  Wetting of liquid droplets on passive surfaces is ubiquitous in our daily
lives, and the governing physical laws are well-understood. When surfaces
become active, however, the governing laws of wetting remain elusive. Here we
propose chemically active wetting as a new class of active systems where the
surface is active due to a binding process that is maintained away from
equilibrium. We derive the corresponding non-equilibrium thermodynamic theory
and show that active binding fundamentally changes the wetting behavior,
leading to steady, non-equilibrium states with droplet shapes reminiscent of a
pancake or a mushroom. The origin of such anomalous shapes can be explained by
mapping to electrostatics, where pairs of binding sinks and sources correspond
to electrostatic dipoles along the triple line. This is an example of a more
general analogy, where localized chemical activity gives rise to a multipole
field of the chemical potential. The underlying physics is relevant for cells,
where droplet-forming proteins can bind to membranes accompanied by the
turnover of biological fuels.
