Friction of thin water films: a nanotribological study

• Published in: Surface science 2002-04, Vol.504 (1-3), p.199-207
• Main Authors: Opitz, A., Ahmed, S.I.-U., Schaefer, J.A., Scherge, M.
• Format: Article
• Language: English
• Subjects: Atomic force microscopy; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Friction; Mechanical and acoustical properties; Physics; Scanning tunneling microscopy; Silicon; Silicon oxides; Solid surfaces and solid-solid interfaces; Solid-fluid interfaces; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Tribology; Water
• ISSN: 0039-6028; 1879-2758
• DOI: 10.1016/S0039-6028(02)01069-5

Lubricant thickness is known to influence the frictional properties between two contacting surfaces in relative motion. In this paper, the dependence of friction on water films with various thicknesses is examined using a scanning force microscopy (SFM) between a hydrophilic silicon tip and silicon flat that was rendered either hydrophilic or hydrophobic. Results indicate that the frictional properties are influenced by the ordering effects of water. The friction of a hydrophilic surface initially covered with 2.6 nm of water was examined as a function of the film thickness, which was separately determined using scanning tunneling microscopy and dynamic SFM. Capillary effects dominate the tribological properties for films that are between 3 and about 1 nm thick. For thinner films the friction properties can be explained by the ordering effect of water on the sample and the tip, which together cause an increased resistance to shear (higher viscosity). Further reduction of film thickness due to water desorption leads to a decrease of the friction force; this regime is dominated by cohesive forces arising from the solid–solid contact. For all applied forces in this study, liquid confinement between tip and surface does not occur. Rather, the sharp SFM tip apparently penetrates the water film with the tip apex making direct contact with the surface. The ordering effect of water occurs on the sample surface and at the sides of the SFM tip. These results, together with previous microtribological studies, highlight the significant differences existing between two contacting surfaces moving in relative motion in the micro- and nanoregime.