The adhesive properties of viscoelastic liquid films

The adhesive characteristics of thin films (0.2 - 2 µm) of linear PDMS liquids with a wide range of molecular weights have been measured using an AFM with a colloid probe (diameters 5 and 12 µm) for different separation velocities. The data were consistent with a residual film in the contact region having a thickness of ~ 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 AFM 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 not significant in the range of separation velocities investigated.