  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.
