1. Introduction
Oiliness agents with polar groups form the adsorption film, reducing friction and wear on sliding surfaces [1]. Previous studies suggested that the friction reduction effect of oiliness agents involves not only the adsorption film but also solvation layer at the friction interface. Watanabe et al. incorporated a frictional system into an SFG spectroscopy system and observed changes in the adsorbed structure due to friction. They demonstrated that in a solution of n-dodecane with stearic acid, n-dodecane molecules were constrained on the adsorption film of stearic acid and aligned in the shear direction [2]. Therefore, to understand the friction reduction of oiliness agents, it is necessary to consider the role of the solvation layer. In this study, we developed a new atomic force microscopy (AFM) system combining FM-AFM and LFM, which measures repulsive force and lateral force simultaneously, to investigate the friction reduction mechanism of n-hexadecane with stearic acid. By combining FM-AFM and LFM, it is also possible to calculate the friction coefficient distribution along the direction of the depths. This report presents the results of simultaneous measurements of solid-liquid interfacial structure and frictional properties using the developed FM-AFM/LFM system.
2. Experimental detail
2.1. FM-AFM/LFM simultaneous measurement method
Fig. 1 shows a schematic of the simultaneous measurement system for FM-AFM and LFM. The equipment consisted of an SPM-8100FM (SHIMADZU, JP) and lock-in amplifier (LI5660, NF, JP). LFM measurements were performed simultaneously with regular FM-AFM measurements. The sample was oscillated in Y direction by XYZ scanner using AC signal from a function generator. The torsion signal of the cantilever, caused by the oscillating sliding of the sample, was input to the lock-in amplifier. Using the reference signal from the function generator, the lock-in amplifier extracted only the lateral force signal from the torsion signal, which contained noise. These measurements were performed with the cantilever constantly excited in Z direction for FM-AFM measurements.
2.2. Experimental conditions
In the FM-AFM/LFM simultaneous measurement method, it is possible to obtain repulsive force and lateral force against Z axis at 25°C using a silicon cantilever (PPP-NCLAuD, spring constant C: 20 N/m, resonance frequency f: 78-80 kHz). The repulsive force was calculated from resonance frequency changes of the cantilever using Sader's formula [3].
3. Result & Discussion
As a result, we were able to measure repulsive force (Fig. 2(a)(b)) and lateral force (Fig. 3(a)(b)) simultaneously. From these results, the friction coefficient against the Z axis (Fig. 4) was obtained using a single test method. The friction coefficient and phase difference between the sample and cantilever (Fig. 5(a)(b)), provide new insights into the low-friction area induced by the solvation structure in n-hexadecane with stearic acid. It was confirmed that the repulsive force, friction force, and phase difference decreased around Z = 4.6. This suggests that in a solution of n-hexadecane with stearic acid, n-hexadecane molecules were constrained on the adsorption film of stearic acid and behaved like a solid, thereby protecting the mica surface and reducing friction.
4. Conclusion
[1] The FM-AFM/LFM simultaneous measurement method enables the acquisition of both repulsive force and lateral force at the same time.
[2] It is possible to investigate the relationship between friction coefficient and solid-liquid interface structure, estimated from repulsive force measurements.
[3] n-hexadecane molecules were constrained on the adsorption film of stearic acid and behaved like a solid, thereby protecting the mica surface and reducing friction.
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