Eliminating the limiting-current phenomenon by geometric field focusing into nanopores and nanoslots

Abstract

A peculiar and undesirable current-voltage characteristic of nanoporous membranes is that it exhibits a voltage window with a near-constant limiting-current density when bulk ions near one surface of the membrane are depleted. We show both theoretically and experimentally that this interval disappears for an isolated circular nanopore (or narrow nanoslot) because radial field focusing at the pore entrance enhances the depletion effect and drives an ejecting hydrodynamic vortex pair that amplifies ion flux into the nanopore. This vortex pair is distinct from the vortex arrays that appear in front of a wide nanoslot or a nanoporous membrane with small inter-nanopore separation. It hence suggests that an optimal pore radius/separation ratio exists for maximum current density across a membrane.

Effect of geometry on concentration polarization in realistic heterogeneous permselective systems

“…This indicates that the conductance of the system may no longer be solely dependent on the permselective region geometry as commonly assumed in microchannel/nanochannel systems [11,12,30,31,36].…”

“…Such a geometry realistically describes systems that have been the subject of numerous recent experimental and numerical works [6,11,12,15,21,[27][28][29][30][31][32][34][35][36][37]41,42]. Additionally, this geometry can also describe a periodic array of permselective regions (e.g.…”

“…1). Solving for a 3 layer system versus the commonly solved 1 layer system, consisting solely of the depleted layer [3,11,19,25,26], will better facilitate the comparison of theoretical models to experiments [5,6,12,13,[27][28][29][30][31][32][33] and simulations [34][35][36][37].…”

“…Yossifon and coworkers [11,12] derived an expression for the conduction (per unit width) of nanoslot-dominated system, valid across the entire range of concentrations, using the well-known Donnan potential equilibrium relations [39,42] …”

“…membranes with a partly conducting surface area [10] or fabricated micro-nanochannel systems [11][12][13][14][15], the electroconvective mechanism induces strong corner vortices that stir the flow and in turn control the length of the diffusion layer. However, aside from these electro-convection effects, field focusing effects alone, stemming from the heterogeneity of 2D [16][17][18] and 3D [19] geometries, can affect the electro-diffusive solution to yield CP with much larger concentration gradients.…”