posted on 2022-12-09, 13:04authored byMark Aarts, Willem Q. Boon, Blaise Cuénod, Marjolein Dijkstra, René van Roij, Esther Alarcon-Llado
Fluidic devices exhibiting
ion current rectification (ICR), or
ionic diodes, are of broad interest for applications including desalination,
energy harvesting, and sensing, among others. For such applications
a large conductance is desirable, which can be achieved by simultaneously
using thin membranes and wide pores. In this paper we demonstrate
ICR in micrometer sized conical channels in a thin silicon membrane
with pore diameters comparable to the membrane thickness but both
much larger than the electrolyte screening length. We show that for
these pores the entrance resistance is key not only to Ohmic conductance
around 0 V but also for understanding ICR, both of which we measure
experimentally and capture within a single analytic theoretical framework.
The only fit parameter in this theory is the membrane surface potential,
for which we find that it is voltage dependent and its value is excessively
large compared to the literature. From this we infer that surface
charge outside the pore strongly contributes to the observed Ohmic
conductance and rectification by a different extent. We experimentally
verify this hypothesis in a small array of pores and find that ICR
vanishes due to pore–pore interactions mediated through the
membrane surface, while Ohmic conductance around 0 V remains unaffected.
We find that the pore–pore interaction for ICR is set by a
long-ranged decay of the concentration which explains the surprising
finding that the ICR vanishes for even a sparsely populated array
with a pore–pore spacing as large as 7 μm.