A cascade of adhesive sliding instabilities unveiled via a postpredictive analysis

The adhesion/friction interplay continues to be an enigma when soft matter is involved. By utilizing a simple two-stage (loading and then unloading) mixed-type (normal and tangential) indentation testing of a transparent layer of adhesive rubber-like gel material, the normal and tangential contact forces acting on a rigid spherical probe are monitored along with the in situ variation of the apparent contact area between the probe and the layer surface. Whereas the force/area relations for the oblique loading and normal unloading do almost coincide, as seen with the naked eye, the postpredictive analysis reveals a drastic difference in variation of the apparent work of adhesion in the concord to fluctuations of the friction force. Each of the interface slips such exposed starts with simultaneous minimum and maximum values of the normal and tangential force sensors, respectively, and proceeds with a non-monotonic variation of the contact area, indicating adhesive reattachment. The occurrence of sliding instabilities upon unloading is supported by the on-the-spot observations of the contact area.