Electrophoresis of Polyelectrolytes in Microfluidic Confinement
The mobility of long polyelectrolytes is constant in free solution because the electric force and drag increase at the same rate. This ensures that electrophoresis cannot sort macromolecules by length in free solution. One way to cause the mobility to change is to confine the charged chains within microfluidic devices. Some experiments find that the mobility decreases but others find that the mobility can increase with further confinement.
Many things can be occuring simultaneously in such experiments: monomers can get momentarily trapped at the surface, they can hydrodynamically interact with the walls (though the walls also screen long-range hydrodynamic interactions), electro-osmotic flow is commonly generated, etc.
To get a foothold on the problem, we simplify the system. We confine a polyelectrolyte (modelled via the mean-field MPCD-MD Debye-Huckel method) in a radially harmonic well, which does not act on the fluid --- only the monomers. We then perform electrophoresis in this simplified system and find that the mobility can indeed increase if the confinement is sufficiently great.