  The spreading of liquid drops on soft substrates is extremely slow, owing to
strong viscoelastic dissipation inside the solid. A detailed understanding of
the spreading dynamics has remained elusive, partly owing to the difficulty in
quantifying the strong viscoelastic deformations below the contact line that
determine the shape of moving wetting ridges. Here we present direct
experimental visualisations of the dynamic wetting ridge, complemented with
measurements of the liquid contact angle. It is observed that the wetting ridge
exhibits a rotation that follows exactly the dynamic liquid contact angle -- as
was previously hypothesized [Karpitschka \emph{et al.} Nature Communications
\textbf{6}, 7891 (2015)]. This experimentally proves that, despite the contact
line motion, the wetting ridge is still governed by Neumann's law. Furthermore,
our experiments suggest that moving contact lines lead to a variable surface
tension of the substrate. We therefore set up a new theory that incorporates
the influence of surface strain, for the first time including the so-called
Shuttleworth effect into the dynamical theory for soft wetting. It includes a
detailed analysis of the boundary conditions at the contact line, complemented
by a dissipation analysis, which shows, again, the validity of Neumann's
balance.
