  Liesegang patterns emerge from precipitation processes and may be used to
build bulk structures at submicron lengthscales. Thus they have significant
potential for technological applications provided adequate methods of control
can be devised. Here we describe a simple, physically realizable
pattern-control based on the notion of driven precipitation, meaning that the
phase-separation is governed by a guiding field such as, for example, a
temperature or a pH field. The phase-separation is modeled through a
non-autonomous Cahn-Hilliard equation whose spinodal is determined by the
evolving guiding field. Control over the dynamics of the spinodal gives control
over the velocity of the instability front which separates the stable and
unstable regions of the system. Since the wavelength of the pattern is largely
determined by this velocity, the distance between successive precipitation
bands becomes controllable. We demonstrate the above ideas by numerical studies
of a 1D system with diffusive guiding field. We find that the results can be
accurately described by employing a linear stability analysis (pulled-front
theory) for determining the velocity -- local-wavelength relationship. From the
perspective of the Liesegang theory, our results indicate that the so-called
revert patterns may be naturally generated by diffusive guiding fields.
