  From hydrogels and plastics to liquid crystals, soft solids cover a wide
array of synthetic and biological materials that play key enabling roles in
advanced technologies such as 3D printing, soft robotics, wearable electronics,
self-assembly, and bioartificial tissues. Their elasticity and stimuli-induced
changes in mechanical, optical, or electrical properties offer a unique
advantage in designing and creating new dynamically functional components for
sensing, micro-actuation, colour changes, information, and mass transport. To
harness the vast potential of soft solids, a thorough understanding of their
reactions when exposed to liquids is needed. Attempts to study the interactions
between soft solids and liquids have largely focused on the wetting of soft
solids and its resulting deformation at equilibrium or in a quasi-static state.
Here, we consider the frequently encountered case of unsteady wetting of a
liquid on a soft solid and show that transient deformation of the solid is
necessary to understand unsteady wetting behaviours. We find that the initial
spreading of the liquid occurs uninterrupted in the absence of solid
deformation. This is followed by intermittent spreading, in which transient
deformation of the solid at the three-phase contact line (CL) causes the CL
motion to alternate alternation between CL sticking and slipping. We identify
the spreading rate of liquids and the viscoelastic reacting rate of soft solids
as the two competing factors in dictating intermittent spreading. We formulate
and validate experimentally the conditions required for the contact line to
transition from sticking to slipping. By considering the growing deformation of
soft solids as dynamic surface heterogeneities, our proposed conditions for
stick-slip transition in unsteady wetting on soft solids broaden the classical
theory on wetting hysteresis on rigid solids.
