ATOMIC-FORCE MICROSCOPY INVESTIGATION OF THE ADHESION BETWEEN A SINGLE POLYMER SPHERE AND A FLAT SURFACE

Adhesional interaction forces between a single 5 mu m radius polystyre ne sphere and an atomically smooth mica surface have been measured usi ng an atomic force microscope. The pull-off force was determined as a function of two factors: the contact time between the surfaces at a co nstant maximum applied load and the applied lend. Careful analysis of the data in the contact zone revealed that the polystyrene underwent s ignificant non-elastic deformations when the contact times were more t han 10 s. These data were rationalized on the basis of viscoelastic de formation of the sphere as the load was applied: the lends used here e xceeded the elastic limit for polystyrene. It appears from the data th at the exact modelling of the sphere deformation is a complex problem involving both surface asperities and bulk material collapse. The pull -off force was seen to increase both with increasing applied load and with longer contact time. Calculation of the applied forces indicated that plastic deformation was likely, at least at the slower loading ra tes, and the relationship between the pull-off force and the applied l oad (P) agreed with a P-1/2 dependence as suggested by Maugis and Poll ock. It was concluded that at the applied loads-used here, the surface asperities were effectively 'squeezed out' resulting in an apparent s mooth sphere contact.