The Simultaneous Determination of the Forces and Viscoelastic Properties of Adsorbed Polymer Layers

The viscoelastic properties of a solvated adsorbed film of gelatin on glass have been determined by modifying a particle forces apparatus which is a derivative of an atomic force microscope (AFM). Here a particle is attached to the end of an AFM lever. Gelatin is allowed to adsorb to the particle and to a flat glass surface. The surface is driven toward the particle using a piezoelectric ceramic and the deflection of the lever monitored using a laser beam and a position-sensitive photodiode. In the experiments reported here a relatively low frequency oscillation (20-1000 Hz) is superimposed on the linear motion. By examining the in-phase and out-of-phase responses of the particle to the applied oscillation, the viscous and elastic properties of the gap may be determined with separation. This information is obtained simultaneously with the forces of interaction between the gelatin-coated glass surfaces. The data show that a viscous response dominates until the gelatin layers interact. After contact the elastic response increases and dominates at high compression of the adsorbed gelatin layers. Problems concerning the quantitative analysis of the data in terms of viscous and elastic moduli are discussed. Copyright 1999 Academic Press.