Fracture of a fatty acid multilayer film

The surface morphology of a fatty acid multilayer film has been studied using atomic force microscopy. Samples were prepared using a range of thermal treatments that generate intrinsic structural defects in the film. Thermal contraction of the cooling film gives rise to tensile surface stresses, which are relieved through the creation and subsequent growth of fractures. The study of these defects elucidates physical properties of the film at the micron and nanometre scale. Following thermal treatment at low temperatures, fractures are found to coexist with quasi-circular hole defects that are localised at the film surface. Fractures and holes result from different nucleation mechanisms and propagate via anisotropic and isotropic growth processes, respectively. The two distinct types of defect indicate that the film exhibits both liquid- and solid-like behaviour and suggests that the division is dependent upon the distance from the graphite substrate. Macroscopic fracture resulting from a decrease in volume caused by thermal contraction or desiccation occurs widely in nature. The work presented here suggests the occurrence of a similar physical process for molecular films on the nanometre scale.