Elasto-plastic and visco-elastic deformations of a polymer sphere measured using colloid probe and scanning electron microscopy

The adhesive interaction energy between a single 27 μm polystyrene sphere and a flat silica surface has been measured, as a function of applied load on the sphere, using an atomic force microscope (AFM). The pull-off force required to remove the sphere from the surface after application of a given load was found to increase as a function of the applied load. These data are indicative of a plastic or elasto-plastic deformation of the sphere. Simple analyses of these data using established elastic/plastic deformation theories indicate that, at the loads used, the system is most probably undergoing an elasto-plastic deformation. Further evidence for some plastic deformation of the sphere was obtained using scanning electron micrographs of the same sphere after an AFM experiment had been completed. Careful analysis of all these data indicated a significant time dependence of these adhesive interactions due to the viscoelastic nature of the polymer bead in question.