Coupled thermo-electro-osmotic characteristics in conical bipolar nanochannels: A comparative study of asymmetric surface charge polarity and electrolyte solutions

Harnessing low-grade waste heat into usable energy at the nanoscale presents a promising approach for sustainable energy generation. Nanofluidic systems are particularly well-suited for understanding and modulating thermally-driven transport through coupled thermo-electro-osmotic phenomena. This study employs a numerical model that integrates electrostatics, ion transport, fluid dynamics, and heat transfer to investigate how nanochannel geometric asymmetry (conicity), asymmetric surface charge distribution (bipolarity), and ion-specific thermodiffusion (Soret effect) influence thermo-electro-osmotic responses. Key findings reveal that these responses yield similar results in specific unipolar and bipolar configurations. This similarity arises from the dominant influence of the surface charge density (Σ) at the narrow end, which governs the direction and magnitude of ion transport under a thermal gradient. In positive unipolar (+Σ) and second bipolar (∓Σ configurations), increasing conicity and Debye parameter significantly enhance the short-circuit current. Conversely, the Seebeck coefficient generally decreases with these parameters, underscoring the Debye length's critical role in modulating the thermo-electric potential. In contrast, these thermo-electric parameters vary non-monotonically for negative unipolar (−Σ) and first bipolar (±Σ) configurations due to charge separation effects. Additionally, the thermo-osmotic coefficient, in both short- and open-circuit scenarios, displays complex, non-monotonic behaviors, including flow reversals driven by net forces under specific conditions. Comparing NaCl and NaI solutions, NaI demonstrates superior performance under certain conditions, attributed to the asymmetric thermophoretic behaviors of their anions (thermophilic I⁻ versus thermophobic Cl⁻). This work's core scientific contribution lies in elucidating non-monotonic transport behaviors, flow reversals, and performance differences under various conditions, which arise from the synergistic effects of geometry, surface charge, and ion-specific thermophoresis. These insights are crucial for understanding how the studied parameters can be tuned to design more efficient nanofluidic devices for thermal energy harvesting and precise fluid control.