Transport processes in nanochannels induced by electric and temperature fields
Creators: name: Hardt, Steffen
Publisher: name: Zenodo
Year: 2024
Subjects: subject: Energy conversion
subject: Energy
subject: Electric energy
subject: Renewable energy
subject: Energy management
subject: Energy technology
subject: Energy utilisation
subject: Energy industry
subject: energy harvesting
subject: Waste heat
subject: waste heat
subject: waste heat recovery
subject: Waste heat utilisation
subject: Waste recovery
subject: nanofluidics
subject: Ion Channels
subject: Ion Transport
subject: Ion Channels/chemistry
subject: ion intercalation
subject: seebeck coefficient
subject: nanochannels
subject: electroosmotic flow
subject: gate electrodes
subject: Electricity generation
subject: electricity
subject: Bioelectric Energy Sources/adverse effects
subject: thermoelectric energy
subject: electrolyte
subject: Electric power
subject: electric double layers

description: Abstract:

Nanofluidics is an emerging field that has rapidly developed in the past few years, partially due to advances in nanofabrication technology. For a number of reasons, transport processes in nanochannels differ from transport through larger-scale channels. First, in very small-scale channels, continuum-mechanical models loose their validity. Second, overlapping electric double layers from opposing channel walls cause effects such as ion exclusion that are absent on larger scales. This presentation will be concerned with the second scenario and will discuss theoretical and simulation results that are based on a continuum-mechanical description for electrolyte-filled nanochannels. 

Nanochannels are true multiphysics entities in which the coupling of a number of different effects needs to be taken into account. Specifically, flow and species transport can either be driven by electric fields or temperature gradients. With respect to electric-field induced driving, two special scenarios are presented: electroosmotic flow (EOF) through conical channels equipped with gate electrodes and EOF through uncharged channels. We show that in channels with gate electrodes, a sinusoidal voltage applied to the electrode results in a net flow, provided that the spatial symmetry is broken. Here, the symmetry breaking is due to the conical shape of the channel. The standard way to induce EOF, however, is to apply an external DC electric field between the channel ends. Here, the common understanding is that the channel walls need to be charged to produce a flow. We show, however, that there can be a significant net EOF even through uncharged channels, an effect that relies on a special type of symmetry breaking. In that case, the symmetry is broken if the anion has a different valence than the cation. 

By contrast to EOF, the flow induced in nanochannels by temperature gradients is usually rather weak. However, if the solid surfaces have a non-zero slip length, the situation changes. We show that very significant thermoosmotic flow velocities can be obtained in nanochannels with realistic slip-length values. Finally, thermoelectric energy conversion processes in nanochannels are considered, that is, the generation of an electric field and/or an electric current upon application of a temperature gradient along the channel. In the past few years, very large values of the thermoelectric field that are hard to explain theoretically have been reported in experiments. Inspired by that, we have considered novel scenarios of thermoelectric energy conversion in nanochannels. On the one hand, we show that large thermoelectric fields can be obtained in channels filled with ionic liquids. The underlying physical mechanism is thermally activated charge carrier generation. On the other hand, we consider ion transport by thermally activated jumping over a potential energy barrier, which also produces unusually large values of the thermoelectric field. 

In all of the above-mentioned scenarios, the governing equations are solved numerically using a finite-element method. In addition, analytical approximations are presented that match well with the numerical results.
description: This presentation was delivered by Dr Steffen Hardt from TU Darmstadt as a part of his invited talk in a mathematics seminar organised by New Jersey Institute of Technology.

TRANSLATE is a €3.4 million EU-funded research project that aims to develop a new nanofluidic platform technology to effectively convert waste heat to electricity. This technology has the potential to improve the energy efficiency of many devices and systems, and provide a radically new zero-emission power source. The TRANSLATE project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement number 964251, for the action of 'The Recycling of waste heat through the Application of Nanofluidic ChannelS: Advances in the Conversion of Thermal to Electrical energy'. More information can be be found on the TRANSLATE project website: https://translate-energy.eu/