Direct measurement of the depletion force using an atomic force microscope

The interactions between colloidal particles immersed in solutions of a nonadsorbing polymer are of interest in a variety of applications such as paint and ink formulations and mineral processing. In this Letter we report the use of an atomic force microscope to study the interaction forces between a silica particle (of radius 3.8 μm) and a planar silica surface, both bearing a terminally grafted n-octadecyl alcohol sheath (SiOinf2/inf-Cinf18/inf), mediated by cyclohexane solutions of poly(dimethyl siloxane) (PDMS). The experimental results are consistent with theories of the depletion interaction between hard, chemically inert surfaces in the presence of free (nonadsorbing) polymer in solution. The length of the measured (attractive) interaction was commensurate with the radius of gyration of the PDMS sample used, and the magnitude of the force increased with increasing polymer concentration. As the surfaces closely approached each other, a short-range steric interaction was observed as the opposing octadecyl chains came into contact, and this interaction was independent of the free polymer concentration.