Comment
Blue lines: Angle of incidence. Red line: Optimal angle.
For the first two angles between normals of spheres (1 and 2) the contact occurs under the conditions of adhesion; tangential stresses are small and the transmitted couple is small as well.
Under the optimal angle (3) the transmitted couple is maximal.
Subsequent increase of the angle (4, 5 and 6) reduces the couple. At the stage of unloading, adhesion zone in the center of contact area and counterslip near its boundary arise (see paper 22). The stress resultant changes its sign, but the total couple taken over the contact interval is however positive.