Forces between surfaces across nanoparticle solutions: role of size, shape, and concentration.

Using a surface force apparatus, we have measured the normal forces between mica surfaces across various types of nanoparticles consisting of ZnS cores coated with a monolayer of physisorbed surfactant, dispersed in organic solvents. We focused on the effects of nanoparticle size, shape, and concentration on the force-distance profiles. Forces were exponentially repulsive when the surfactant layers were strongly bound to the nanoparticles and were roughly linear when there was adhesion between the nanoparticle cores, i.

Properties of confined and sheared rhodamine B films studied by SFA-FECO spectroscopy.

We have used a surface forces apparatus and multiple beam interferometry to measure the absorbance of thin films of rhodamine B in water/ethylene glycol solutions while applying and measuring normal and lateral (shear) forces. Both normal and shear forces induced changes in the absorption spectra indicating a change in molecular alignment, and rhodamine-rhodamine and rhodamine-surface interactions. We also measured differences in the absorbance spectra in different regions of the contact indicating, as expected, that the stresses are not uniform throughout the contact area.

Triboelectrification between smooth metal surfaces coated with self-assembled monolayers (SAMs).

Using a modified surface forces apparatus, we have simultaneously measured the friction and triboelectrification between both similar and dissimilar molecularly smooth hexadecanethiol-coated metal surfaces on mica substrates. On shearing dissimilar surfaces, the tribocurrent increases dramatically as the load or pressure is increased, with large fluctuations about the mean. Neither charge transfer nor fluctuations are observed when the symmetric surfaces are sheared against each other.

Friction and tribochemical reactions occurring at shearing interfaces of nanothin silver films on various substrates.

The tribological and tribochemical properties of 5-10 nm thick Ag films sliding on various metal and inorganic substrates were measured using a surface forces apparatus coupled with ex situ x-ray photoelectron spectroscopy. We observed enhanced chemical reactivity in the sheared regions compared to the unsheared regions, which we attribute to significant frictional heating in agreement with two recent simulations. It is also found that the initial topography (roughness) of the films plays a significant role in determining the friction, wear, and tribochemical reactions.