Understanding the coupled thermo-electro-osmotic transport in asymmetrically charged nanochannels

Abstract

Nanofluidic thermo-diffusion, encompassing both thermo-electric and thermo-osmotic effects, is gaining increasing attention for applications in low-grade thermal energy conversion, bio-molecular sensing, charge separation, and desalination. However, the influence of asymmetric surface charge density in thermally-driven nanochannel configurations has remained largely unexplored. This study presents a computational investigation using the extended Nernst–Planck–Poisson–Navier–Stokes and energy equations to examine the effects of Debye length and surface charge configurations (unipolar vs. bipolar) on thermo-electro-osmotic characteristics in nanochannels. Two electrolyte solutions, NaCl and NaI, were assessed, with a focus on ion-specific thermophobic and thermophilic behaviors. The results reveal that unipolar channels show a strong dependence on the Debye length, with significant effects on short-circuit current and Seebeck coefficient, while bipolar configurations exhibit rectified and monotonic behavior that is largely independent of surface charge density. Thermo-osmotic coefficients, evaluated under both short- and open-circuit conditions, demonstrate that bipolar channels accumulate responses with decreasing Debye length, contrasting with the discrete shifts observed in unipolar channels. The superior thermophoretic properties of ${\text{I}}^{-}$ in NaI solutions consistently lead to higher performance compared to NaCl, particularly in specific bipolar configurations. These findings underscore the critical role of surface charge polarity and ionic mobility in influencing the magnitude and direction of thermo-electro-osmotic responses, highlighting the potential of asymmetric nanochannels to achieve controlled ionic transport and rectification. This work provides essential insights for the design of next-generation nanofluidic devices and advances our understanding of thermally-driven transport phenomena in nanofluidic systems.