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45101-G5
Friction of Self-Assembled, Polyaromatic Monolayers
This project focuses on fundamental aspects of friction of polyaromatic self-assembled structures at solid-liquid interfaces interfaces. Aromatic and polyaromatic systems have thus far not been extensively studied despite their importance for the natural lubricity of mineral oils and fuels. Materials and Methods: Self-assembled monolayers (SAMs) of a simple aromatic and a polyaromatic thiol (thiophenol and 2-napthalenethiol, respectively) on template-stripped gold have been studied in single-asperity contacts with an atomic force microscope (AFM) in friction mode. The AFM tips were functionalized with the same monolayers. The strength of adhesion was altered immersing the systems in ethanol (low adhesion) or in dry N2 gas (higher adhesion). A recent contact mechanics model by Sridhar, Johnson and Fleck (SJF) [Johnson, K. L.; Sridhar, I: J. Phys. D: Appl. Phys. 2001, 34, 683; Sridhar et al., J. Phys. D: Appl. Phys. 2004, 37, 2886] for a thin, compliant elastic film confined between stiffer substrates was used to evaluate the data in systems with higher adhesion (in dry N2). Experimental results: In ethanol, the interfacial energy was low, γ < 1 mJ/m2, and a linear dependence of the friction force (F) on load (normal force, L), with F → 0 at L → 0 was found in both monolayer systems. The same friction force was found at low loads for tip radii of 52 and 157 nm, i.e., the friction force did not depend on the contact area, which is consistent with previous observations on friction in contacts with low adhesion [Ruths, M. J. Phys. Chem. B 2006, 110, 2209]. The friction force was also measured in dry N2 gas (r.h. <1.5%), where the adhesion is higher (Figure 1). The same tips were used as for the measurements in ethanol. The values of F and the shape of the F vs. L curves in Figure 1 are different from the linear F vs. L dependence observed in ethanol, which has also been observed previously for certain other aromatic and alkanethiol monolayers [Ruths, M., ibid.]. The data appears to have an area-dependence not seen in systems with low adhesion. Comparison to SJF model: The radius of the contact area is comparable in size to the thickness of the monolayer, whose elastic modulus is 10-50 times lower than that of the confining surfaces. The SJF model was applied to the data obtained in dry N2. The solid curves in Figure 1 (intended as fits to the low-load part of the experimental data, L <150 nN) represent F = ScA with the area A calculated using an estimated film modulus of E' = 2 GPa, giving a critical shear stress Sc = 400 and 200 MPa for thiophenol and 2-napthalenethiol, respectively. These values suggest a lower friction in the 2-napthalenethiol system, which was also obtained when measuring linear friction F = μL in ethanol. The value for thiophenol is in good agreement with a previous result [Ruths, M., ibid.] of Sc = 320 MPa obtained with a tip with a different radius, R = 17 nm. Discussion: Low adhesion (in ethanol) resulted in a linear increase in friction force with load, i.e., F = μL, whereas higher adhesion (in N2 gas, Figure 1) gave an apparent area-dependence of the form F = ScA. Using the SJF model, and estimated values of the elastic modulus of the SAM, we obtained Sc values at low load in the adhesive systems. The experimental data at the highest loads frequently agree less well with the models, i.e., with the linear dependence in ethanol (not shown) or with the solid curves in Figure 1. In some cases, a distinct onset of another region is seen as in Figure 1a. Measurements with tips with smaller radii show onsets of this divergence at lower loads, [Ruths, M. ibid.]. In work on alkanethiol monolayers [Liu, G.-y.; Salmeron, M. B. Langmuir 1994, 10, 367], it has been suggested that this could be caused by a reversible displacement of molecules in the monolayer at a certain pressure. However, in our work, the friction at high load is not the same as for a bare substrate, and it is therefore unlikely that the monolayer is completely removed from the contact. This project provides one Graduate Research Assistant with the opportunity to become familiar with surface modification and characterization techniques. Results are expected to provide a more in-depth understanding of the interactions in aromatic monolayers and insight into the relationship between monolayer structure, adhesive conditions, and friction response. Figure 1. F vs. L measured in dry N2 gas. (a) Thiophenol, R = 52 nm. (b) 2-Napthalenethiol, R = 103 nm. Curves are calculated according to the SJF model with E' = 2 GPa.
