  We report on a novel and flexible experiment to investigate the
non-equilibrium melting behaviour of model crystals made from charged colloidal
spheres. In a slit geometry polycrystalline material formed in a low salt
region is driven by hydrostatic pressure up an evolving gradient in salt
concentration and melts at large salt concentration. Depending on particle and
initial salt concentration, driving velocity and the local salt concentration
complex morphologic evolution is observed. Crystal-melt interface positions and
the melting velocity are obtained quantitatively from time resolved Bragg- and
polarization microscopic measurements. A simple theoretical model predicts the
interface to first advance, then for balanced drift and melting velocities to
become stationary at a salt concentration larger than the equilibrium melting
concentration. It also describes the relaxation of the interface to its
equilibrium position in a stationary gradient after stopping the drive in
different manners. We further discuss the influence of the gradient strength on
the resulting interface morphology and a shear induced morphologic transition
from polycrystalline to oriented single crystalline material before melting.
