Massively Enhanced Electroosmotic Transport in Nanochannels Grafted with End-Charged Polyelectrolyte Brushes

We establish that nanochannels grafted with pH-responsive, end-charged polyelectrolyte (PE) brushes demonstrate a massive augmentation in the strength of the electroosmotic (EOS) transport in the presence of an external electric field. This contradicts the existing understanding that the EOS transport is severely retarded in channels grafted with the PE brushes due to the brush-induced enhanced drag force. Our mathematical model, developed on the basis of the fact that the ion concentration polarization (ICP) effect can be neglected, explains this enhancement of the EOS transport by noting that the end-charged PE brushes demonstrate a unique ability of localizing the electric double layer (or the EDL) or equivalently localizing the maximum charge density of the electrolyte ions at the location of its end, i.e., away from the grafting surface. Accordingly, the maximum EOS driving force on the liquid, which is proportional to this charge density, can be maximum at a location far away from the wall. As a consequence, the resulting local EOS velocity suffers very little retardation due to the wall shear stress enabling such massive augmentation of the EOS transport. We anticipate that the present paper will unravel a completely new paradigm in the employment of functionalized interfaces in regulating the nanofluidic transport for a myriad of applications.