  The interfacial mechanics of soft elastic networks play a central role in
biological and technological contexts. Yet, effects of solid capillarity have
remained controversial, primarily due to the strain-dependent surface energy.
Here we derive the equations that govern the selection of contact angles of
liquid drops on elastic surfaces from variational principles. It is found that
the substrate's elasticity imposes a nontrivial condition that relates pinning,
hysteresis and contact line mobility to the so-called Shuttleworth effect. We
experimentally validate our theory for droplets on a silicone gel, revealing an
enhanced contact line mobility when stretching the substrate.
