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Non-monotonic salt dependence of electro-osmotic flow in pH-regulated nanochannels

Published online by Cambridge University Press:  12 March 2025

Mingyu Duan Open the ORCID record for Mingyu Duan [Opens in a new window] ,
Luanzhe Xu Open the ORCID record for Luanzhe Xu [Opens in a new window] ,
Jiadong Chen Open the ORCID record for Jiadong Chen [Opens in a new window]  and
Guang Chen Open the ORCID record for Guang Chen [Opens in a new window]
Mingyu Duan
Affiliation:
Department of Advanced Manufacturing and Robotics, College of Engineering, Peking University, Beijing, 100871, PR China
Luanzhe Xu
Affiliation:
Department of Advanced Manufacturing and Robotics, College of Engineering, Peking University, Beijing, 100871, PR China
Jiadong Chen
Affiliation:
Department of Advanced Manufacturing and Robotics, College of Engineering, Peking University, Beijing, 100871, PR China
Guang Chen*
Affiliation:
Department of Advanced Manufacturing and Robotics, College of Engineering, Peking University, Beijing, 100871, PR China
*
Corresponding author: Guang Chen, guangc@pku.edu.cn
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Abstract

Electro-osmotic flow (EOF) in nanochannels exhibits a puzzling non-monotonic dependence on salt concentration, which contrasts with observations in microchannels and remains not fully understood. In this work, we address this phenomenon through a theoretical investigation of EOF in $\mathrm{pH}$-regulated channels. New analytical approximations for electrostatic potential, EOF profile and electro-osmotic mobility beyond the Debye–Hückel limit are derived through asymptotic analysis. Our findings reveal that the surface electrostatic potential is independent of the channel size only when the half-channel size exceeds the Gouy–Chapman length. In contrast, surface ionization and net charge distribution play more crucial roles in EOF at the nanoscale, as they govern both the magnitude and the spatial distribution of the Coulomb driving force. As salt concentration increases, EOF velocity initially rises due to enhanced surface ionization, followed by a decline attributed to increased wall shear stress. This work provides key insights for EOF applications in nanofluidics and biomedical devices, and deepens the understanding of electrokinetic phenomena influenced by $\mathrm{pH}$-regulation effects.

JFM classification

MHD and Electrohydrodynamics: Electrokinetic flows
Type
JFM Rapids
Information
Journal of Fluid Mechanics , Volume 1007 , 25 March 2025 , R2
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Copyright
© The Author(s), 2025. Published by Cambridge University Press

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