Dynamic contacts on viscoelastic films: Work of adhesion

Dynamic mechanical contacts with nanometer to millimeter dimensions are important in scanned probe microscopy, contact mechanics analysis, ultralow load indentation, microelectromechanical systems, compact disks, biological systems, pressure sensitive adhesives and so forth. The response of these contacts is poorly understood if they involve adhesive viscoelastic materials. We have used indentation to study contacts to styrene-butadiene latex films with a range of glass transition temperatures. Contact times were in the range 0.01-200 s and loads were in the micronewton to millinewton range. Diamond probes with Berkovich and spherical end shapes were used. Load versus displacement data showed substantial adhesion hysteresis between the loading and unloading portions. The hysteresis is at least partially due to creep as indicated by the continued increase in penetration after the start of unloading. We show that an extended Johnson-Kendall-Roberts (JKR) model due to Johnson provides a robust way to extract works of adhesion from data obtained at low loading. The range of the interaction potential between the probe and substrate is also obtained from the fits. Because this model neglects long-range creep effects, it breaks down at high loading rates.