  In this work, we investigate dissipative effects involved during the
detachment of a smooth spherical glass probe from a viscoelastic silicone
substrate patterned with micro-asperities. As a baseline, the pull-off of a
single asperity, millimeter-sized contact between a glass lens and a smooth
poly(dimethylsiloxane) (PDMS) rubber is first investigated as a function of the
imposed detachment velocity. From a measurement of the contact radius a(t) and
normal load during unloading, the dependence of the strain energy relase rate G
on the velocity of the contact line vc = da/dt is determined under the
assumption that viscoelastic dissipation is localized at the edge of the
contact. These data are incorporated into Muller's model (V.M. Muller J Adh Sci
Tech (1999) 13 999-1016) in order to predict the time-dependence of the contact
size. Similar pull-off experiments are carried out with the same PDMS substrate
patterned with spherical micro-asperities with a prescribed height
distribution. From in situ optical measurements of the micro-contacts, scaling
laws are identified for the contact radius a and the contact line velocity vc.
On the basis of the observed similarity between macro and microscale contacts,
a numerical solution is developed to predict the reduction of the contact
radius during unloading.
