On the Jump-to-Contact of Micr/Nano Contact

Atomic force microscopy (AFM) is widely used in nanotechnology. The AFM can not only be used to image the topography of solid surfaces at high resolution, but also measure the force of the tip on the substrate. In AFM measurement, jump-to-contact and jump-off-contact are well-known phenomena. Jump-to-contact may cause complete breakdown of the oscillation of the AFM measurement in non-contact mode. Therefore, the jump-to-contact is a topic of interest in AFM measurement. This research will investigate this issue. For AFM, the force-distance curves are the fundamental tools in research. For an apparatus of finite stiffness, jump-to-contact occurs when the force gradient exceeds the elastic constant of the cantilever. For a fixed-grips device, jump-to-contact occurs at the vertical tangents of the S-shape force-approach curve. Self-consistent numerical analysis for the adhesive contact between elastic spheres was done by Greenwood [2] and Feng [8]. In the numerical simulation, the deformation of the sphere is taken into account, and the force-displacement relation can be obtained numerically. Using this force-displacement relation, the jump-to-contact distance can be obtained. The jump-to-contact formula was firstly proposed by Israelachvili and Tabor. They assumed a rigid sphere and van der Waals force. Their formula works well for small Tabor parameters, but does not work well for large Tabor parameters. This research will use the Lennard-Jones potential and take the deformation into account. By using the path-following method and the Newton-Raphson method, the force-approach relation will be investigated. Semi-empirical formula for the approaching part of the force-approach curve will be derived. The jump-to-contact distance can be predicted by the proposed formula. The result can apply in the AFM measurement. It will make an important contribution to nanotechnology.

Project ID:PB9907-10759
External Project ID:NSC99-2221-E182-015