  The contact line of a liquid drop on a solid exerts a nanometrically sharp
surface traction. This provides an unprecedented tool to study highly localised
and dynamic surface deformations of soft polymer networks. One of the
outstanding problems in this context is the stick-slip instability, observed
above a critical velocity, during which the contact line periodically depins
from its own wetting ridge. Time-resolved measurements of the solid deformation
are challenging, and the mechanism of dynamical depinning has remained elusive.
Here we present direct visualisations of the dynamic wetting ridge formed by
water spreading on a PDMS gel. Unexpectedly, it is found that the opening angle
of the wetting ridge increases with speed, which cannot be attributed to bulk
rheology, but points to a dynamical increase of the solid's surface tensions.
From this we derive the criterion for depinning that is confirmed
experimentally. Our findings reveal a deep connection between stick-slip
processes and newly identified dynamical surface effects.
