  We assemble charged colloidal spheres at deliberately chosen locations on a
charged unstructured glass substrate utilizing ion exchange based
electro-osmotic micro-pumps. Using microscopy, a simple scaling theory and
Brownian Dynamics simulations, we systematically explore the control parameters
of crystal assembly and the mechanisms through which they depend on the
experimental boundary conditions. We demonstrate that crystal quality depends
crucially on the assembly distance of the colloids. This is understood as
resulting from the competition between inward transport by the electro-osmotic
pump flow and the electro-phoretic outward motion of the colloids. Optimized
conditions include substrates of low and colloids of large electro-kinetic
mobility. Then a sorting of colloids by size is observed in binary mixtures
with larger particles assembling closer to the ion exchanger beads. Moreover,
mono-sized colloids form defect free single domain crystals which grow outside
a colloid-free void with facetted inner crystal boundaries centred on the ion
exchange particle. This works remarkably well, even with irregularly formed ion
exchange resin splinters.
