Anti-Electrophoretic Response Driven Bending-Tilting Deformation of Cationic Polyelectrolyte Brushes Drives Massively Non-Linear Electroosmotic Transport in Brush-Grafted Nanochannels

The interplay of the electric field-driven response of charged polyelectrolyte (PE) brushes and the supported counterions and water molecules dictate the intriguing fluid dynamics in a PE-brush-grafted nanochannel. Such combined brush-ion-fluid dynamics can be captured in unprecedented atomistic details using all-atom molecular dynamics (MD) simulations unraveling a plethora of new and fascinating nanoscale science. In this paper, we use such simulations to describe a most remarkable non-linearly enhanced large electroosmotic (EOS) flow, where, in a nanochannel grafted with cationic PMETAC ([Poly(2-(Methacryloyloxy)Ethyl) Trimethylammonium Chloride]) brushes, a two-fold increase in the electric field strength leads to a several-fold (much more than two-fold) increase in the EOS flow strength and volume flow rate. The electric field enforces the PMETAC brushes to undergo a bending-tilting driven deformation with a significant portion of the brush layer becoming parallel to the grafting surface. In response, a substantial fraction of the counterions come out of the brush layer (hence become more mobile), but instead of going into the bulk, accumulate at the brush-bulk interface, i.e., stay in close proximity of the brush segments aligned parallel to the grafting surface. This creates a fascinating situation, where the counterions are not completely within the brush layer, yet they fully screen the brush charges. Such “freer” conditions enable the counterions to achieve very high velocity, which eventually ensures that the water solvating the counterions themselves move very fast triggering the significantly augmented EOS transport. Probing deeper we can identify that the bending-tilting driven brush deformation, enforcing the brushes to align parallel to the substrate, results from the anti-electrophoretic behavior of the brushes, where despite being positively charged, the brushes move against the electric field direction: such a highly unprecedented behavior can be associated with the very fast velocities of the negatively charged counterions and the electrostatic coupling of the counterions with the positive functional groups of the brushes. We anticipate that the findings of this paper will not only open up novel strategies for flow enhancement in functionalized nanochannels and shed light on a myriad of applications associated with the anti-electrophoretic response of charged polymer chains, it will kindle interests in the need for understanding the detailed chemical architecture of polyelectrolyte systems (beyond coarse-graining based simplifications) in nanoscale science and engineering.