Thermoelectric effect and temperature-gradient-driven electrokinetic flow of electrolyte solutions in charged nanocapillaries

TL;DR

This paper presents a comprehensive theoretical analysis of thermoelectric effects and electrokinetic flows in charged nanocapillaries driven by temperature gradients, highlighting mechanisms, synergy conditions, and potential applications.

Contribution

It develops a semianalytical model solving coupled non-isothermal electrokinetic equations, clarifying the interplay of thermoelectric mechanisms and proposing synergy conditions for electrolyte transport.

Findings

Flow is nearly unidirectional with velocity varying along the axis.

Seebeck coefficient and flow velocity depend on electrolyte parameters.

Thermoelectric mechanisms can cooperate or counteract, affecting flow direction and magnitude.

Abstract

A systematic theoretical study of thermoelectric effect and temperature-gradient-driven electrokinetic flow of electrolyte solutions in charged nanocapillaries is presented. The study is based on a semianalytical model developed by simultaneously solving the non-isothermal Poisson-Nernst-Planck-Navier-Stokes equations with the lubrication theory. Particularly, this paper clarifies the interplay and relative importance of the thermoelectric mechanisms due to (a) the convective transport of ions caused by the fluid flow, (b) the dependence of ion electrophoretic mobility on temperature, (c) the difference in the intrinsic Soret coefficients of cation and anion. Additionally, synergy conditions for the three thermoelectric mechanisms to fully cooperate are proposed for thermo-phobic/philic electrolytes. The temperature-gradient-driven electrokinetic flow is shown to be a nearly unidirectional flow whose axial velocity profiles vary with the axial location. Also, the flow can be regarded as a consequence of the counteraction or cooperation between a thermoelectric-field-driven electroosmotic flow and a thermo-osmotic flow driven by the osmotic pressure gradient and dielectric body force. Moreover, the Seebeck coefficient and the fluid average velocity are demonstrated to be affected by electrolyte-related parameters. The results are beneficial for understanding the temperature-gradient-driven electrokinetic transport in nanocapillaries and also serve as theoretical foundation for the design of low-grade waste heat recovery devices and thermoosmotic pumps.