Electrokinetic migration of surface-modified particles under a direct current electric field in viscoelastic fluids

Direct current (DC) electromigration of three surface-modified polystyrene (PS) particles in straight microchannels was investigated using buffer-based polyethylene oxide (PEO) solutions with concentrations cPEO from 0 to 0.1 wt. % and a viscosity-matched 21 wt. % glycerol reference. The pH was adjusted to keep particle zeta potential ζp comparable across PEO concentrations, highlighting viscoelastic effects on electromigration. Linear electrokinetic behavior at low electric fields and non-linear behavior at higher fields were analyzed. The results suggest that the particle's overall velocity Vp decreases with cPEO. For PS-NH2 particles, direction reversal occurs at a lower critical field as cPEO increases. In the low-field region, the linear electrokinetic mobility μEK decreases with cPEO, with sensitivity ordered as sulfonate-modified PS particle (PS-SO3H) > amino-modified PS particle (PS-NH2) > unmodified PS particle (PS-Plain). For viscosity-matched 0.1 wt. % PEO solution and 21 wt. % glycerol, Vp in PEO is slightly higher than in glycerol. At high fields, the non-linear electrophoretic mobility μEP, NL decreases as |ζp| and cPEO increase. The absolute non-linear index |n| increases with |ζp| when |ζp| < 25 mV and approaches 2.0 at higher |ζp|. Increasing cPEO shifts |n| toward lower values. The key points include the pH-controlled ζp and a unified linear to non-linear framework, which quantifies how viscoelasticity affects electrokinetic mobility and non-linear responses. These results provide a practical basis for DC-driven separation strategies that combine surface modifications with fluid rheology.