Direct measurement of structural forces between two surfaces in a nonpolar liquid

The force as a function of separation has been measured between two molecularly smooth surfaces immersed in the liquid octamethylcyclotetrasiloxane (OMCTS) whose molecules are quasispherical of diameter ∼1.0 nm. The force is an oscillatory function of distance, varying between attraction and repulsion, with a periodicity equal to the size of the liquid molecules to within the experimental resolution of ∼0.1 nm. The oscillations decay rapidly with distance: their measurable range is 6–10 molecular diameters and their magnitude exceeds that of conventional van der Waals forces at small distances. The magnitude of the oscillations is insensitive to changes in temperature, but sensitive to the chemical nature of the surfaces, and very sensitive to the presence of water. The results are considered, qualitatively, within the context of current theories of the liquid state near solid interfaces, and some implications for surface chemistry and colloid science are discussed.