  The steady state structure of an interface in an Ising system on a square
lattice placed in a {\em non-uniform} external field, shows a commensurate
-incommensurate transition driven by the velocity of the interface. The
non-uniform field has a profile with a fixed shape which is designed to
stabilize a flat interface, and is translated with velocity $v_{e}$. For small
velocities the interface is stuck to the profile and is rippled with a
periodicity which may be either commensurate or incommensurate with the lattice
parameter of the square lattice. For a general orientation of the profile, the
local slope of the interface locks in to one of infinitely many rational
directions producing a devil's staircase structure. These ``lock-in'' or
commensurate structures dissappear as $v_e$ increases through a kinetics driven
commensurate - incommensurate transition. For large $v_e$ the interface becomes
detached from the field profile and coarsens with Kardar-Parisi-Zang exponents.
The complete phase-diagram and the multifractal spectrum corresponding to these
structures have been obtained numerically together with several analytic
results concerning the dynamics of the rippled phases. Our work has
technological implications in crystal growth and the production of surfaces
with various desired surface morphologies.
