Application of Colloid Probe Atomic Force Microscopy to the Adhesion of Thin Films of Viscous and Viscoelastic Silicone Fluids

The adhesive characteristics of thin films (0.2-2 mu m) of linear poly(dimethylsiloxane) (PDMS) liquids with a wide range of molecular weights have been measured using an atomic force microscope with a colloid probe (diameters 5 and 12 mu m) for different separation velocities. The data were consistent with a residual film in the contact region having a thickness of similar to 6 nm following an extended dwell time before separation of the probe. It was possible to estimate the maximum adhesive force as a function of the capillary number, Ca, by applying existing theoretical models based on capillary interactions and viscous flow except at large values of Ca in the case of viscoelastic fluids, for which it was necessary to develop a nonlinear viscoelastic model. The compliance of the atomic force microscope colloid beam was an important factor in governing the retraction velocity of the probe and therefore the value of the adhesive force, but the inertia of the beam and viscoelastic stress overshoot effects were significant in the range of separation velocities investigated.