Highly enhanced liquid flows via thermoosmotic effects in soft and charged nanochannels

Abstract

Enhancing nanoscale liquid flows remains an existing challenge in nanofluidics. Here we propose the generation of highly augmented thermoosmotic (TOS) liquid flows in soft nanochannels (or nanochannels functionalized by grafting with end-charged polyelectrolyte or PE brushes) by employing an axial temperature gradient. The TOS transport is a combination of the induced-electric-field electroosmotic (EOS) transport and a thermo-chemioosmotic (TCOS) transport with the latter resulting from an induced pressure gradient on account of the changes associated with the imposition of the axial temperature gradient. The end-charged brushes develop an electric double layer (EDL) localized at the charged, non-grafted brush end. Depending on the system parameters, this EDL localization massively augments the influence of the EOS body force and the induced pressure-gradient resulting in a TOS transport in soft nanochannels that can be more than one order of magnitude larger than that in brush-free nanochannels. Given the existing notion that the presence of the brushes invariably reduces the flow strength, this result of massive flow augmentation is extremely significant and non-trivial serving as a paradigm shift in the study of liquid transport in brush-grafted nanochannels.

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

“…This study (Maheedhara et al 2018a) also showed enhanced transport, as the localization of the EOS body force is equally prevalent for the case with end-charged brushes. However, this study (Maheedhara et al 2018a) reported a less realistic flow field stemming from the fact that the brushes were modelled using the simplistic Alexander-de Gennes model (De Gennes 1976b;Alexander 1977), which in turn meant that the brushes had a uniform density distribution along their height leading to an over prediction of the brush-induced drag force (Sivasankar et al 2020a,b). On the other hand, this current paper that describes the brushes using an augmented SST considers a much more realistic monomer density distribution (having a much larger density at near-wall locations) and hence provides a more accurate description of the brush-induced drag force.…”

“…the induced EOS flow). Over the past several years, such TOS transport in nanofluidic systems has been extensively studied (Dietzel & Hardt 2016, 2017 and has been identified to be useful in applications ranging from generating electricity in nanocellulose channels using waste heat (Li et al 2019), energy conversion and storage (Chen et al 2019), enhanced nanofluidic transport in presence of functionalization with PE brushes (Maheedhara et al 2018a), etc. For the past decade and a half, functionalizing nanochannels with PE brushes has been employed for applications such as analyte and biomolecule sensing (Ali et al 2011), fabrication of ionic nanofluidic diodes and current rectifiers (Ali et al 2009(Ali et al , 2013Yameen et al 2009Yameen et al , 2010Lin et al 2016), etc.…”

“…The DNA is a charged polymer (or a PE) molecule and along that argument it is not too unreasonable an assumption that there will be a finite slip along the surface of the PE brushes. In our previous work (Maheedhara et al 2018a), we have analysed the TOS transport in a nanochannel grafted with end-charged PE brushes. This study (Maheedhara et al 2018a) also showed enhanced transport, as the localization of the EOS body force is equally prevalent for the case with end-charged brushes.…”

“…In our previous work (Maheedhara et al 2018a), we have analysed the TOS transport in a nanochannel grafted with end-charged PE brushes. This study (Maheedhara et al 2018a) also showed enhanced transport, as the localization of the EOS body force is equally prevalent for the case with end-charged brushes. However, this study (Maheedhara et al 2018a) reported a less realistic flow field stemming from the fact that the brushes were modelled using the simplistic Alexander-de Gennes model (De Gennes 1976b;Alexander 1977), which in turn meant that the brushes had a uniform density distribution along their height leading to an over prediction of the brush-induced drag force (Sivasankar et al 2020a,b).…”

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.…”