surfaces are in contact. Xyloglucan also increases the repulsion on approach of the cellulose surfaces, and the friction is markedly reduced. The apparently incompatible observations of decreased friction in combination with increased adhesion fulfills many of the necessary criteria for a papermaking additive.

Friction force measurements have been conducted with a colloid probe on mica and silica (both hydrophilic and hydrophobized) after long (24 h) exposure to high-humidity air. Adhesion and friction measurements have also been performed on cellulose substrates. The long exposure to high humidity led to a large hysteresis between loading and unloading in the friction measurements with separation occurring at large negative applied loads. The large hysteresis in the friction - load relationship is attributed to a contact area hysteresis of the capillary condensate which built up during loading and did not evaporate during the unloading regime. The magnitude of the friction force varied dramatically between substrates and was lowest on the mica substrate and highest on the hydrophilic silica substrate, with the hydrophobized silica and cellulose being intermediate. The adhesion due to capillary forces on cellulose was small compared to that on the other substrates, due to the greater roughness of these surfaces.

Four different cellulose model surfaces, and one silica surface, have been studied by means of atomic force microscopy (AFM). The normal interactions have been found to consist of a longer range double layer force with a short range steric interaction, the nature of which is extensively discussed. Both the surface charge and range of the steric force depend on the type of cellulose substrate used, as does the magnitude of the adhesion. Studies of friction reveal that surface roughness is the determining factor for the friction coefficient, with which it increases monotonically. The absolute value, however, is determined by the surface chemistry. All studied cellulose surfaces show similar behavior in response to xyloglucan addition.

 Presented here is a novel technique for the in situ calibration and measurement of friction with the atomic force microscope that can be applied simultaneously with the normal force measurement. The method exploits the fact that the cantilever sits at an angle of about 10 degrees to the horizontal, which causes the tip (or probe) to slide horizontally over the substrate as a normal force run is performed. This sliding gives rise to an axial friction force (in the axial direction of the cantilever), which is measured through the difference in the constant compliance slopes of the inward and outward traces. Traditionally, friction is measured through lateral scanning of the substrate, which is time consuming, and requires an ex situ calibration of both the torsional spring constant and the lateral sensitivity of the photodiode detector. The present method requires no calibration other than the normal spring constant and the vertical sensitivity of the detector, which is routinely done in the force analysis. The present protocol can also be applied to preexisting force curves, and, in addition, it provides the means to correct force data for cantilevers with large probes.