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Abstract:
The contact line of a liquid drop on a solid exerts a nanometrically sharp surface traction. This provides
an unprecedented tool to study highly localized 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.
