Variation of frictional forces in air with the compositions of heterogeneous organic surfaces

The frictional properties of heterogeneous organic materials have been inve stigated by atomic force microscopy. There is a good correlation between me asurements of the friction coefficient in air and the cosines of the advanc ing water contact angles for mixed self-assembled monolayers composed of po lar and nonpolar groups. The friction force is proportional to the surface free energy for these chemically heterogeneous materials. For mixed monolay ers containing short and long chain thiols with methyl terminal groups ther e is a large increase in friction, attributed to alkyl chain disorder rathe r than a surface energy effect. There is a nonlinear relation between frict ion coefficient measurements in air and the cosines of the advancing water contact angles on plasma-treated polyester films. This is thought to reflec t a contribution from the changed mechanical properties of the plasma-treat ed film surface in addition to the observed surface energy increase on thes e more complex materials.