Mechanics of Nanoscale Single Asperity Contacts in Friction Force Microscopy

Many new technologies are relying upon nanostructured devices and materials to deliver new properties and improved performance. Many everyday technologies also depend upon the organisation of materials at the nanometre scale. A good illustration of this is the action of hair and fabric conditioners, which rely for their performance on the ability to control the distribution of conditioner molecules at complex, curved surfaces (hair fibres are only 100 micrometres in diameter, and textile fibres may be much smaller) with uniform distribution being required on very small length scales. In all of these areas, there is an urgent need for information about the distribution of molecules at the surface with a resolution of nanometres. However, there are very few ways of doing this. Friction force microscopy (FFM), which uses a sharp tip attached to a flexible microscopic cantilever to measure surface friction, provides one solution to this problem. In addition to providing a means of mapping surface composition, FFM also provides an ideal model system for understanding the types of sliding contacts that occur in new miniaturised technologies such as microelectromechanical devices, where tiny sliding contacts require lubrication but where conventional lubricants fail.The principal difficulty with using FFM to solve these varied problems is that we still lack an adequate understanding of the fundamental principles that underpin its mechanism of operation. In order to obtain quantitative information about surface friction, we must first be able to understand the contact mechanics associated with the tip-sample interaction - the microscopic physical interactions that determine the strength of the frictional interaction. There has been a great deal of debate about this. Some researchers have favoured the use of a very old physical law, Amontons' law, which states simply that the friction force is proportional to the load applied perpendicular to the sample surface. Others have suggested that more complex laws apply. Recent significant progress was made by the applicants, who showed for the first time that not just the strength of the frictional interaction, but also the type of mechanics that applied seemed strongly influenced by the environment in which the FFM experiment was conducted. The objective of this proposal is to build on these preliminary findings, by building a broad understanding of the mechanics of FFM. Such a venture will provide an interpretational framework for the technique that is grounded in solid experimental data. In addition to developing a better understanding of fundamental principles, we also aim to apply the technique to two important classes of materials: organic polymers (polystyrene and polymethylmethacrylate), where the molecular weight determines many of the mechanical properties of the material; and polymer brushes, new materials that are attracting enormous interest because of the potential they offer for control of interfacial interactions such as adhesion. In both cases, FFM may provide a quicker and easier method for exploring molecular structure and properties than other techniques currently available, and it could prove a valuable tool to researchers working on a variety of problems.