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

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 (VG) would generate heterogeneous surface charge at the channel walls which can be modulated by VG, i.e., a moderate VG (0.7–0.9 V) causes charge inversion in diverging sections of the channel (Dmax), while VG > 0.9 enables charge inversion in the entire channel, but it is still non-uniform in each section. The results show that zeta (ζ) potential is a function of VG which shows a linear to nonlinear 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 ζ potential. It is also shown that the difference in ζ potential across the channel (Δζ) increases by dilution of electrolyte and utilizing a more symmetric electrolyte with lower valances. For the first time, it is shown that Δζ presents a maximum with the VG. The VG corresponding to the maximum Δζ 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.