Effect of electrolyte concentration and symmetry on the heterogeneous surface charge in an electrically gated nanochannel

TL;DR

This study explores how electrolyte concentration, symmetry, and gate voltage influence surface charge heterogeneity and fluid flow in nanochannels, revealing new insights into charge inversion, potential modulation, and flow control for nanofluidic devices.

Contribution

It demonstrates for the first time how gate voltage and electrolyte properties can be used to control surface charge heterogeneity and fluid flow in nanochannels with complex geometries.

Findings

Gate voltage induces charge inversion in nanochannels.

Electrolyte dilution affects zeta potential and charge difference.

Applying gate voltage is more effective than electrolyte modification for flow control.

Abstract

The present study aims to investigate utilizing field-effect for inducing heterogeneous surface charge and consequently changing the fluid flow in a solid-state nanochannel with converging-diverging periodicity. It is shown that the combination of geometry and applied gate voltage (V_{G}) would generate heterogeneous surface charge at the channel walls which can be modulated by V_{G}, i.e. a moderate V_{G} (0.7-0.9 V) causes charge inversion in diverging sections of the channel while V_{G} > 0.9 enables charge inversion in the entire channel but it is still non-uniform in each section. The results show that zeta ({\zeta}) potential is a function of V_{G} which shows a linear to non-linear transition due to dilution of electrolyte in agreement with density functional theory and Monte Carlo simulations. In contrast, electrolyte symmetry has a minor effect on the variation of {\zeta} potential. It is also shown that the difference in {\zeta} potential across the channel ({\Delta}{\zeta}) increases by dilution of electrolyte and utilizing a more symmetric electrolyte with lower valances. For the first time, it is shown that {\Delta}{\zeta} presents a maximum with the V_{G}. The V_{G} corresponding to the maximum {\Delta}{\zeta} decreases with both dilution of electrolyte and higher anion valance. This is of practical importance to overcome leakage current problem of field-effect fluidic devices. It is also shown that the velocity field can be altered by changing both electrolyte concentration and symmetry. However, applying VG was found to be a more efficient way than electrolyte modifications. This includes generating circulation inside the channel which is of prime importance for applications such as mixing or separation/trapping.