Mechanics of Nanoscale Single Asperity Contacts in Friction Force Microscopy

Lead Research Organisation: University of Sheffield
Department Name: Chemistry


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.


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Description We have investigated the mechanics of interaction in sliding contacts between an atomic force microscope (AFM) probe and a counter-surface. The AFM functions a little like the stylus of a record player, responding to changes in surface contours. As the probe is scanned across the surface, the changes in topography yield a changing signal that is used to map the surface. The frictional resistance to motion of the probe may also be measured, but the meaning of the signals has been controversial because the mechanics of interaction have not been well understood. We have measured the contact mechanics for probes and surfaces coated with thin films of organic molecules in a range of liquid media, including mixtures of two different liquids. In the case of liquid mixtures, we have been able to model the relationship between the adhesion and friction forces and the applied load, and compare the results with predictions made using a model for bulk-phase hydrogen-bond thermodynamics. The most important findings are: (i) that the friction force may be treated as the sum of two components, a load-dependent term characterised by a "coefficient of friction" (attributed to "molecular ploughing" - tip-induced deformations of molecules in the contact area) and an area-dependent shear term characterised by a surface shear stress; and (ii) that both the adhesion force and the surface shear strength, obtained by analysis of friction-load plots, are proportional to the free energy of interaction of the molecules attached to the AFM probe and counter-surface. Indeed, we are able to predict the magnitude of key thermodynamic parameters (for example, equilibrium constants for association at the surface) by analysis of the friction-load plots. A strong correlation has been established between the state of solvation of the sliding surfaces and the friction-load behaviour: for weakly solvated surfaces, the shear term dominates and the interaction is, for organic monolayers, described by Derjaguin-Muller-Toporov mechanics. However, the load-dependent term dominates for highly solvated surfaces, and the friction force is proportional to the load. These findings assist greatly in interpreting the apparently contradictory findings in a large body of literature.
Exploitation Route These findings provide a fundamental basis for understanding the basic interactions between the tip and the surface in FFM. We now have a realistic model to begin to measure the thermodynamics of non-covalent interactions at surfaces in a direct and quantitative fashion. We believe that these findings are of potentially very widespread importance.
Sectors Chemicals,Electronics,Energy,Healthcare,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology,Transport

Description The project was aimed at the provision of underpinning udnerstanding in the area of nanotribology. As such, there was not immediate commercial impact. However, in the period directly after the end of the grant we begun speaking to a number of companies about ways that we could use the knowledge acquired to build collaborative research programmes. We are optimistic that some of these will come to fruition soon. We have also carried out several pieces of work on behalf of industrial partners who have paid to access our expertise.
First Year Of Impact 2012
Sector Healthcare,Manufacturing, including Industrial Biotechology
Impact Types Economic