Electrokinetics of non-Newtonian fluids in poly-electrolyte grafted nanochannels: Effects of ion-partitioning and confinement

Abstract

Fluid flow inside nano-channels coated with a polyelectrolyte brush layer (PEL) is dictated by the brush structure and its interaction with the counter-ion dominated electrical double layer (EDL) near the walls of the PEL channel. In this work, we bring out the confluence of spatial permittivity variation, the influence of proximity between ions in charged PEL channels and non-Newtonian constitutive behavior of the fluid towards altering the electrokinetics of power-law fluids as well as their mechanical to electrical energy conversion capabilities. Our estimates are based on considering the fluidic drag experienced by the ions mediated by effective confinement induced by other ions and fluid rheology. For small electrical double layer (EDL) thicknesses (corresponding to high ionic concentrations), the energy conversion efficiency is shown to be higher for shear-thinning fluids, with a crossover in these characteristics for higher EDL thicknesses. Results show that lower bulk to PEL charge density ratio corresponds to higher conversion efficiencies. PEL also dictates ionic drag in that higher PEL thickness corresponds to higher ionic drag. It has been observed that although the electrokinetic parameters such as streaming potential (electrical potential generated across a channel due to a pressure-driven flow) and energy conversion efficiency (based on the ratio of electrical energy harvested to mechanical energy input) are significantly influenced by the fluid and the nanochannel properties, there exists a cross-over EDL thickness for the variation in the conversion efficiency, which is solely dependent on the properties of the nanochannel. The consideration of the ionic confinement effect allows us to have a more realistic estimate of energy conversion efficiency and yields better estimates of critical regimes of operation depending on EDL thickness for optimal efficiency. These results may yield insight towards designing functional nanofluidic devices with biological interfaces.