  The most direct measurement of adhesion is the pull-off force, i.e. the
tensile force necessary to separate two solids in contact. For a given
interface, it depends on various experimental parameters, including separation
speed, contact age and maximum loading force. Here, using smooth contacts
between elastomer spheres and rigid plates, we show that the pull-off force
also varies if the contact is sheared prior to separation. For shear
displacements below a critical valueabout 10% of that necessary to yield gross
sliding, the pull-off force steadily decreases as shear increases. For larger
shear, the pull-off force remains constant, at a residual value 10%--15% of its
initial value. Combining force measurements and in situ imaging, we show how
the unloading path leading to contact separation is modified by the initial
shear. In particular, we find that the residual pull-off force prevails if the
contact reaches full sliding during unloading. Based on those observations, a
first modeling attempt of the critical shear displacement is proposed,involving
a competition between jump instability and transition to sliding. Overall,
those results offer new insights into the interplay between adhesion and
friction, provide new constraints on adhesion measurements and challenge
existing adhesive models. They will be useful wherever soft contacts undergo
both normal and shear stresses, including tire grip, soft robotics, haptics and
animal locomotion.
