  This paper studies mechanism of preconcentration of charged particles in a
straight micro-channel embedded with permselective membranes, by numerically
solving coupled transport equations of ions, charged particles and solvent
fluid without any simplifying assumptions. It is demonstrated that trapping and
preconcentration of charged particles are determined by the interplay between
drag force from the electroosmotic fluid flow and the electrophoretic force
applied trough the electric field. Several insightful characteristics are
revealed, including the diverse dynamics of co-ions and counter ions,
replacement of co-ions by focused particles, lowered ion concentrations in
particle enriched zone, and enhanced electroosmotic pumping effect etc.
Conditions for particles that may be concentrated are identified in terms of
charges, sizes and electrophoretic mobilities of particles and co-ions.
Dependences of enrichment factor on cross-membrane voltage, initial particle
concentration and buffer ion concentrations are analyzed and the underlying
reasons are elaborated. Finally, post priori a condition for validity of
decoupled simulation model is given based on charges carried by focused charge
particles and that by buffer co-ions. These results provide important guidance
in the design and optimization of nanofluidic preconcentration and other
related devices.
