Tribology is the study of sliding contacts between materials, and includes the phenomena of friction and wear. Our particular interest is in nanometre-scale contacts between molecular materials. Nanometre-scale sliding contacts are important for a variety of reasons; the following are examples:
- Miniaturised devices, such as microelectromechanical systems (MEMS), rely upon tiny mechanical contacts (see http://mems.sandia.gov/gallery/images.html for a wonderful collection of micrographs), and the lubrication of these presents substantial problems - a drop of oil will not suffice! Organic thin films and monolayers may provide solutions.
- Macroscopic materials may have properties that depend on microscopic sliding interactions. For example, conditioners in laundry formulations are used to modify the sliding interactions at tiny contacts between between textile fibres that are only a few micrometres in diameter (see below).
- Measurements of the tribological properties of surfaces at nanometre length scales provide a powerful means for mapping variations in chemical structure and composition; friction force microscopy is an invaluable tool for the characterisation of molecular nanostructures (see nanofabrication page).
- Studies of tribological phenomena may yield insights into the fundamental nature of forces at surfaces; a current focus in our research is exploring the relationship between nanotribology and the thermodynamics of non-covalent molecular interactions.
Self-assembled monolayers are organised assemblies of organic molecules that form spontaneously when a suitable substrate (for example, gold) is immersed in a solution containing an appropriate adsorbate (for example, a dilute solution of an alkylthiol in ethanol). They enable the precise control of interfacial chemistry and provide ideal model systems for exploring the relationship between surface molecular structure and tribological interactions.
Our measurements are made using an atomic force microscope (AFM). In the AFM, a sharp tip attached to a flexible cantilever is scanned across a sample surface. As the interaction force between the probe and the sample changes, the deflection of the cantilever changes. The deflection is proportional to the force (the cantilever behaves like a Hookean spring). Lateral and vertical deflections may be measured, to yield the forces normal to and parallel to the plane of the sample surface. Molecular interactions may be studied by adsorbing a self-assembled monolayer onto the AFM probe and the sample surface, and nanoscale variations in friction used to map the surface chemical composition [1-3].
The lateral deflections of the AFM cantilever are influenced by the friction force acting at the tip-sample contact. The friction force can be determined readily by subtracting friction traces acquired with the probe travelling in opposite directions (to eliminate topographical contributions to friction). This approach is called friction force microscopy (FFM). It has been known for two decades that the nature of any molecular species at this interface has an effect on the magnitude of the frictional force, but the relationship between the friction force and the load has been the subject of a great deal of controversy. For example, larger coefficients of friction have been measured for adsorbates that form hydrogen bonds than for hydrocarbon films. Many workers have used Amontons' law - that the friction force is proportional to the applied load, with the constant of proportionality being equal to the coefficient of friction, to analyse friction-load data acquired by FFM. However, the AFM probe should resemble an idealised asperity, whereas Amontons' law represents contacting surfaces that consist of arrays of large numbers of asperities that rub against each other.
An alternate approach is to use a single asperity contact mechanics model such as the Johnson-Kendall-Roberts model (JKR) or the Derjaguin-Muller-Toporov model (DMT). These models predict the area of contact between the asperity and the countersurface, and it is assumed that the friction force is propertion to the area of contact between the two. In contrast to Amontons' law, these models predict a sub-linear relationship between the friction force and the load. The JKR model has been widely used to model pull-off force measurements in AFM, but for molecular systems, there is little direct empirical evidence to support its application to sliding contacts.
The absence of a clear basis for quantitative intepretation of FFM data has been a substantial problem. Evidence from this laboratory suggests that the nature of the medium in which measurements are made can have a profound effect on the mechanics of the tip-sample interaction [4,5]. Recently we have begun exploring these correlations in detail.
In collaboration with Chris Hunter we have established a direct link between solution-phase thermodynamics and hydrogen bonding in nanoscale molecular contacts, based upon extensive studies of the behaviour of molecular films in a range of liquid media. A distinctive feature of these studies has been the systematic investigation of the correlation between contact mechanics and the composition of the liquid medium, using extensive models that Chris has developed for non-covalent interactions in condensed phases. In previous work the importance of the medium has been ignored largely. However, we have found that measurements in different liquids, and in liquid mixtures consisting of a hydrocarbon (S1) and a polar molecule that is a hydrogen bond acceptor (S2), unlock the connection between bulk-phase thermodynamics and the mechanics of nanometre-scale contacts. For molecules that form hydrogen bonds, the strength of adhesion between the probe and the surface does not correlate with the dielectric properties of the medium. Instead, the adhesion force correlates remarkably closely with logKa, where Ka is the equilibrium constant for the formation of a hydrogen-bonded complex between a donor (D) and an acceptor (A) in the bulk phase. In the diagram below, the left-hand figure shows the variation in logKa predicted by Chris Hunter's model. The right-hand figure shows the variation in the experimental pull-off force for a carboxylic acid functionalised AFM probe interacting with a carboxylic acid functionalised surface.The value of logKs can be predicted from the intersection between the horizontal and sloped parts of the plot, and the values obtained have been found to be in remarkably close agreement with values predicted by Chris's model and confirmed in bulk-phase measurements.
The dependence of the friction force on the load has been found to be strongly dependent on the thermodynamics of interaction at the tip-sample contact. We find that linear friction-load relationships are observed where unusually weak tip-sample adhesive interactions occur. For example, in acetone-heptane mixtures a linear friction-load relationship is observed for carboxylic acid and hydroxyl terminated monolayers at compositions that yield extensive surface solvation [6,7]. As the concentration of the hydrogen bond acceptor is reduced, non-linear friction-load relationships are observed that may be modelled using DMT mechanics. These observations are rationalised by assuming, as others have previously suggested, that the friction force is the sum of a load-dependent and a shear term:
The load-dependent term represents energy dissipation in ploughing, probably in the form of deformations of adsorbate alkyl chains. The shear term results from adhesive interactions between the probe and the surface. Modelling of the friction-load relationships yields a “coefficient of friction” that does not vary with the adhesion force, and a surface shear strength that correlates closely with the free energy of interaction between the hydrogen bonding functional groups. The data below are for hydroxyl functionalised probes interacting with hydroxyl (HUT) and phosphonate diester (DPTS) films . Comparative data are also shown for a hydrocarbon SAM.
A non-linear friction-load relationship is thus “normal”, with linearity representing a limiting form of behaviour where unusually weak adhesion occurs (eg a highly solvated surface). Observations of the solvent-dependence of the pull-off force and the surface shear strength enable the prediction of Ks, the equilibrium constant for solvation of the hydrogen bonding functional groups on the surface by the polar solvent. Remarkably close agreement has been found between Ks values determined from modelling the contact mechanics in this way and from bulk phase thermodynamics.
- N. J. Brewer, B. D. Beake and G. J. Leggett, "Friction Force Microscopy of Self-Assembled Monolayers: Influence of Adsorbate Alkyl Chain Length, Terminal Group Chemistry and Scan Velocity", Langmuir 2001, 17, 1970. http://pubs.acs.org/doi/abs/10.1021/la001568o
- N. J. Brewer and G. J. Leggett, "Chemical Force Microscopy of Mixed Self-assembled Monolayers of Alkanethiols on Gold: Evidence for Phase Separation", Langmuir 2004, 20, 4109. http://pubs.acs.org/doi/abs/10.1021/la036301e
- T. J. Whittle and G. J. Leggett, "Quantitative Measurement of the Kinetics of Reaction of Trifluoroacetic Anhydride with Self Assembled Monolayers of Mercaptoundecanol Using Friction Force Microscopy", Langmuir 2009, 25, 9182-9188. http://pubs.acs.org/doi/abs/10.1021/la900741y
- C. R. Hurley and G. J. Leggett, "Influence of the Solvent Environment on the Contact Mechanics of Tip-Sample Interactions in Friction Force Microscopy of Poly(ethylene terephthalate) Films", Langmuir 2006, 22, 4179-4184. http://pubs.acs.org/doi/abs/10.1021/la053176t
- T. J. Colburn and G. J. Leggett, "Influence of Solvent Environment and Tip Chemistry on the Contact Mechanics of Tip-Sample Interactions in Friction Force Microscopy of Self Assembled Monolayers of Mercaptoundecanoic Acid and Dodecanethiol", Langmuir 2007, 23, 4959-4964. http://pubs.acs.org/doi/abs/10.1021/la062259m
- K. Busuttil, M. Geoghegan, C. A. Hunter and G. J. Leggett,"C ontact Mechanics of Nanometer-Scale Molecular Contacts: Correlation between Adhesion, Friction, and Hydrogen Bond Thermodynamics ", J. Am. Chem. Soc. 2011, 133, 8625. http://pubs.acs.org/doi/abs/10.1021/ja2011143
- K. Busuttil, N. Nikogeorgos, Z. Zhang, M. Geoghegan, C. A. Hunter and G. J. Leggett, "The mechanics of nanometre-scale molecular contacts ", Faraday Disc. 2012, 156, 325. http://pubs.rsc.org/en/content/articlelanding/2012/fd/c2fd00133k
- N. Nikogeorgos, C. A. Hunter and G. J. Leggett, Langmuir 2012, 28, 17709. http://pubs.acs.org/doi/abs/10.1021/la304246e