  We consider the equilibrium behaviour and dynamics of liquid drops on a
superhydrophobic surface patterned with sawtooth ridges or posts. Due to the
anisotropic geometry of the surface patterning, the contact line can
preferentially depin from one side of the ratchets, leading to a novel,
partially suspended, superhydrophobic state. In both this configuration, and
the collapsed state, the drops show strong directional contact angle hysteresis
as they are pushed across the surface. The easy direction is, however,
different for the two states. This observation allows us to interpret recent
experiments describing the motion of water drops on butterfly wings.
