Flow and streaming potential of an electrolyte in a channel with an axial temperature gradient

TL;DR

This paper investigates how axial temperature gradients influence flow profiles and streaming potentials in electrolyte-filled channels, deriving analytical models that account for multiple thermal and electrokinetic effects, including thermoelectric fields and thermoosmosis.

Contribution

It provides a comprehensive analytical framework for understanding non-isothermal electrokinetic phenomena in channels, highlighting the interplay of various thermal effects on flow and electric fields.

Findings

Thermophoretic ion motion increases EDL thickness with temperature.

Temperature-dependent permittivity shrinks the EDL.

Thermoelectric fields can be generated by temperature gradients, especially under confinement.

Abstract

The effect of an axial temperature gradient on the flow profile and the induced streaming potential of a pressure-driven symmetric electrolyte in a slit channel is investigated. Based on the non-isothermal Nernst-Planck equations as well as the Poisson equation in the lubrication approximation, expressions for the ion distribution in the electric double layer (EDL) are derived. It is found that thermophoretic ion motion and a temperature-dependent electrophoretic ion mobility increase the local EDL thickness with temperature, whereas a temperature-dependent permittivity shrinks the EDL. Within the Debye-H\"uckel approximation, the Navier-Stokes equation with the corresponding electric body force terms is solved. Analytical expressions for the flow profile and the induced (streaming) field under non-isothermal conditions are derived. It is shown that for such a situation the induced electric field is the linear superposition of at least seven individual contributions. For very wide channels, only the thermoelectric field typically present in bulk electrolytes when subjected to a temperature gradient (Soret equilibrium) as well as the conventional pressure-induced streaming field are of importance. Under extreme confinement, selective thermo-electro-migration driven by the interplay between the temperature-dependent electrophoretic ion mobility and the interaction of the ions with the surface wall charge causes a thermoelectric field of non-advective origin. For wider channels and besides the well-known thermoosmosis due to the temperature dependence of the dielectric permittivity, it is demonstrated that a temperature gradient renders the ion cloud in the EDL out of mechanical equilibrium. This leads to a thermoosmotic flow, and the ion advection affiliated with it may induce a thermoelectric field of similar order of magnitude as the one caused by more conventional thermal effects.