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

Abstract

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.

Thermo-osmotic transport in nanochannels grafted with pH-responsive polyelectrolyte brushes modelled using augmented strong stretching theory

“…This issue is the presence of possible molecular slip that the fluid flow experiences along the PE brushes. The presence of this slip will imply that the brushes are not strictly behaving as rigid solid cylinders, which in turn, coupled with the localization of the EDL body force away from the nanochannel wall, is responsible for the flow enhancement that we observe (both in this paper as well as all our previous papers (Chen & Das 2017;Chen et al 2018;Maheedhara et al 2018a,b;Sachar et al 2019a;Sivasankar et al 2020a,b)). How can one justify the presence of such intermolecular slip along the brushes?…”

“…Such reduced fluid flows are considered as a critical advantage of such brush-functionalized nanochannel systems: weak flow strength ensures that the differences in the ionic migration of the different ions, which is the key to several of the above applications, does not get masked by the background advective transport that acts equally on all the different types of ions. Very recently, in a series of papers (Chen & Das 2017;Chen, Sachar & Das 2018;Maheedhara et al 2018a,b;Sachar, Sivasankar & Das 2019a;Sivasankar et al 2020a,b), however, we have established that such brush-induced weakening of flow strength in PE-brush-grafted nanochannels might not always be true. We showed that for certain cases where nanochannels are grafted with end-charged PE brushes (Chen & Das 2017;Chen et al 2018;Maheedhara et al 2018a,b), some particular combination of grafting density, brush properties and salt concentration will lead to significantly enhanced nanofluidic EOS or induced EOS transport.…”

“…Very recently, in a series of papers (Chen & Das 2017;Chen, Sachar & Das 2018;Maheedhara et al 2018a,b;Sachar, Sivasankar & Das 2019a;Sivasankar et al 2020a,b), however, we have established that such brush-induced weakening of flow strength in PE-brush-grafted nanochannels might not always be true. We showed that for certain cases where nanochannels are grafted with end-charged PE brushes (Chen & Das 2017;Chen et al 2018;Maheedhara et al 2018a,b), some particular combination of grafting density, brush properties and salt concentration will lead to significantly enhanced nanofluidic EOS or induced EOS transport. We argued that such enhancement stemmed from the localization of the EDL charge density (and hence the EOS body force) away from the nanochannel wall (or the location of the maximum drag force).…”

“…Nanofluidic transport involves the flow of liquids and flow-driven transport of ions, solutes, bioanalytes and other species in nanochannels and nanopores (Eijkel & Van Den Berg 2005;Sparreboom et al 2009;Das et al 2012;Ziemys et al 2012;Koltonow & Huang 2016;Gao et al 2017;Zhu et al 2019). Such transport has received immense attention over the past few decades motivated by its applications in fabricating sensors that require very little sample volumes (Venkatesan & Bashir 2011;Miles et al 2013), devices capable of ionic gating (Liu et al 2015;Fang et al 2016), and platforms capable of a variety of biomedical applications (Hood et al 2017;Weerakoon-Ratnayake et al 2017), water filtration and desalination (Chen et al 2017;Anand et al 2018), oil recovery (Zhang et al 2019), etc. The significantly large interfacial effects in such nanofluidic systems have enabled the utilization of novel and unconventional flow-driving mechanisms such as electro-osmotic (EOS) transport (Eijkel & Van Den Berg 2005).…”

Ionic current in nanochannels grafted with pH‐responsive polyelectrolyte brushes modeled using augmented strong stretching theory

“…In a recent set of papers, we made the first attempt to describe the ionic and liquid transport in PE brush–grafted nanochannels in a framework that accounts for the PE brush configuration while describing the PE brush EDL electrostatics . In other words, the PE brush EDL electrostatics is obtained by minimizing the free energy of the entire system that also yields the corresponding PE brush configuration.…”