The influence of contact pressure and saponification on the sliding behavior of stearic acid monolayers

Abstract

This paper describes the influence of the degree of calcium ion saponification and the contact pressure on the character of sliding of monomolecular layers of stearic acid. We also studied the effect of sliding velocity on the shear strength for the pure acid and the pure soap. The layers were sheared between curved mica surfaces following the techniques described initially by Bailey and Courtney-Pratt [ Proc. Roy. Soc. London Ser. A 227, 500 (1955)], and more recently by Israelachvili and Tabor [ Wear 24, 386 (1974)]. In the present experiments we increased the range of contact pressures (up to 5 × 10 8 Pa) and conducted our measurements over a wider range of sliding velocities (0.3 to 300 μm s −1). We observed two distinct types of sliding behavior: smooth sliding and oscillatory motion, involving “stick-slip” or relaxation oscillations. At low contact pressures and high acid contents, we observed smooth sliding, whereas at high pressures and high soap content, we obtained stick-slip. We found a marked difference between the friction-velocity relationships of pure acid and pure soap films. The connection between the occurrence of stick-slip and the friction-velocity relationship is well established [Bristow, Proc. Roy. Soc. London Ser. A 189, 88 (1946)]. Our observations of the variation of friction with sliding velocity and the character of sliding are in agreement with theoretical analyses of stick-slip motion. We have no unequivocal explanation for the sensitivity of the friction-velocity relationship to changes in degree of saponification and pressure.