Friction forc emicroscopy of mixed Langmuir-Blodgett films

Thin films of mixtures of fluorocarbons and hydrocarbons are investigated by friction force microscopy. For a mixture of equal molar parts of arachidic acid and partially fluorinated carboxylic acid, phase separation manifests itself as circular domains. By shortening the length of the hydrocarbon chain, the disorder of the system is increased and phase separation takes on a fractal geometry. The frictional forces are found to be material specific and distinguish between domains of fluorocarbons and hydrocarbons. With higher normal forces hydrocarbon particles are easily sheared away while the fluorocarbon region remains unbroken. The results are discussed in relation to the phenomenon of boundary lubrication.