Quasi-Equilibrium AFM Measurement of Disjoining Pressure in Lubricant Nano-Films I:  Fomblin Z03 on Silica

Abstract

We have identified conditions in which the atomic force microscope can be used to stretch a meniscus of a perfluoropolyether (PFPE) lubricant pinned between an AFM tip and a nanometer-thick PFPE film to obtain the disjoining pressure of the film. Under quasi-equilibrium conditions, the chemical potential of the film can be equated to that of the stretched meniscus. A theory is presented that provides a complete description of the capillary force of a stretched meniscus. Fits of the theory to quasi-equilibrium force-extension curves yield the effective meniscus curvature and, by extension, the disjoining pressure of the underlying film. AFM force curves collected at varying film thicknesses compare very well with predictions from Lifshitz theory of dispersive interactions in thin films, with no adjustable parameters. This complete description of meniscus deformation during atomic force microscopy force-extension experiments makes possible the measurement of unknown disjoining pressures as required for screening of lubricant-overcoat combinations required for next-generation data storage systems.

Gas flow near a smooth plate

“…This agreement extends to the region when the gas is confined to a film that has a thickness that is similar to the mean free path of the gas. This ability to extend continuum force and flow models to molecular dimensions is seen elsewhere [10,21] and is useful for modeling. The experiment is sufficiently sensitive to distinguish between damping from asymmetric pairs of surfaces.…”

Improved Substrate Protection and Self-Healing of Boundary Lubrication Film Consisting of Polydimethylsiloxane with Cationic Side Groups

“…The higher the ionic content, the higher viscosity of the bulk liquid is. The higher viscosity would result in slower lateral flow [25]. The working duration cycle increases sharply when the cycle interval is longer than 35-40 s, indicating that the CPL film is almost fully recovered before the next sliding cycle begins.…”

“…The only difference is that the CPL lateral flow is driven by a disjoining pressure gradient of the film while molecular diffusion is driven by a concentration gradient [21,34]. The unit of the characteristic spread rate constant is the same as the diffusion coefficient [25]. Since we cannot quantitatively correlate the duration cycle to the film thickness, we cannot use analytical mass transfer model with moving boundary conditions.…”

“…For all experiments, the balls were preconditioned by contacting the substrate with no applied load for 30 min. This allows transfer of the CPL film from the substrate to the ball surface [25,26]. The lateral force exerted to the pin during the slide was measured with a strain gauge sensor.…”

Thermodynamics of capillary adhesion between rough surfaces

“…It is then straightforward to calculate the adsorbed layer thickness [43]. It can also be determined from AFM measurements [63,64]. For other substances electronic and structural components must also be considered to find A H .…”