We report on the multi-contact frictional dynamics of model elastomersurfaces rubbed against bare glass slides. The surfaces consist of layerspatterned with thousands spherical caps (radius of curvature 100 $\mu$m)distributed both spatially and in height, regularly or randomly. Use ofspherical asperities yields circular micro-contacts whose radius is a directmeasure of the contact pressure distribution. In addition, optical tracking ofindividual contacts provides the in-plane deformations of the tangentiallyloaded interface, yielding the shear force distribution. We then investigatethe stick-slip frictional dynamics of a regular hexagonal array. For all stickphases including the initial one, slip precursors are evidenced. They are foundto propagate quasi-statically, normally to the iso- pressure contours. A simplequasi-static model relying on the existence of interfacial stress gradients isderived and predicts qualitatively the position of slip precursors.
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