(468f) Time-Dependent Effects in Afm Measurements of Disjoining Pressure of Pfpe Films

An important parameter for quantifying the spreading behavior of a perfluoropolyether (PFPE) film is its disjoining pressure. Previous work in this research group has described a method for calculating the disjoining pressure indirectly from AFM measurements. This work focused on looking at the disjoining pressure qualitatively with the goal of using this information to screen hard disk lubricants in a high-throughput manner. Here, we present new procedures for reducing time-dependent kinetic effects. Minimizing these effects is critical to obtaining accurate quantitative data from the AFM method. The accuracy of this data is verified by comparison with Lifshitz theory.

The AFM method is based upon interpretation of force-distance curves obtained by probing silicon substrates dip-coated with the PFPE, Fomblin Z 03. Experiments are performed by bringing a spherical cantilever tip into contact with the film. Contact causes a meniscus of PFPE to form due to capillary forces. After contact is made, the AFM probe and the sample are separated until the meniscus between them is broken. Analyzing the stretched meniscus region of force curves with a model describing the Laplace pressure of a PFPE meniscus yields the effective radius of curvature of the meniscus. This radius of curvature is related to the disjoining pressure when the film on the surface is in equilibrium with the film in the meniscus. Equilibrium is defined as the state when there is no net flow rate to or from the meniscus.

To reach equilibrium, it is necessary to consider time-dependent effects. Initial contact between the cantilever and the surface depletes the film in a region around the cantilever tip. To account for this depletion, a cantilever preconditioning step is used. This step involves leaving the probe in contact with the film for a prolonged period of time, then moving it to a new location for force curve measurements. This preconditioning wets the AFM probe with PFPE so less film is drawn from the surface at the location the force curve measurements are performed. Additionally, a closed-loop AFM system is used to hold the stage displacement at discrete locations during the force curve collection. The waiting periods imposed at these locations allow the film time to equilibrate as the meniscus region of the force curve is traversed. Experiments have shown that stretching the PFPE meniscus at the slowest constant velocities possible with our AFM is insufficient for reaching equilibrium.

Including these two modifications gave superior results. A plot of disjoining pressure versus film thickness (for 1-7 nm films of Fomblin Z 03) demonstrated behavior in accordance with Lifshitz theory; namely, thin films yielded extremely large disjoining pressures while thicker films had smaller disjoining pressures. A similar suite of samples was obtained using Fomblin Zdol 2000 as the lubricant. Zdol more closely matches commercial lubricants, and has terminal hydroxyl groups that strongly bind many polar surfaces. Force curves on Zdol show a pronounced inflection near contact, signaling that structural forces operate near probe-surface contact. The effect of this non-van-der-Waals interaction on disjoining pressure profiles is discussed.